EARLY-LIFE STAGES IN CHESAPEAKE BAY: MECHANISMS AND IMPLICATIONS. Elizabeth Watkins North, Doctor of Philosophy, 2001

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1 Title of Dissertation: TRANSPORT AND RETENTION OF FISH EARLY-LIFE STAGES IN CHESAPEAKE BAY: MECHANISMS AND IMPLICATIONS FOR RECRUITMENT Elizabeth Watkins North, Doctor of Philosophy, 2001 Dissertation directed by: Professor Edward D. Houde Marine-Estuarine-Environmental Sciences Mechanisms of transport and retention of fish eggs and larvae that result from small-scale biological-physical interactions are described for three species of estuarine-spawning fish: bay anchovy (Anchoa mitchilli), white perch (Morone americana), and striped bass (M. saxatilis) in Chesapeake Bay. Research on bay anchovy early-life stages identified factors that influenced their small-scale distribution and potential transport. Depth-stratified sampling at a fixed station demonstrated that the pycnocline was an important physical feature structuring the planktonic community. Most bay anchovy early-life stages, copepods (prey) and ctenophores (predators) occurred above it. Variations in organism abundances were associated with the advection of water masses past the fixed station. Variable windforced circulation patterns, below-pycnocline dissolved oxygen concentrations, ontogenetic and diel changes in larval swimbladder inflation, and diel changes in

2 vertical distribution of larvae in relation to copepod prey have important consequences for potential transport of bay anchovy larvae. Results on white perch and striped bass demonstrated the importance of the estuarine turbidity maximum (ETM) region as a nursery area. Surveys of the upper bay and fixed-station sampling within the ETM were conducted in May 1998 (three cruises) and May 1999 (two cruises). In high-flow conditions (1998), fish early-life stages were abundant and concentrated in the ETM region that overlapped the salt front. Fish eggs, yolk-sac larvae and Eurytemora affinis copepods appeared to be retained passively in the convergence zone within the salt front and ETM. Feedingstage larvae probably accumulated in the ETM by tracking their passively retained prey (copepods). In low-flow conditions (1999), feeding-stage larvae were significantly less abundant and the salt front was up-estuary of the ETM. Freshwater flow was positively correlated with juvenile recruitments (1968/ ) and explained a large portion of variability in spawner-recruit relationships ( ). The close coupling between physical conditions, larval concentrations and prey distributions between cruises and years suggested that annual differences in freshwater flow influence larval retention, survival and recruitment by controlling the physicalbiological characteristics of the ETM region. Differences in life-history strategies, tradeoffs between optimal zones of feeding success or retention in the ETM, and episodic wind/flow events modulate the relationship between larval survival and freshwater flow.

3 TRANSPORT AND RETENTION OF FISH EARLY-LIFE STAGES IN CHESAPEAKE BAY: MECHANISMS AND IMPLICATIONS FOR RECRUITMENT by Elizabeth Watkins North Dissertation submitted to the Faculty of the Graduate School of the University of Maryland, College Park in partial fulfillment of the requirements for the degree of Doctor of Philosophy 2001 Advisory Committee: Professor Edward D. Houde, Chair/Advisor Professor William C. Boicourt Dr. Michael J. Fogarty Professor Michael R. Roman Professor Estelle Russek-Cohen

4 Copyright by Elizabeth Watkins North 2001 ii

5 Dedicated to my grandmothers, Flora Hillow and Jane Watkins, for Thursdays of blue crabs and yellow perch, weeks of seashells, snook, and mangroves, and a lifetime of encouragement and inspiration. iii

6 ACKNOWLEDGEMENTS I am deeply grateful for the opportunities, valuable guidance, and inspiration that my advisor, E. D. Houde, has given me. I am sure that it will be many years before I fully recognize the many doors he has opened in my career and mind. In addition, I am indebted to members of my graduate committee. W. B. Boicourt s advice and willingness to share his knowledge of physics have been indispensable. In many ways, his perspectives on water movement have guided my thinking and underlie much of the research in this dissertation. I appreciate M. J. Fogarty s insight and guidance on quantitative population dynamics and sample design. M. Roman s thought provoking questions and helpful discussions have provided me entry into the world of zooplankton and have certainly enhanced my research. I am very grateful for E. Russek-Cohen s statistical advice, and for the substantial time and effort that she has spent on training this fishhead to think statistics. My saints include L. Beaven, J. Boynton, and S. Jones for their companionship and help with ichthyoplankton sorting and zooplankton counting. I can t imagine where I would be without their assistance and friendship. I appreciate the significant guidance and valuable input that L. Sanford, T. Miller, D. Secor, W. Boynton, and S. Suttles have given to me. I am also grateful for the assistance and advice of B. Pearson, C. Derry, G. C. Rilling, S. Leach, J. Hagy, S. Smith, M. Mallonee, R. Marinelli, J. Love, L. Cyrus, M. Santangelo, N. Mielcarek, R. Sink, and D. Morrin; and thank L. Sanford, iv

7 A. Valle-Levinson, E. Setzler-Hamilton, L. Harding and W. Boynton for loaning me equipment. Thanks also to J. Stone, A. Beaven, M. Rhodes and L. Pignolet for their help in the laboratory and to E. Durrell and L. Fegley from Maryland Department of Natural Resources for providing young-of-the-year and spawning stock abundance data. I would like to acknowledge the Captains and crew of the RV Henlopen and RV Orion. This research would not have been possible without their hard work, competence and ingenuity. In addition, I would like to thank the many cruise participants who have logged long hours at sea on my behalf, especially S. Leach, G. C. Rilling, A. Madden, S. Jung, and D. O Brien. I appreciate the friendship, support and advice of K. Warner, L. Wainger, and M. Haasch. Thanks to all who have practiced Tai-chi with me it was great to have your company. I would also like to acknowledge the CBL community: the Sorting Center crew, students, FRAs, secretaries, maintenance personnel, and staff of Chesapeake Biological Laboratory. Although there are too many of you to name here, I will not forget your kindness and assistance. At the end and at the beginning, I have my family to thank for their support, encouragement, and patience. I don t think that my parents thought that a medical doctor crossed with an artist would result in a fisheries scientist, but they have been amazing parents and wonderful supporters anyway. My grandparents have encouraged v

8 and inspired me, and my brother, Charlotte and Maeve have kept me grounded and laughing. I am especially grateful for my husband s support, practical advice, and kindness. He is my storm anchor, even in rough seas. This research was supported by the National Science Foundation (Grants No. NSF OCE and DEB ), the EPA Science-To-Achieve-Results Fellowship Program (Fellowship No. U ), and a Chesapeake Biological Laboratory Graduate Research Assistantship. vi

9 TABLE OF CONTENTS List of Tables ix List of Figures xiv Introduction Literature Cited Chapter One: Transport and dispersal of bay anchovy (Anchoa mitchilli) eggs and larvae in Chesapeake Bay Abstract Introduction Methods Results Discussion Tables Figures Literature Cited Chapter Two: Retention of white perch and striped bass larvae: biological-physical interactions in Chesapeake Bay estuarine turbidity maximum Abstract Introduction Methods Results and Discussion Tables Figures Literature Cited Chapter Three: Time, space, food and physics: the temporal and spatial distribution of anadromous fish larvae in an estuarine turbidity maximum Abstract Introduction Methods Results Discussion Tables Figures Literature Cited vii

10 Chapter Four: Retention mechanisms of white perch and striped bass earlylife stages in an estuarine turbidity maximum: an integrative mapping and Eulerian approach Abstract Introduction Methods Results Discussion Tables Figures Literature Cited Conclusion Literature Cited Appendix I (Chapter One) Appendix II (Chapter Three) Appendix III (Chapter Four) Bibliography viii

11 LIST OF TABLES Table 1.1. Fixed-station organism occurrences. Percent organisms above the pycnocline during the entire fixed-station sampling (total), as well as the first and second occupation. Mean percentages were calculated for each set of depth-stratified samples using organism concentrations (no. m -3 for eggs and larvae, no. L -1 for copepod stages) and biovolume (ml m -3 for gelatinous zooplankton). Standard error (+/- one stderr) and sample number (n) are reported in parentheses below mean percentages. Sets of depth-stratified samples in which no organisms were captured were excluded Table 1.2. Mean abundance (no. m -2 for fish early-life stages, ml m -2 for ctenophores, no. x 10 3 m -2 for copepods) of organisms for the entire fixed-station sampling (total) and for four time periods with different physical characteristics. Standard errors (+/- 1 stderr) are presented in small text below the mean. Range of sample sizes (n) for means in each time period are located at the bottom of the table Table 1.3. Results of correlation analysis of fixed-station organism abundances (no. m -2 or ml m -2 ). Significant Pearson correlation coefficients (r) are presented with stars to indicate probability levels (*P<0.05, **P<0.01, ***P<0.001). Sample sizes ranged from n = 33 to n = Table 1.4. Results of correlation analysis of mean depths of organisms, pycnocline depth, 3.0 mg L -1 oxycline depth, and time of day. Significant Pearson correlation coefficients (r ) are presented with stars to indicate significance test probability levels (*P<0.05, **P<0.01, ***P<0.001). Due to potential confounding related to net evasion during the day by anchovy larvae > 6 mm, only mean depths of larvae > 6 mm caught during the night were included in this analysis. Sample sizes ranged from n = 6 to 13 for bay anchovy larvae > 6 mm, and from n = 22 to 38 for other organisms and size classes Table 1.5. Fixed station regression table: results of repeated measures multiple regression analysis on log e -transformed organism abundances (no. m -2 or ml m -2 for ctenophores). The regression analysis was conducted to determine whether advection (displacement), photoperiod (day-night), or time periods of differing physical conditions explained a significant amount of variability in organism abundance. Parameter estimates and their standard errors are reported for significant effects ( = 0.05). Mean displacement in the lower-layer was used for sciaenid eggs; upper-layer mean displacement was used for other organisms. The day-night variable was coded ix

12 day = 0 night = 1 (night: sunrise to sunset). Period represents the four time periods when physical conditions differed (1 = 2-20 hrs, 2 = hrs, 3= hrs, 4= hrs) Table 1.6. Potential transport of bay anchovy larvae by size class during the first station occupation (0 to 83 hrs) under the hypothesis that larvae made diurnal vertical migrations that became deeper during the day as larvae developed. In this hypothetical analysis, potential transport was calculated by summing the cumulative displacement (km) at the 5-m depth interval during night (sunset to sunrise) with the cumulative displacement at depths specific to each length class during the day (sunrise to sunset). Positive potential transport indicated hypothetical transport down-estuary Table 2.1. Mean percent eggs and larvae collected during cruises in a) 1998 and b) 1999 in relation to physical parameters or depth intervals. Standard errors are reported below the means. YSL = yolk-sac larvae. In 1999, only 2 of 34 net tows contained striped bass YSL and zero tows contained striped bass post-yolk-sac larvae. Net tows were not made in salinities < 1 psu during the second cruise in Table 3.1. Mean percent eggs, larvae by size-class, and zooplankton prey collected during cruises in a) 1998 and b) 1999 in relation to physical factors or depth intervals. Standard errors are reported below the means. YSL = yolk-sac larvae. In 1999, only 2 of 34 net tows contained striped bass YSL and white perch post yolk-sac larvae 8-10 mm, and zero tows contained striped bass post-yolk-sac larvae. Sampling was not conducted in salinities < 1 psu during the second cruise in Zooplankton prey includes Bosmina cladocera, Eurytemora affinis copepodites and males (Eury. copep), E. affinisadult females (Eury. female), and all E. affinis copepods combined (All Eury.) Table 3.2. Summary of multiple regression analysis. Multiple regression models were fit to zooplankton prey and early life stages of white perch and striped bass. Parameter estimates and standard errors (in parentheses) are reported for explanatory variables that described a significant ( = 0.05) amount of variability in organism concentrations (shaded boxes). Stars reflect probability levels (* = P < 0.05, ** = P < 0.001, *** = P < ). Other symbols indicate surface depth (s), middle depth (m), and variables that were not included in each model (x). All organism concentrations were loge transformed. Regression tables with F and P values for each model are in Appendix III, Table III-1. Temp. = temperature, TSS = total suspended solids, axial distance = distance from Susquehanna River mouth. Salinity and TSS were modeled as dichotomous variables (0 when salinity < 1 psu or TSS < 47 mg L -1 and 1 otherwise). n (sample size) = 49 for all models x

13 Table 3.3. Parameters (a,b,c) and model fit information for white perch and striped bass spawner-recruit models. Ricker spawner-recruit models ( R = a S e bs ) and Ricker models that incorporated spring freshwater discharge ( R = a S e bs cd ) were fit to indices of upper Chesapeake Bay young-of-the-year recruitment (R) and spawning stock abundance (S) from (D = discharge). Standard errors of the parameter estimates are reported in small text. Sample size (n) was Table 4.1. Mean percent overlap (Schoener overlap index) between potential prey and fish early-life stages. Means are calculated from the percent overlap of each cruise in Standard errors (+/- 1 stderr) are reported in small text below each mean. Bold numbers highlight percent overlap indices > Table 4.2. Mean salinity of occurrence of fish early-life stages from a) fixed-station sampling within the ETM and from b) gradient-mapping above, within, and below the ETM. Mean salinity of occurrence was derived from the average salinity of water filtered by each net tow weighted with concentration of copepods, fish early-life stages for each year, species, and size class. For copepods and fish early-life stages, N refers to the number of tows in which eggs or larvae were present. A star (*) indicates that the mean salinity of occurrence is significantly different from the unweighted mean salinity of the net tows (two-tailed t test, = 0.05, df = Nnet_tow + Norganism - 2); ns indicates not significant Table 4.3. Pearson correlation coefficients (r) for correlation between mean depth of occurrence (by species and stage) and current velocity, maximum depth of larval visual feeding, 1 psu isohaline depth, and mean depth of prey in Significant correlation coefficients are reported (P<0.05). Bold numbers indicate P<0.01 and '-' indicates that the correlation was not significant (n = 23 for white perch, Eurytemora and striped bass eggs, n = 22 for striped bass larvae). Correlation analysis results excluding data from the second cruise in 1998 are presented in small text (n = 15 for white perch, Eurytemora and striped bass eggs, n = 14 for striped bass larvae) Table 4.4. One-tailed paired t-test results, testing the hypothesis that the mean depth of occurrence of eggs, larvae and copepods at 1998 fixed stations was deeper than 1 psu isohaline depth (i.e., the difference between the 1 psu isohaline depth and the mean depth of occurrence was less than zero). Bold probabilities represent significant differences (P < 0.05). Stations up-estuary of the salt front were excluded from this analysis. Results of tests excluding data from the second cruise are presented in small text xi

14 Table 4.5. Summary of multiple regression analysis. Multiple regression models were fit to zooplankton prey and early life stages of white perch and striped bass. Parameter estimates and standard errors (in parentheses) are reported for explanatory variables that described a significant ( = 0.05) amount of variability in organism concentrations (shaded boxes). Stars reflect probability levels (* = P < 0.05, ** = P < 0.01, *** = P < 0.001). Other symbols indicate surface depth (s), mid depth (m), and variables that were not included in each model (x). All organism concentrations were log e transformed except for white perch that were square root transformed. N = 68 for all models. Regression tables with F and P values for each model can be found in Appendix III, Table III-1. Salinity, TSS (total suspended solids), salt front location and 1 psu & light overlap were modeled as dichotomous variables (0 when salinity < 1 psu, TSS < 47 mg L -1, station were located up-estuary of the salt front, and the maximum depth of larval visual feeding did not overlap the 1 psu isohaline. The dichotomous variables equaled 1 otherwise) Table I-1. Laboratory methods for counting microzooplankton collected in 10-L Niskin bottles Table II-1. Laboratory methods for counting microzooplankton collected in 1-m 2 Tucker Trawls Table II-2. Multiple regression tables for potential prey (a-c) and striped bass (d-g) and white perch (h-k) early-life stages from gradient-mapping. NDF = numerator degrees of freedom, DDF = denominator degrees of freedom, F = F statistic, and P = probability Table II-3. Summary data on striped bass catches by year in Upper Chesapeake Bay from Maryland Department of Natural Resources spring drift gillnet survey. Data from the gillnet survey was used to estimate spawning stock abundance Table II-4. Summary data on white perch catches by year in Upper Chesapeake Bay from Maryland Department of Natural Resources spring drift gillnet survey. Data from the gillnet survey was used to estimate spawning stock abundance Table II-5. Total catches of female striped bass > 500 mm by mesh size in upper Chesapeake Bay Maryland DNR spring drift gill net survey from 1987 to Table also contains percent of total catch by mesh size and the weights used to calculate effort estimates. Weights (proportion of total fish captured) were used to calculate effort to account for differences in efficiencies between mesh sizes xii

15 Table II-6. Total white perch catches by mesh size in upper Chesapeake Bay Maryland DNR spring drift gill net survey from 1987 to Table also contains percent of total catch by mesh size and the weights used to calculate effort estimates. Weights (proportion of total fish captured) were used to calculate effort to account for differences in efficiencies between mesh sizes Table III-1. Multiple regression tables for potential prey (a-c) and striped bass (d-g) and white perch (h-k) early-life stages from fixed-station sampling. NDF = numerator degrees of freedom, DDF = denominator degrees of freedom, F = F statistic, and P = probability. N = 68 for all models xiii

16 LIST OF FIGURES Figure 1.1. Chesapeake Bay, USA. Location of fixed station sampling ( ), current velocity measurements at Chesapeake Bay Observing System (CBOS) buoy ( ), and water level records ( ) at Baltimore, MD, and Chesapeake Bay Bridge Tunnel (CBBT), VA. Open circles ( ) represent axial sampling stations on June Fig Salinity (psu) contour lines and dissolved oxygen (mg L -1 ) shaded contours along the axis of Chesapeake Bay for a) June 10-11, 1996, and b) mean June measurements from Data from Chesapeake Bay Monitoring Program. Black dots indicate depths of CTD measurements. Dissolved oxygen data up-estuary of 50 km were not available in June 1996 (panel a). The location of the fixed sampling station in this study is marked by an arrow in the upper panel Figure 1.3. a) Chesapeake Bay sampling stations for the June 1996 axial survey. Bar graphs depict b) bay anchovy egg and c) bay anchovy larvae abundances (no m -2 ), d) gelatinous zooplankton biovolume (ml m -2 ) and e) sciaenid egg abundances (no. m -2 ) above (upper-layer) and below (lower-layer) the pycnocline. Fixed-stations ampling occurred near 201 km. Stations from km were sampled at night. 55 Figure 1.4. Time series of fixed station CTD and ADCP measurements. a) Salinity (psu) contour lines and temperature ( o C) shaded contours, b) dissolved oxygen (mg L -1 ) contour lines and fluorescence (RFU) shaded contours, and c) along-channel current velocity (cm s -1 ) for first (2-83 hr) and second ( hr) station occupations. Actual measurements indicated by small black dots. White and gray bars at the top of b) indicate day and night. Negative current velocity is up-estuary Figure 1.5. a) Wind velocity (10 m s -1 ) at mid-bay CBOS buoy, b) filtered water level (cm) at Baltimore, MD, and Chesapeake Bay Bridge Tunnel (CBBT), VA, and c) CBOS filtered along-channel current speed (cm s -1 ) in the upper and lower layers. Negative current speed is up-estuary. Shaded areas indicate timing of fixed-station sampling Figure 1.6. Potential displacement (km) over time for the first and second fixed-station occupations. Lines represent potential displacement of water at each 1-m depth interval. Displacement at each time point was calculated by multiplying the alongchannel current velocity estimate at that time by the duration between velocity estimates ( velocity (km s -1 ) * time (s) = displacement (km) ) and then summing with xiv

17 displacement estimates from all previous time points. Specific depths are marked by open circles and labeled. Negative displacement is up-estuary Figure 1.7. Fixed station temperature-salinity plots of a) water column profiles during three time periods (see legend for time periods), b) bay anchovy egg concentrations (no. m -3 ), c) copepod nauplii concentrations (no. L -1 ), and d) ctenophore biovolumes (ml m -3 ). Each line in a) represents data from an individual CTD cast. Panels b-d show data from all depth intervals sampled during both occupations of the fixed station..60 Figure 1.8. Time series of a) bay anchovy and b) sciaenid egg concentrations (no. m -3, shaded contours) and c) ctenophore biovolume (ml m -3, shaded contours) with salinity (psu) contour lines for the first (2-83 hr) and second ( hr) fixed-station occupations. Mid-point of trawl depth intervals indicated by small black dots. White and gray bars indicate day and night Figure 1.9. Time series of bay anchovy larvae concentrations (no. m -3, shaded contours) with salinity (psu) contour lines. a) Yolk-sac larvae, b) larvae 3-6 mm, and c) larvae 6-9 mm for the first (2-83 hr) and second ( hr) station occupations. Mid-point of trawl depth intervals indicated by small black dots. Trawls in which no larvae were captured are marked by an 'x'. White and gray bars indicate day and night Figure Time series of bay anchovy larvae concentrations (no. m -3, shaded contours) with salinity (psu) contour lines. a) Larvae 9-13 mm, and b) larvae >13 mm for the first (2-83 hr) and second ( hr) station occupations. Mid-point of trawl depth intervals indicated by small black dots. Trawls in which no larvae were captured are marked by an 'x'. White and gray bars indicate day and night Figure Time series of copepod prey concentrations (shaded contours) with salinity (psu) contour lines. a) Copepod nauplii (no. L -1 ), b) Acartia tonsa copepodites (no. L -1 ), and c) A. tonsa adults (no. L -1 ) for the first (2-83 hr) and second ( hr) station occupations. Small black dots indicate depth of Niskin bottle collection. White and gray bars at top of panels indicate day and night Figure Bay anchovy egg abundances (no. m -2 ) during the first fixed-station occupation and average potential displacement (km) in the upper 10 m of the water column (right Y-axis). Rectangles at the top of the plot represent night (sunset to sunrise) and arrows indicate peaks in egg abundance that were used in mortality rate calculations. The shaded area shows the time period used to calculate the egg xv

18 mortality rate during the third period (73 to 82 hrs). Comparing the shaded area with potential displacement shows that samples collected at 73 and 82 hrs were probably within 0.5 km of each other Figure Fixed-station occupation depth distributions. Mean depth (m) of a) bay anchovy and sciaenid eggs, b) bay anchovy larvae by size class (mm), and c) ctenophores, copepod nauplii, and A. tonsa copepodites and adults. Shaded area represents water below the pycnocline. The 3.0 mg L -1 oxycline (heavy gray line) and night (sunset to sunrise: boxes at top of panel) are depicted Figure Percent of bay anchovy larvae with inflated swimbladders by length class (mm) during day (open circles) and night (closed circles) on left Y-axis. Total larvae captured (no. m -3 ) in each size class during day (light gray line) and night dark gray heavy line) on right Y-axis. Day defined as 20 minutes after sunrise and night as 20 minutes after sunset. Length classes are in 1 mm intervals for larvae <15 mm, and in 3 mm size classes for larvae > 15 mm. Data points are located at the midpoints of length class intervals Figure 2.2. Thirty-year mean monthly Susquehanna River discharge at Conowingo ( ) on a logarithmic scale. Also depicted are 30-year maximum and minimum and 1998 and 1999 mean monthly discharge values Figure 2.3. Wind velocity at BWI, water level at Baltimore, Susquehanna River discharge at Conowingo, and water temperature from the CBOS northern Bay buoy in May Shaded areas indicate dates of research cruises Figure 2.4. Wind velocity at BWI, water level at Baltimore, Susquehanna River discharge at Conowingo, and water temperature from the CBOS northern Bay buoy in May Shaded areas indicate dates of research cruises Figure 2.5. Contour plots of total suspended solids (mg L -1 ) and salinity (psu) contour lines from the axial CTD surveys of the five research cruises. The lower left corner of xvi

19 each panel contains the date of the survey. The location of each CTD cast is marked by a letter that represents the stage of the predicted tide (e = ebb, s = slack, f = flood) Figure 2.6. Contour plots of white perch yolk-sac larvae concentrations (no. m -3 ) and salinity (psu) contour lines from gradient mapping surveys of ichthyoplankton abundance on a) 4-5 May 1998, b) May 1998, c) May 1998, d) 4-5 May 1999, and e) May Black dots indicate the midpoints of ichthyoplankton tow depth intervals. The location of each CTD cast is marked by a letter that represents the stage of the predicted tide (e = ebb, s = slack, f = flood) Figure 2.7. Contour plots of white perch post-yolk-sac larvae concentrations (no. m -3 ) and salinity (psu) contour lines from gradient mapping surveys of ichthyoplankton abundance on a) 4-5 May 1998, b) May 1998, c) May 1998, d) 4-5 May 1999, and e) May Black dots indicate the midpoints of ichthyoplankton tow depth intervals. The location of each CTD cast is marked by a letter that represents the stage of the predicted tide (e = ebb, s = slack, f = flood) Figure 2.8. Contour plots of striped bass egg concentrations (no. m -3 ) and salinity (psu) contour lines from gradient mapping surveys of ichthyoplankton abundance on a) 4-5 May 1998, b) May 1998, c) May 1998, d) 4-5 May 1999, and e) May Black dots indicate the midpoints of ichthyoplankton tow depth intervals. The location of each CTD cast is marked by a letter that represents the stage of the predicted tide (e = ebb, s = slack, f = flood) Figure 2.9. Contour plots of striped bass yolk-sac larvae concentrations (no. m -3 ) and salinity (psu) contour lines from gradient mapping surveys of ichthyoplankton abundance on a) 4-5 May 1998, b) May 1998, and c) May Black dots indicate the midpoints of ichthyoplankton tow depth intervals. The location of each CTD cast is marked by a letter that represents the stage of the predicted tide (e = ebb, s = slack, f = flood) Figure Contour plots of striped bass post-yolk-sac larvae concentrations (no. m -3 ) and salinity (psu) contour lines from gradient mapping surveys of ichthyoplankton abundance on a) 4-5 May 1998, b) May 1998, and c) May Black dots indicate midpoints of ichthyoplankton tow depth intervals. The location of each CTD cast is marked by a letter that represents the stage of the predicted tide (e = ebb, s = slack, f = flood) xvii

20 Figure Mean abundance (no. m -2 ) of white perch a) yolk-sac and b) post-yolksac larvae, and striped bass c) yolk-sac and d) post-yolk-sac larvae during each cruise in 1998 (98-1, 98-2, 98-3) and 1999 (99-1, 99-2). Error bars indicate +/- 2 standard errors of the mean Figure Abundance indices of young-of-the-year (no. haul -1 ) white perch and striped bass from in the upper Chesapeake Bay versus spring Susquehanna River discharge (March to May, 100 m 3 s -1 ). Young-of-the-year were collected in the upper Chesapeake Bay during the Striped Bass Juvenile Index Seine Survey (Maryland Department of Natural Resources) at seven stations that were sampled three times between July and September each year. Each symbol represents an individual year. Young-of-the-year are approximately d posthatch Figure Conceptualization of differences in salinity structure (contour lines), ETM location (black dots) and larval preferred salinity/temperature zones (light gray area) during a high freshwater flow (a) and a low freshwater flow (b) spring. Arrows indicate direction and relative strength of gravitational circulation Figure 3.1. Chesapeake Bay, USA. a) Axial CTD survey stations. b) An example of gradient-mapping stations in the May 1998 research cruise. Shaded area represents the ETM. Conowingo is location of freshwater discharge measurements. Latitudes and longitudes are in decimal fractions Figure 3.2. a) Thirty-year mean monthly Susquehanna River discharge at Conowingo ( ) on a logarithmic scale. Also depicted are 30-year maximum and minimum, and 1998 and 1999 mean monthly discharge values. b) Daily Susquehanna River discharge in May 1998 and Boxes indicate dates of gradient mapping cruises Figure 3.3. Contour plots of total suspended sediment (mg L -1 ) and salinity (psu) contour lines from the gradient mapping surveys of the five research cruises. The upper left corner of each panel indicates the date of each survey. The location of each CTD cast is marked by a letter that represents the stage of the predicted tide (e = ebb, s = slack, f = flood) Figure 3.4. Salinity (psu) contour lines and shaded contours of a-d) striped bass and e-h) white perch larvae concentrations (no. m -3 ) from the 4-5 May 1998 cruise. The upper left corner of each panel contains stage and size-class information. Black dots xviii

21 and 'x's indicate the midpoints of net tow depth intervals; 'x's indicate tows in which no organisms were captured Figure 3.5. Salinity (psu) contour lines and shaded contours of a-b) striped bass post yolk-sac larvae, c) Eurytemora copepodites and adult male copepods, d) Eurytemora adult female copepods, e-g) white perch post yolk-sac larvae, and h) Bosmina cladocera concentrations (no. m -3 ) from the May 1998 cruise. The upper left corner of each panel contains species and size-class information. Black dots and 'x's indicate the midpoints of net tow depth intervals; 'x's indicate tows in which no organisms were captured Figure 3.6. Salinity (psu) contour lines and shaded contours of a) white perch larvae < 5 mm, b) Eurytemora copepodites and adult male copepods, c) Eurytemora adult female copepod, and d) Bosmina cladocera concentrations (no. m -3 ) from the 4-5 May 1999 cruises. Black dots and 'x's indicate the midpoints of net tow depth intervals; 'x's indicate tows in which no organisms were captured Figure 3.7. White perch and striped bass abundance (no. m -2 ) by early-life stage versus distance from maximum total suspended solids (TSS) concentrations for each cruise in 1998 (98-1, 98-2, 98-3) and 1999 (99-1, 99-2). Negative numbers indicate distance upestuary of maximum TSS Figure 3.8. Salinity (psu) contour lines and shaded contours of a) Eurytemora copepodites and adult male copepods, b) Eurytemora adult female copepod, and c) Bosmina cladocera concentrations (no. m -3 ) from the 4-5 May 1998 cruise. Black dots and 'x's indicate the midpoints of net tow depth intervals; 'x's indicate tows in which no organisms were captured Figure 3.9. a) Eurytemora copepodites and adult males, b) Eurytemora adult females, and c) Bosmina cladocera abundance (number under 1 m 2 ) relative to distance from maximum TSS measurements for each cruise in 1998 (98-1, 98-2, 98-3) and 1999 (99-1, 99-2). Negative numbers indicate distance up-estuary of maximum TSS Figure Abundance indices of young-of-the-year (no. haul -1 ) white perch and striped bass from in the upper Chesapeake Bay versus spring Susquehanna River discharge (March to May, 100 m 3 s -1 ). Young-of-the-year were collected in the upper Chesapeake Bay during the Striped Bass Juvenile Index Seine Survey (Maryland Department of Natural Resources) at seven stations that were sampled three xix

22 times between July and September each year. Each symbol represents an individual year. Young-of-the-year are approximately d posthatch Figure Surface plots of Ricker stock-recruit models incorporating an environmental parameter (mean spring Susquehanna discharge (m s -1 ) for upper Chesapeake Bay a) white perch and b) striped bass. Young-of-the-year (YOY, no. haul -1 ) and spawning stock abundance (SSA, no. yrds -2 day -1 ) indices for upper Chesapeake Bay stocks were calculated with seine net (YOY) and drift gillnet (SSA) survey data provided by Maryland Department of Natural Resources. Actual data points used to fit the model are plotted. The size of data points represent YOY abundance index in five classes, with white perch symbols ranging from 0 to 107 haul -1 and striped bass from 0 to 25 haul -1. Model equations are depicted Figure 4.1. Sampling locations in Chesapeake Bay. a) CTD survey stations. Additional stations in 1999 indicated by open circles. B) Location of Chesapeake Bay Observing System buoy and fixed station locations (1998: solid circles, 1999: open circles) Figure 4.2. Contour plots of total suspended solids (mg L -1 ) and salinity (psu) from CTD surveys in 1998 (a - c) and 1999 (d,e). Lines represent salinity, shaded contours represent TSS. CTD cast locations are marked with symbols ( ) at the top of each plot. Arrows indicate the location of fixed station sampling during each cruise Figure 4.3. Fixed-station data. Along-channel current velocity (cm s -1 ) bar graphs, salinity (psu) contour lines, and shaded contours of total suspended sediment (TSS) concentrations (mg L -1 ) from the a) first, b) second, and c) third research cruises in CTD cast locations are marked with symbols ( ) at the top of each contour plot. Night indicated by dark gray bars in the current velocity graphs Figure 4.4. Fixed-station data. Along-channel current velocity (cm s -1 ) bar graphs, salinity (psu) contour lines, and shaded contours of total suspended solids (TSS) concentration (mg L -1 ) during the a) first and b) second research cruise in CTD cast locations are marked with symbols ( ) at the top of each contour plot. Night indicated by dark gray bars in the current velocity graphs Figure 4.5. Fixed-station analysis of mean abundance (no. m -2 ) of early-life stages of a) white perch and b) striped bass in 1998 and Error bars represent +/- 2 standard errors of the mean. Stars indicate that 1999 mean is significantly different xx

23 from 1998 mean (**: p<0.01, ***: p<0.001) using t-tests with unequal variances (equal variances for striped bass eggs) Figure 4.6. Fixed-station data. Along-channel current velocity (cm s -1 ) bar graphs, salinity (psu) contour lines, and shaded contours of white perch and striped bass yolksac larvae concentration (no. m -3 ) from the a,b) first, c,d) second, and e,f) third research cruises in CTD cast locations are marked with symbols ( ) at the top of each contour plot. Black dots mark the mid-point depth of the net tows. Night indicated by dark gray bars in the current velocity graphs Figure 4.7. Fixed-station data. Along-channel current velocity (cm s -1 ) bar graphs, salinity (psu) contour lines, and shaded contours of white perch yolk-sac larvae concentration (no. m -3 ) during the a) first and b) second research cruise in CTD cast locations are marked with symbols ( ) at the top of each contour plot. Black dots mark the mid-point depth of the net tows. Night indicated by dark gray bars in the current velocity graphs Figure 4.8. Fixed-station data. Along-channel current velocity (cm s -1 ) bar graphs, salinity (psu) contour lines, and shaded contours of 8-10 mm white perch and striped bass larvae concentration (no. m -3 ) from the a,b) first, c,d) second, and e,f) third research cruises in CTD cast locations are marked with symbols ( ) at the top of each contour plot. Black dots indicate the mid-point depth of the net tows. Night indicated by dark gray bars in the velocity graphs Figure 4.9. Foods of white perch and striped bass larvae. Percentage of white perch (a) and striped bass (b) larvae by size class with prey present in their guts. Only larvae with prey present in their guts were included in this analysis. Legend contains the number of larvae (n) examined in each size class Figure Mean depth of larval occurrence (m) versus current velocity (cm s -1 ) for white perch (a,b) and striped bass (c,d) yolk-sac larvae and 8-10 mm larvae and Eurytemora (e,f) small (copepodites and males) and adult female copepods. Data from research cruises in 1998 (98-1, 98-2, 98-3) are represented by different symbols. Negative current velocity is up-estuary (i.e., flood tide) Figure Fixed-station data. Mean depths of occurrence (m) of (a-c) Eurytemora copepodites and males (copep) and adult (Efem) females and (d-f) striped bass eggs (sbegg) and white perch yolk-sac larvae (wpysl) during fixed station sampling for three research cruises in 1998 (98-1, 98-2, 98-3). Also depicted are 1 psu isohaline xxi

24 depth, the maximum depth for larval visual feeding (light max), and the maximum depth of bottom net tow (tow max). Night occurs when maximum depth of larval visual feeding is zero. Shaded area represents salinities > 1 psu Figure Fixed-station data. Mean depths of occurrence (a-c) white perch postyolk-sac larvae (post-ysl) and (d-f) striped bass yolk-sac and post-yolk-sac larvae during fixed station sampling during three research cruises in May 1998 (98-1, ). Post-yolk-sac larvae are in the following size classes: < 5 mm (wp_sm), 5-8 mm (wp_med, sb_med), and 8-10 mm (wp_lg, sb_lg). Also depicted are 1 psu isohaline depth, the maximum depth for larval visual feeding (light max), and the maximum depth of net tows (tow max). Night occurs when maximum depth of larval visual feeding is zero. Shaded area represents salinities > 1 psu Figure Gradient-mapping data. Mean depths of occurrence (m) of (a-c) white perch yolk-sac larvae (wpysl) and striped bass eggs (sbegg), (d-f) white perch postyolk-sac larvae (postlarvae), and (g-i) striped bass post-yolk-sac larvae (post-ysl) from gradient sampling during three research cruises in May 1998 (98-1, ). Post-yolk-sac larvae are in the following size classes: < 5 mm (wp_sm), 5-8 mm (wp_med, sb_med), and 8-10 mm (wp_lg, sb_lg). The depths of the 1 psu isohalines are depicted and labled in the upper panels. Shaded areas represent the region of the salt front in which fixed station sampling occurred based on displacement calculations and 1 psu isohaline depth. Symbol size indicates order of magnitude differences in larval abundance (no. m -2 ) within size classes. Gradient-mapping surveys were conducted at night Figure Fixed-slab advection analysis. This analysis was conducted to determine the influence of advection on changes in 1 psu depth during 98-3 fixed-station sampling. a) The slab (salt front) was defined as the depth of the 1 psu isohaline from the axial CTD survey on 19 May Panel b) depicts the 1 psu isohaline depth over time measured at the fixed station on May 1998 (actual) as well as the estimated 1 psu isohaline depth of the slab. The 1 psu isohaline depth of the slab was estimated by matching the slab and the actual fixed station 1 psu isohaline depths at time = 0, moving the slab by the displacement (km) between fixed station measurements, and recording the 1 psu isohaline depth of the slab at the time of each fixed station measurement. Displacement was calculated by integrating polynomial equations fit to adjusted CBOS current velocities between each fixed station CTD measurement. This analysis demonstrates that most of the changes in 1 psu isohaline depth at the fixed station can be explained by advection of the sloping salt front past the fixed station xxii

25 Figure Conceptualization of physical conditions in an ETM nursery area. Arrows indicate hypothetical residual current velocity and dots mark null zones where net transport is neither up- nor down-estuary (as diagramed by Sanford et al in press, Fig. 11). Maximum depth of larval visual feeding (dashed line) and 1 psu isohaline depth (solid line) are depicted. Zones indicate areas of similar physical conditions. Shaded area represents region of high turbidity Figure I-1. Estimated current velocity measurements (solid circles) compared to actual current velocities (open circles) at three depths for the a) first and b) second fixed- station occupations. See Methods for a description of the interpolation technique used to derive estimated current velocities. These graphs demonstrate that the interpolation technique used to estimate current velocities between actual ADCP measurements provided a reasonable fit to the measured data. Estimated current velocities were used to calculate potential displacement. Negative velocity is upestuary Figure II-1. Scatterplots of prey and larval concentrations by size class versus explanatory variables that were significant in multiple regression models for gradientmapping surveys. Although not significant, the factor that explained the most variability in striped bass yolk-sac larval concentrations is also plotted. ln = log e Figure II-2. Salinity (psu) contour lines and shaded contours of a-b) striped bass postyolk-sac larvae, c) Eurytemora copepodites and adult male copepods, d) Eurytemora adult female copepod, e-g) white perch post-yolk-sac larvae, and h) Bosmina cladocera concentrations (no. m -3 ) from the May 1998 cruise. The upper left corner of each panel contains species and size-class information. Black dots and 'x's indicate the midpoints of net tow depth intervals; 'x's indicate tows in which no organisms were captured Figure II-3. Salinity (psu) contour lines and shaded contours of a) white perch larvae < 5 mm, b) Eurytemora copepodites and adult male copepods, c) Eurytemora adult female copepod, and d) Bosmina cladocera concentrations (no. m -3 ) from the May 1999 cruises. Black dots and 'x's indicate the midpoints of net tow depth intervals; 'x's indicate tows in which no organisms were captured Figure III-1. Filtered current velocity (cm s -1 ) and wind speed (m s -1 ) during May Current velocity measurements were from the Chesapeake Bay Observing System (CBOS) buoy located up-estuary of the salt front in Wind speed measurements were taken by the National Ocean and Atmospheric Administration at xxiii

26 BWI Airport. Both filtered time series were courtesy of W. C. Boicourt and C. Derry. Positive current velocities are in the down-estuary direction Figure III-2. Filtered current velocity (cm s -1 ) and wind speed (m s -1 ) during May Current velocity measurements were from the Chesapeake Bay Observing System (CBOS) buoy located down-estuary of the salt front in Wind speed measurements were taken by the National Ocean and Atmospheric Administration at BWI Airport. Filtered time series were courtesy of W. C. Boicourt and C. Derry. Positive current velocities are in the down-estuary direction Figure III-3. Displacement of the water mass over time during fixed station sampling for each cruise in Data points indicate times of fixed station sampling. Displacement estimates are scaled so that mean displacement for each cruise equals zero Figure III-4. Scatterplots of prey and larval concentrations by size class versus explanatory variables that were significant in multiple regression models for fixedstation data. Although not significant, the relationship between salinity and Eurytemora copepodites and males also is plotted to demonstrate the 1 PSU cut off for coding the salinity variable in the model. Eurytemora copepdites are also plotted by cruise with a trend line that demonstrates increasing concentrations from the first to the third cruise. ln = log e xxiv

27 INTRODUCTION This research addressed the fundamental question of how organisms interact with the physical environment, a critical issue in estuaries, the site of many fish spawning grounds and nursery areas. Because the net flow of water in estuaries is seaward, fish that use estuaries as spawning and nursery areas most likely have mechanisms to retain or disperse their progeny within them. This research described and evaluated mechanisms of transport or retention of fish eggs and larvae that resulted from smallscale biological-physical interactions. The interaction of estuarine physics and biology during early life was hypothesized to control fish egg and larval distributions, potential survival, and juvenile recruitment. Biological-physical interactions were investigated in Chesapeake Bay for three estuarine spawners: bay anchovy (Anchoa mitchilli), white perch (Morone americana), and striped bass (M. saxatilis). The theory that year-to-year changes in environmental conditions may be one of the underlying causes of recruitment variability has become a fundamental paradigm in fisheries science (Sissenwine 1984). Because recruitment level is determined during early life, evaluating the influence of physical and biological conditions on fish eggs and larvae is an important component of the research on the recruitment variability question. In estuaries such as Chesapeake Bay, seasonal and annual variations in salinity, temperature, dissolved oxygen and circulation patterns, as well as day to weekly variability induced by wind and discharge events, could control survival of fish early-life stages and potential recruitment. The interaction between physics and biology during early-life could control survival and recruitment a) by affecting the 1

28 timing and extent of prey availability (Hjort 1914; Cushing 1975), b) by influencing growth rates and trophic interactions that shape temporal and spatial co-occurrence with predators (Houde 1989; Bailey and Houde 1989), c) by regulating larval feeding success via turbulence-induced encounter rates (Rothschild and Osborn 1988), and d) by controlling dispersal to, or retention within, nursery areas (Iles and Sinclair 1982). In the variable estuarine environment, life-history strategies that incorporate retention and dispersal may have adaptive value by retaining early-life stages of fish in, or transporting them to, optimal conditions in relation to physical factors, food availability, habitat requirements, and predation mitigation (Strathmann 1982). Throughout the four chapters of this dissertation, research results demonstrate that physical conditions and prey concentrations influenced the distribution and potential transport or retention of bay anchovy, white perch, and striped bass larvae at both large and small, and daily to annual, scales. Field research surveys and fixed-station sampling were employed to determine the spatial and vertical distribution of fish early-life stages and also their predators and prey in relation to physical factors. Environmental monitoring data were evaluated to understand the physical forces that influenced field observations and to relate short-term research cruise results to the temporal scales of population-level processes. Results of field studies combined with environmental monitoring data were evaluated to address the recruitment variability issue by relating freshwater flow to striped bass and white perch juvenile recruitment. The goal of Chapter One was to identify mechanisms and processes that control the small-scale distribution and potential dispersal of bay anchovy early-life stages in Chesapeake Bay. Stratified conditions during fixed-station sampling highlighted the 2

29 importance of the pycnocline and 3.0 mg L -1 oxycline in structuring the biological community. In addition, the light/dark cycle and changes in water masses associated with wind-forcing events affected larval distributions and their co-occurrence with predators and prey, and had important implications for larval transport and dispersal. The remaining three chapters summarize research on white perch and striped bass early-life stages in the upper Chesapeake Bay estuarine turbidity maximum (ETM). The ETM is an important physical feature near the head of coastal plain estuaries where sediment is trapped in a convergence zone and continually resuspended by tidal action (Schubel 1968). Chapter Two examines the influence of wind and freshwater flow on ETM physical characteristics and on eggs and larval distributions. Spatial patterns in larval distributions changed in association with shifts in ETM and salt front location between cruises and years. Larvae were most abundant in the ETM region during the high freshwater flow year, and were scarce, or virtually nonexistent (striped bass), in samples collected during the low flow year. It was hypothesized that variability in the physical and biological characteristics of the ETM region, induced by changes in freshwater flow, are linked to larval survival and juvenile recruitment. Chapter Three undertakes additional analysis of the data in Chapter Two, focusing here on ontogenetic differences in larval distributions in relation to physical conditions and concentrations of zooplankton that are larval prey. Results confirmed that the ETM region is an important white perch and striped bass nursery area where larvae are retained in a region of elevated zooplankton prey concentrations. Weekly and annual differences between larval and prey distributions and abundances were examined in relation to changing physical conditions. In addition, a strong relationship between 3

30 post-yolk-sac larvae and prey concentrations was demonstrated. Finally, the link between variability in physical conditions and recruitment success was quantified by incorporating freshwater discharge into a stock-recruit model. The goal of Chapter Four was to identify mechanisms that white perch and striped bass early-life stages use for retention within the ETM region. The small-scale distribution of larvae and potential zooplankton prey were determined with time series of measurements at a fixed station within the ETM. Striped bass yolk-sac larvae, white perch larvae < 5 mm, and the copepod prey of post-yolk-sac larvae appear to be passively retained in the convergence zone associated with the salt front. Striped bass and white perch post-yolk-sac larvae > 5 mm appear to track the distributions of copepod prey, which could promote retention within the ETM region. Trade-offs between zones of high larval retention and high feeding success within the ETM nursery area are discussed. 4

31 LITERATURE CITED Bailey, K. M., and E. D. Houde Predation on eggs and larvae of marine fishes and the recruitment problem. In: Blaxter, J. H. S, and A. J. Southward (eds.), Advances in Marine Biology 25: Cushing, D. H Marine Ecology and Fisheries. Cambridge University Press. pp Hjort, J Fluctuations in the great fisheries of Northern Europe. Rapp. Proces- Verb. Cons. int. Explor. Mer. 20: Houde, E. D Subtleties and episodes in the early life of fishes. Journal of Fish Biology 35(Supplement A): Iles, T. D., and M. Sinclair Atlantic herring: stock discreteness and abundance. Science 215: Rothschild, B. J., and T. R. Osborn Small-scale turbulence and plankton contact rates. Journal of Plankton Research 10: Schubel, J. R Turbidity maximum of the northern Chesapeake Bay. Science 161: Sissenwine, M. P Why do fish populations vary? In: May, R. E. (ed.), Exploitation of Marine Communities. Berlin, Heidelberg, New York, Tokyo: Springer-Verlag. pp Strathmann, R. R Selection for retention or export of larvae in estuaries, p In Kennedy, V. S. (ed.) Estuarine Comparisons. Academic Press, New York. 5

32 CHAPTER ONE Transport and dispersal of bay anchovy (Anchoa mitchilli) eggs and larvae in Chesapeake Bay ABSTRACT Mechanisms and processes that control the small-scale distribution and potential dispersal of bay anchovy (Anchoa mitchilli) early-life stages are poorly known. Fixed station sampling was conducted to determine the depth-specific distributions of egg and larval stages in Chesapeake Bay. Coupled with an axial abundance survey and environmental monitoring data, depth-stratified sampling results were used to determine how wind, currents, time of day, physics, ontogeny (egg and larval stages), and prey and predator abundances interacted to affect the distribution and potential transport of eggs and larvae. Wind-forced circulation patterns altered the depthspecific physical conditions at the fixed station and significantly influenced organism distributions and potential transport. The pycnocline proved to be an important physical feature that structured the planktonic community, including ichthyoplankton: >77% of bay anchovy early life stages (no. m -3 ), 72% of ctenophores (ml m -3 ), and > 69% of early copepod stages (no. L -1 ) occurred above the pycnocline. In contrast 90% of sciaenid eggs were found below the pycnocline in waters where dissolved oxygen concentrations were as low as 0.2 mg L -1. The day-night cycle also influenced organism abundance and distributions, including diel periodicity in concentrations of bay anchovy and sciaenid eggs and of bay anchovy larvae > 5 mm, likely reflective of nighttime spawning (eggs) and net evasion during the day (larvae). Diel periodicity in 6

33 bay anchovy swimbladder inflation was also observed, indicating that larvae may migrate to surface waters at dusk to fill their swimbladders by gulping air. Study results suggest that wind-forced circulation patterns, lower-layer dissolved oxygen conditions, and diel changes in the vertical distribution of larvae and their copepod prey have important consequences for potential transport of bay anchovy early-life stages. INTRODUCTION Many factors, including physics, ontogeny, food abundance, and predation act and interact to affect the small-scale distributions and potential dispersal of bay anchovy (Anchoa mitchilli) early life stages in Chesapeake Bay. The question of how organisms respond or react to the biological and physical environment is critical in estuaries, important spawning grounds and nursery areas for many fish species. Results of this research demonstrate the influence of changing physical conditions on organism distributions and illustrate the potential for varying circulation patterns to restructure the planktonic community and affect bay anchovy larval transport and dispersal. Bay anchovy is the most abundant fish in Chesapeake Bay. It is a pelagic, small (<110 mm), short-lived (<3 years) clupeoid fish that plays an important ecological role in Chesapeake Bay as a major prey for piscivores such as striped bass (Morone saxatilis), bluefish (Pomatomus saltatrix), and weakfish (Cynoscion regalis) (Baird and Ulanowicz 1989). The anchovy is a pelagic, serial spawner with a reproductive season in Chesapeake Bay that extends from May to September with a peak in July (Luo and 7

34 Musick 1991; Zastrow et al ). Spawning occurs at salinities from 0 to 32 psu (Dovel 1971; Olney 1983; Houde and Zastrow 1991) and peaks at temperatures from 26 to 28 C (Houde and Zastrow 1991). Bay anchovy spawns between 1800 and 0100 hours (Luo and Musick 1991; Houde and Zastrow 1991; Zastrow et al ), producing daily cohorts of eggs that hatch into yolk-sac larvae about hours after fertilization, depending upon temperature (Houde and Zastrow 1991). The larvae begin feeding about 2.5 days after hatching (Houde and Zastrow 1991) and grow at a rate of about 0.5 to 0.7 mm/day (Rilling and Houde 1999b). The spatial and vertical distribution of bay anchovy early life stages potentially are influenced by physical conditions. Houde and Zastrow (1991) suggested that the occurrence and survival of eggs below the pycnocline might depend on dissolved oxygen concentrations because the viability of bay anchovy early life stages may be limited at concentrations <3.0 mg L -1. In addition, bay anchovy larvae strongly avoided waters with dissolved oxygen concentrations < 1 mg L -1 in laboratory experiments (Breitburg 1994). Keister et al. (2000) demonstrated that bay anchovy larvae concentrations were reduced in waters where oxygen concentrations were < 2.0 mg L - 1. Temperature also potentially can influence the distribution of bay anchovy early life stages, especially in highly stratified conditions. The reported range of temperature is C for eggs and C for larvae (Houde and Zastrow 1991). Prey abundance also may influence the spatial occurrence and vertical distribution of bay anchovy eggs and larvae. Adult bay anchovy may spawn where food abundance is high, leading to an association between high concentrations of eggs and larvae and 8

35 concentrations of copepods that serve as prey for adult anchovy (Peebles et al. 1996). The location of larval prey also may influence larval distributions. Larval anchovy first feed on microzooplankton such as copepod nauplii, rotifers, and tintinnids, and shift to larger copepodites and adult copepods as they grow (Houde and Zastrow 1991). Acartia tonsa, the dominant copepod in mid and lower Chesapeake Bay during spring and summer, makes diel vertical migrations, moving down in the water column during the day and up at night (Roman et al. 1993). If larvae track copepod prey, they may have similar patterns in distribution. Predation may influence the distribution of bay anchovy eggs and larvae by 1) direct mortality, 2) stimulating predator-avoidance movements by larvae, and/or 3) influencing adult spawning-site selection (Dorsey et al. 1996). Major predators of bay anchovy eggs and larvae include the gelatinous zooplankton scyphomedusan (Chrysaora quinquecirrha) and the lobate ctenophore (Mnemiopsis leidyi) (Purcell et al ). Dorsey et al. (1996) reported a negative relationship between gelatinous zooplankton biovolume and bay anchovy egg concentrations and suggested that adults may avoid spawning in areas of gelatinous zooplankton abundance. Ontogenetic changes in swimming ability and buoyancy regulation may also influence larval distributions by allowing larvae to control their vertical position in the water column (Boehlert and Mundy 1988). In estuaries with two-layer circulation like the Chesapeake Bay, vertical changes of < 5 m could result in dispersal either up- or down-estuary. During the 30 to 60 day duration of the larval stage (Jones 1978), bay anchovy larvae develop capabilities that enable them to migrate vertically. Improved 9

36 swimming abilities begin evolving with the onset of fin formation at 5 mm (total length), improve further when fin ray counts are complete at 11 mm (Jones 1978), and are well developed by 15 to 20 mm when larvae begin to exhibit schooling behavior (E. D. Houde, personal communication). Potential for buoyancy regulation increases when the swimbladder first inflates, although precise control may not be possible until later as noted for northern anchovy (Engraulis mordax) whose swimbladder is first inflated when larvae are ~10 mm in length (O Connell 1981). Pelagic fish larvae affect transport within or out of estuaries by regulating their vertical position in the water column in relation to currents rather than by swimming horizontally (Norcross and Shaw 1984; Miller 1988). Larvae may use tidally-timed vertical migration, also referred to as selective tidal stream transport, to move upestuary by swimming up into high velocity currents at flood tide and down to depths of slower currents during ebb tide. In estuaries with two-layer flow, larvae could affect retention or transport by maintaining their position in landward flowing bottom waters or by migrating around the depth of null velocity. In addition, diel vertical migration could result in transport if larvae move into landward-flowing waters during summer days when days are longer than nights. Several of these transport mechanisms have been documented for larval fish: for example, selective tidal stream transport (Rowe and Epifanio 1994), migration around the depth of null velocity (Fortier and Leggett 1983), and migration in relation to both time of day and tides (Weinstein et al. 1980). Although these mechanisms seem plausible, it is also possible that random movements 10

37 of larvae coupled with frequent spawning could lead to a sizable fraction of surviving larvae being retained within the estuary. The objective of this research was to identify the mechanisms and processes controlling small-scale vertical distributions and potential transport of bay anchovy eggs and larvae. The study was designed to determine how currents, time of day, physics (temperature, salinity, dissolved oxygen), ontogeny (egg and larval stages), food abundance, predation, and weather act or interact to affect egg and larval distributions. Study design was guided by the hypothesis that bay anchovy larvae make tidally-timed vertical migrations that improve with development. The research consisted of 1) an initial survey in Chesapeake Bay to determine areas of maximum egg and larvae abundance, 2) depth-stratified sampling at a fixed location to describe vertical distributions of early-life stages in relation to physical and biological factors, 3) length measurements of larvae to classify larvae by ontogentic stage, and 4) an analysis of environmental data to evaluate the factors that influenced physical conditions and organism distributions during field collections. METHODS Data were collected during ten days of intense sampling in Chesapeake Bay from June 1996 on the 120 ft RV Cape Henlopen. The first two days of the cruise consisted of an ichthyoplankton and CTD survey along the axis of the Chesapeake Bay (Fig. 1.1). After the initial survey, sampling effort was concentrated at a fixed station 11

38 located in mid-bay south of the Rappahannock River (37 o 45 N) from 20 to 23 June and from 26 to 27 June (Fig. 1.1). Axial survey. The initial survey along the axis of Chesapeake Bay was conducted to determine the spatial distributions and abundances of fish early-life stages and gelatinous zooplankton and to locate areas suitable for intensive depth-stratified collections. Thirteen stations at 15 nautical-mile (~26 km) intervals were occupied from the Bay mouth (37 o 00 N) to near the head of the Bay (39 o 26 N). At each station, a CTD cast (measuring water-column temperature, salinity, dissolved oxygen, and fluorescence) preceded ichthyoplankton collections. A 1 m 2 mouth-opening, opening-closing Tucker Trawl with 280-µm mesh nets was used to collect ichthyoplankton and gelatinous zooplankton. A pair of 2-min tows was made in a single deployment at each station, one from near bottom (within 1.5 m) to the pycnocline, and the second from the pycnocline to the surface. Tow depth was controlled by wire lengths and angles. A flowmeter in the trawl mouth recorded the volume of water sampled and a Depth and Temperature Data Recorder provided a sequential record of the depth of the net during each tow. On average, each tow filtered approximately 110 m 3 of water. Catches from the Tucker Trawl were processed on deck. Gelatinous zooplankton were separated from the sample with large strainers (~5 mm pores). Gelatinous zooplankton species were identified and the bio-volumes of each species in the total catch were determined. Remaining plankton was preserved in a 1% Formalin plus ethanol solution. Samples were transferred to fresh ethanol for storage within six days after collections. 12

39 Fixed station sampling. The objective of activities at the fixed sampling station was to determine the vertical distribution and abundance of fish early-life stages, microzooplankton, and gelatinous zooplankton in relation to each other and water column hydrography. The site of the fixed station was determined based on coarse information from egg and larval abundances observed in the axial survey (from visual inspection of samples) and with consideration of water-column depth (30 m), shipping traffic, and gelatinous zooplankton concentrations. The first station occupation of the station (20 to 23 June) was for 81 hrs and the second (26 to 27 June) was for 28.5 hrs. At the fixed station, depth-stratified sampling for ichthyoplankton, microzooplankton, and gelatinous zooplankton was conducted at 5-m intervals from the surface to 25-m (near bottom) in conjunction with physical measurements of the water column. On each tide (every ~6 hrs), current velocity was measured with an Acoustic Doppler Current Profiler (ADCP). Following the ADCP run, a CTD was deployed to measure water-column temperature, salinity, dissolved oxygen, and fluorescence, and also to collect microzooplankton in 10-liter Niskin bottles on the CTD rosette. Then, ichthyoplankton and gelatinous zooplankton were collected in two sets of depth-stratified Tucker-trawl tows. These tows were followed by another CTD cast and microzooplankton collections, and an additional ADCP run. An entire suite of measurements was completed in ~2.7 hrs. Current velocity measurements. Current velocity was determined with the RV Cape Henlopen s hull-mounted 1200 khz Acoustic Doppler Current Profiler. Raw data was collected in 1-m depth intervals (from 4 to 25 m) during ADCP runs (

40 minutes) with the ship at constant speed and heading (parallel to the channel) every ~2.7 hrs during the first station occupation, and every ~1.7 hrs during the second occupation. Six-minute averages of ADCP raw data with '94-100% good' depth intervals were used as instantaneous measures of current velocity, except for one run with only 3.8 min of acceptable data, and one where only processed data ('97-100% good' depth intervals) was available. Percent-good is a data-quality measure calculated by RD Instruments ADCP software packages that indicates the percentage of good data collected in each depth interval. Current velocities were rotated in the along-channel direction to maximize variance in along-channel velocities based on a principal axis analysis (W. C. Boicourt, personal communication). Microzooplankton. Depth-stratified microzooplankton samples were collected during the CTD casts with 10-liter Niskin bottles. Samples were collected at 2.5, 7.5, 12.5, 17.5, and 22.5 m depths. They were filtered onto a 35-µm mesh, and then preserved in 5% Formalin-seawater solution. In the laboratory, zooplankton (copepod nauplii, rotifers, tintinnids, cyclopoid copepods, harpacticoid copepods, and Acartia tonsa copepodites and adults) were identified and enumerated under a dissecting microscope. All organisms were enumerated in samples from 12.5, 17.5, and 22.5 m depths (79% of the samples). High numbers of organisms in 21% of the samples, usually from 2.5 and 7.5 m depths, necessitated dilution to 25 ml, from which three 1 ml aliquots were enumerated, averaged, and multiplied by 25 ml to determine the total number of organisms in the sample. A detailed description of zooplankton counting methods is in Appendix I, Table I-1. 14

41 Ichthyoplankton and gelatinous zooplankton. Depth-stratified collections of ichthyoplankton and gelatinous zooplankton at the fixed station were made in the 1-m 2 Tucker Trawl. This gear was deployed as in the initial axial survey except that the trawl was fished with a single net at m, m, and m depth intervals. To minimize potential bias due to time and tidal stage effects during a station occupation, the order of depth intervals sampled was randomly assigned in each set of stratified samples, except that the 10-5 m and 5-0 m depth intervals were always collected during one tow, and in that order. The depth recorder on the Tucker Trawl was checked immediately after each set of tows to ensure that the trawl had fished the nominal depth interval. Tows that did not were repeated. Catches were processed as at the axial survey stations, except that a) gelatinous zooplankton processing was limited to total biovolume measurements (species were identified at three stations), and 2) plankton samples were preserved in a 2% Formalin plus ethanol solution when gelatinous zooplankton biovolumes were especially large. The Tuckertrawl samples were transferred to fresh ethanol for storage within four days after collection. In the laboratory, fish eggs and larvae were removed from plankton samples (both the axial survey and fixed station sampling). They were identified and counted under a dissecting microscope. Although entire samples were sorted for larvae, eggs were counted in a subsample (1/2 to 1/8 of the whole sample, divided with a plankton splitter) when numbers were > 200. Larvae from the fixed-station sampling were 15

42 separated into two groups based on swimbladder inflation (inflated or not inflated). All larvae were measured with a computer-based digitizing system. Analysis. The time series of depth distributions of physical factors, ichthyoplankton (no. m -3 ) and zooplankton (no. L -1 ) concentrations, and gelatinous zooplankton biovolume (mg m -3 ) at the fixed station was contoured using Surfer version 6.01 software. In these plots, time is depicted in hours since midnight of the first day of fixed station occupation. The gridding method was kriging with an isotropic linear variogram model. Grid-line geometry was no finer than half the average distance between measurements in the Y-direction (depth), and no finer than the average distance between measurements in the X-direction (time). Abundance (per m 2 ) of ichthyoplankton, mircozooplankton, and biovolumes of gelatinous zooplankton in each tow were estimated by multiplying organism concentrations by the sampled depth interval. For fixed-station samples, the abundance estimates from net tows in 10 5, and 5 0 m tows were adjusted for a 1-m overlap between the sampled depth intervals using the equation: A i = 4*c i + 1*(c i + c j )/2 where A i = abundance in i th depth interval, c i = concentration in i th depth interval, and c j = concentration in the j th depth interval. Abundances in each depth interval were then summed to determine total water column abundance (no. m -2 or ml m -2 ). Mean depths of organism occurrence at the fixed station were calculated for each set of depth-stratified samples by averaging net tow depths (the midpoint of each depth interval) weighted by the concentration of fish eggs and larvae using the equation: 16

43 D m = ( d i * c i )/ c i where D m = mean depth of occurrence, d i = depth of the i th net tow, and c i = organism concentration in the i th net tow. Associations between the abundance and means depths of occurrence of organisms were evaluated with Pearson correlation coefficients (SAS, PROC CORR). Correlation analyses of mean depths of occurrences also included pycnocline depth and the depth of the 3.0 mg L -1 oxycline. Dissolved oxygen concentrations below 3.0 mg L -1 caused reduced survival of bay anchovy eggs and larvae in laboratory experiments (Chesney and Houde 1989; Breitburg 1994). Pycnocline depth was assigned as the depth at which the Brunt-Väisällä frequency was maximum during each CTD cast (Mann and Lazier 1996). The depth of the 3.0 mg L -1 oxycline was calculated by linear interpolation between CTD measurements that were adjacent to 3.0 mg L -1. Potential advection of water (and organisms within it) during fixed-station sampling was estimated by calculating displacement (km) using ADCP current velocities. Depth-specific along-channel current velocities were iteratively interpolated over time until time steps were ~12 min (Surfer version 6.01 software). The gridding method was kriging with an isotropic linear variogram model. In the initial grid file, grid-line geometry was 1-m in the depth-direction and 1.67 hrs in the time-direction. The initial grid file was output as ASCII text and then re-interpolated with half the distance between grid points in the time-direction. This process was repeated until the distance between grid points in the time-direction was ~12 min. This iterative 17

44 technique prevented over-interpolation at time scales finer than the resolution of ADCP measurements and preserved the approximate smoothed tidal variation from the initial interpolation of current velocity data. Although coarse, this technique fit raw data well (Appendix I, Fig. I-1). Potential displacement during each time step (i) was calculated by multiplying interpolated velocity (v) at each 1-m depth by the time-step duration (t = 720 s). Cumulative potential displacement (D c ) at time c was calculated by summing displacement at t = c with displacement during all previous time steps (D c = v i t for t = 0 to t = c). This potential displacement analysis assumed that current velocities at specific depths did not differ within 40 km of the fixed station. This assumption may not be realistic because changes in bathymetry (Fig. 1.2a) likely resulted in nonuniform current velocities within that 40-km distance. However, estimates of lowerlayer water mass movement based on displacement calculations in this study (18.5 cm s -1 ) were in the range of those reported at the mouth of the Choptank River, a tributary of Chesapeake Bay (20 cm s -1 ) (Sanford and Boicourt 1990). Despite its associated uncertainties, the potential displacement analysis provided a valuable and reasonable assessment of potential transport of planktonic organisms during this study. Potential displacement estimates were used to calculate hypothetical transport of bay anchovy larvae by size class under the hypothesis that larvae made diurnal vertical migrations that became deeper during the day as larvae developed. All size classes of larvae were hypothesized to remain at a mean depth of 5 m during the night. During the day, larvae 4-5 mm in length were hypothesized to remain at 5 m, while the mean 18

45 depth of successive 1-mm size classes was 1 m deeper than previous size classes. Potential transport (km) during the first station occupation was estimated by calculating cumulative displacement for the 5-m depth interval during night (sunset to sunrise) and adding it to cumulative displacement at depths specific for each length class during the day (sunrise to sunset). Mortality rates of anchovy eggs were calculated using the exponential model N t = N o e -Zt where N o = number at peak spawning event, N t = number at time t, Z = instantaneous mortality coefficient (per h). Egg-stage mortality (Z e ) of bay anchovy and sciaenid eggs for the entire fixed-station sampling data set was estimated by rearranging the preceding equation: Z e = -log e (N t / N 0 ) = -log e (N ysl / N egg ) with N t = N ysl = c xt, the sum of yolk-sac larval concentrations at depth x and time t, and N o = N egg = c xt, the sum of egg concentrations at depth x and time t, using the entire fixed-station data set. A statistical analysis was conducted to determine if advection (displacement), photoperiod (day-night), or time periods of differing physical conditions described a significant amount of variability in the water column abundance of organisms. Time periods of differing physical conditions were chosen based upon water column hydrography. A repeated measures, multiple regression analysis was conducted on log e transformed abundances (no. m -2 or ml m -2 ) with displacement, day-night, and time period as fixed effects and station occupation (first or second) as a random effect (SAS 19

46 v 6.12, PROC MIXED, betwithin degrees of freedom method). A first order autoregressive covariance structure was used to account for covariance in time. The day-night variable was coded day = 0 and night = 1 where night represented time from sunset to sunrise. Environmental monitoring data. Physical conditions during the research cruise were evaluated in connection with environmental monitoring data from the Chesapeake Bay Observing System (CBOS), NOAA National Ocean Service (NOS), and the Chesapeake Bay Program (CBP) Monitoring Program. Wind (RMP) and current velocities (at 2.4 and 18.9 m depths) from the mid-bay CBOS buoy in June 1996 were used in conjunction with NOS water level measurements at Baltimore and at the Chesapeake Bay Bridge Tunnel (Fig. 1.1) to identify forces that influenced physical conditions during fixed-station sampling. Water level measurements and CBOS current velocities were filtered with a Lanczos low pass filter with 34-hr halfpower point to reveal low-frequency water level and circulation patterns (courtesy of W. C. Boicourt and C. Derry). June salinity (psu) and dissolved oxygen (mg L -1 ) measurements from channel stations of the CBP Monitoring Program (Fig. 1.2) were used to provide historical context to the field sampling results in June Mean salinity and dissolved oxygen concentrations at each depth from June surveys were contoured in Surfer version 6.01 software and compared to similar plots for June 1996 CBP monitoring results. The gridding method for the contour plots was kriging with an isotropic linear variogram model. Grid-line geometry was 2-m in the depth-direction and 13.8 km in the along-channel direction. 20

47 RESULTS Historical context. Freshwater flow in the Chesapeake Bay during the spring of 1996 was far above the long-term average (Boynton et al. 1997). The high input of low salinity water greatly affected spring-summer salinity structure in mid-chesapeake Bay, as can be seen in a comparison of the mean June salinity from (Fig. 1.2b) with June salinity in 1996 (Fig. 1.2a). Above pycnocline salinities upestuary of 200 km were lower in June 1996 than the mean of the 10 prior years. In fact, the intersection of the 10 psu isohaline with the surface was >80 km down-estuary in June 1996 compared to its mean location in At the fixed station, June 1996 surface salinity was ~ 5 psu lower than the long-term average. Although surface dissolved oxygen concentrations were similar in the June average and June 1996 comparison, dissolved oxygen concentrations <1 mg L -1 were more prevalent in deep waters in Axial survey. Results of the CTD survey were similar with respect to salinity and dissolved oxygen to those of the CBP Monitoring Program (Fig. 1.2a) so are not illustrated here. Peak abundances of bay anchovy eggs and larvae (Fig. 1.3b,c), sciaenid eggs (Fig. 1.3e), and gelatinous zooplankton (Fig. 1.3d) occurred downestuary of the Potomac River (174 km), although gelatinous zooplankton occurred in significant biovolumes as far up-estuary as 58 km from the head of the Bay. Bay anchovy eggs were two orders of magnitude more abundant than anchovy larvae, and one order of magnitude more abundant than sciaenid eggs. Although the sciaenid eggs potentially were spawned by 4 to 7 different species (Olney 1983, Daniel and Graves 21

48 1994), most eggs probably were those of weakfish (Cynoscion regalis) because most sciaenid larvae that were positively identified during the cruise were weakfish (50 out of 72 post-yolk-sac larvae). Biovolumes of gelatinous zooplankton were mostly composed of the ctenophore, Mnemiopsis leidyi, (99.3%) with the hydromedusa Nemopsis bachei constituting the remainder. Within lower Chesapeake Bay (> 174 km), bay anchovy eggs were most abundant where ctenophore biovolume was minimal (230 and 285 km), and were virtually absent when ctenophore biovolume peaked (257 km). Anchovy larvae were found throughout the lower Bay and also outside the mouth of the Bay (309 km), including at stations where egg abundances were low. Anchovy eggs, larvae, and ctenophores were most abundant above the pycnocline at all stations. In contrast, sciaenid eggs were abundant in both layers at the most down-bay stations but were predominantly in below-pycnocline waters up-estuary of 257 km, apparently most associated with salinities > 20 psu. Fixed station sampling. Fixed-station sampling was conducted near 201 km where depths were > 25 m, anchovy larval abundances peaked and gelatinous zooplankton biovolumes were moderate (Fig. 1.3c,d). Maximum abundance of bay anchovy larvae (17.9 no. m -2 ) during the fixed station occupation was comparable to peak abundance of larvae collected during the axial survey (11.5 m -2 ). The combined means for axial and fixed-station sampling for anchovy egg (183.0 m -2 +/ s.e.) and larvae (4.04 m -2 +/ s.e.) abundances were lower than mean baywide abundances (eggs: /- 95.8, larvae: /- 6.4 m -2 ) reported by Rilling and 22

49 Houde (1999a) during June Although egg abundances were not significantly different (t-test, t = 1.28, n.s., df = 78), larval abundances were significantly lower (ttest, t = 18.81, P< , n = 78). The paucity of anchovy eggs (max: 34.6 m -2 ) and the absence of anchovy larvae in the upper Bay (< 89 km in Fig. 1.3) during the axial survey compared with mean abundances (eggs: 838.5, larvae: 75.8 m -2 ) found by Rilling and Houde (1999a) in the upper-bay during June 1993 suggest that June baywide production of larvae was limited in 1996 and restricted to the lower Bay. Physics. Contour plots of physical factors at the fixed station show a well-defined pycnocline below which temperature (Fig. 1.4a) and dissolved oxygen concentrations (Fig. 1.4b) were lower than those above the pycnocline. Dissolved oxygen concentrations < 3.0 mg L -1, a level below which bay anchovy early-life stage growth and survival may be impaired (Houde and Zastrow 1991), generally occurred just below the pycnocline. Dissolved oxygen levels continued to decrease with depth, frequently measuring mg L -1 near bottom. Highest fluorescence values (RFU, raw fluorescence units) occurred within and above the pycnocline and peaked in surface waters during the afternoons (Fig. 1.4b). Peaks in dissolved oxygen concentrations corresponded to those of fluorescence, a result of photosynthesis in the upper-layer. During the initial 30 hrs of the fixed-station occupation, the pycnocline was shallow (~6 m), temperatures in the upper-layer were the highest of the time series, and those below the pycnocline were the lowest (Fig. 1.4a). Tidal currents were reversing in the upper and lower layer (Fig. 1.4c). After 30 hrs, conditions changed. 23

50 The pycnocline deepened (~9 m), water temperatures and salinity increased in the lower layer and decreased in the upper layer. These changes corresponded to strong up-estuary currents below the pycnocline where tidal currents were no longer reversing. This change in physical conditions probably was related to a wind event that occurred from hrs during the fixed-station occupation (Fig. 1.5a, shaded area, June 20-23). Strong winds blowing from the north reduced water levels in the upper Bay (Fig. 1.5b, Baltimore record), and enhanced two-layer estuarine circulation. This response can be seen in the CBOS filtered current record where residual surface velocities increased in the down-estuary direction while lower-layer velocities increased in the up-estuary direction (Fig. 1.5c). Just prior to the second station occupation (Fig. 1.5, shaded area, June 26-27), another strong wind event from the north (Fig. 1.5a) reduced water levels in the upper Bay (Baltimore record, Fig. 1.5b), increased water level in the lower Bay (CBBT record, Fig. 1.5b), and resulted in enhanced two-layer circulation (Fig. 1.5c). At the start of the second occupation of the fixed station, the pycnocline had deepened (~ 15 m) and intensified, probably a result of mixing during the wind event (Fig. 1.4a). The 3.0 mg L -1 oxycline also was driven deeper during the wind event as was fluorescence (Fig. 1.4b). Unlike the first fixed-station occupation, the 2.0 and 3.0 mg L -1 oxyclines now were in close proximity. The strong up-estuary current velocity throughout the water column (Fig. 1.4c, hrs) likely was due to flood tide combined with a barotropic response to the removal of wind stress, producing an up-estuary pulse of water (a seiche effect). The subsequent gradual elevation in pycnocline depth during 24

51 the second fixed-station occupation may have resulted from a decrease in wind mixing as well as the barotropic response to the removal of wind stress. Potential displacement during the two fixed-station occupations demonstrates the dynamic changes in physical conditions and potential consequences for organism transport (Fig. 1.6). For the first 20 hrs of the time series, displacement in both the surface- (< 9 m) and lower-layer (> 9 m) was within that expected of tidal excursions. Between 25 and 45 hrs, enhanced gravitational circulation related to the northern wind event resulted in potential displacement of the surface layer ~10 km down-estuary and the lower-layer ~10 km up-estuary. The upper-layer was nearly stationary between 45 and 75 hrs, suggesting that collections in the upper layer during this time period were sampling the same water mass and its associated organisms, which had originated 8-16 km up-estuary at the initiation of fixed-station sampling. In contrast, lower-layer displacement was non-stationary after 30 hrs, indicating that up-estuary residual currents continually displaced water and organisms within it past the fixed station. By the end of the first fixed-station occupation, Tucker-trawl tows in the upper layer sampled water that potentially originated 20 km up-estuary while tows near-bottom potentially sampled water from 40 km down-estuary, a net difference of 60 km. During the second fixed-station occupation, the effects of enhanced two-layer circulation were immediately apparent in the divergent displacement of near-surface and near-bottom waters during the 150 to 160-hr period. The strong baroclinic response is also evident in the up-estuary displacement throughout the water column between 160 and 170 hrs. From the beginning to end of the second fixed-station 25

52 occupation, net displacement of water deeper than 6 m was up-estuary. Maximum displacement was > 20 km at 17-m depth in < 30 hrs, while near-surface waters (4 to 6 m depths) were displaced 7 km down-estuary. To examine the influence of advecting water masses on biological collections at the fixed-station, the fixed-station time series was grouped into four periods, each with different physical characteristics. During the first period, 0-20 hrs, the pycnocline depth was shallow and displacement was near zero in both the upper and lower layer. The second period, hrs, was characterized by changing physical conditions (decreasing temperature and salinity in the upper-layer; increasing in the lower layer) and rapid displacement both up-estuary in the lower layer and down-estuary in the upper-layer. From hrs, the third period, upper-layer displacement was near zero when salinities were < 11 psu in the upper layer, pycnocline depth was stable, and lower-layer salinity continually increased, as did lower-layer displacement up-estuary. Finally, the second fixed-station occupation can be considered a fourth period that was characterized by rapidly changing physical conditions: increasing pycnocline depth, a strong barotropic up-estuary response throughout the water column, and a transition in the upper-layer from wind-mixed towards restratification. A temperature-salinity plot of CTD casts (Fig. 1.7a) clearly shows differences between water masses during the first, third and fourth periods. Biology. All stages of bay anchovy (Fig ), ctenophores (Fig. 1.8c), copepod nauplii (Fig. 1.11a), and A. tonsa copepodites (Fig. 1.11b) were found in highest concentrations above the pycnocline during both the first and second fixed- 26

53 station occupations (Table 1.1). In contrast, less than 10 % of sciaenid eggs were found above the pycnocline (Table 1.1, Fig. 1.8b) and only about half of A. tonsa adults (49%) were found above the pycnocline during the first occupation, but most (85%) were above the pycnocline during the second occupation. Concentrations of both anchovy and sciaenid eggs peaked at night (Fig. 1.8), presumably because adults spawn at night. Advection into the study area is not a likely cause because egg development times are short: bay anchovy eggs hatch in hrs (Houde and Zastrow 1991) and sciaenid eggs hatch in hrs (Holt et al. 1985), depending upon temperature. High concentrations of sciaenid eggs were found near the base of the pycnocline around sunset, whereas bay anchovy eggs peaked near surface at sunset and at night. Mean abundances of anchovy eggs were highest during the first period (0-20 hrs) while concentrations of sciaenid eggs were highest during the second period (20-50 hrs) (Table 1.2). Egg abundance of both anchovy and sciaenids declined an order of magnitude by the fourth period. Abundance (Table 1.2) and upper-layer concentrations (Fig. 1.9) of bay anchovy yolk-sac and 3-6 mm larvae peaked during the first period (0-20 hrs) and were low after 40 hrs. Abundance was at least an order of magnitude lower in the third and fourth periods compared to the first, despite the presence of anchovy eggs during the third period. Larvae 6-13 mm were present during all four periods. Unlike larvae < 9 mm, the abundance of bay anchovy larvae > 13 mm increased from the first to the fourth period (Table 1.2). Eighty-five percent of larvae captured during the fourth period were > 9 mm. These large larvae probably were not survivors from eggs and 27

54 small larvae during the first period but rather a new group advected into the region. Lengths of larvae from eggs spawned during the first period could not have been more than ~ 6 mm by the middle of the fourth period (bay anchovy larvae hatch ~1.9 mm standard length (Houde and Zastrow 1991) and grow at a rate of ~0.6 mm d -1 (Rilling 1996)). Larvae > 6 mm showed clear peaks in concentration during night collections (Fig. 1.9c, 1.10), suggesting that net evasion during the day is a significant confounding factor in determining the diel vertical distribution of large larvae. In fact, 67% of larvae > 5 mm, 75% of larvae > 9 mm, and 95% of larvae > 14 mm were collected at night. Copepod nauplii and A. tonsa copepodites, the most prevalent prey items for bay anchovy larvae in Chesapeake Bay (T. Auth, personal communication), peaked in concentration and abundance in the upper layer during the first two periods (Fig. 1.11a,b, Table 1.2). Nauplii were concentrated in the upper 5 m of the water column, while copepodites were found throughout the upper layer at night. It is likely that most nauplii were A. tonsa because the majority of copepods (96%) collected during fixedstation sampling were this species. Peak concentrations of adult A. tonsa copepods were shallower during the night and deeper during the day, often occurring within or below the pycnocline during day. Like those of nauplii and copepodites, abundances of adult A. tonsa declined between the second and fourth periods. Most gelatinous zooplankton captured during fixed station sampling were Mnemiopsis leidyi. This species comprised 98% of gelatinous zooplankton in depth- 28

55 stratified samples for which species were identified. Nemopsis bachei comprised the remaining 2%. Ctenophores were present throughout the upper layer and peaked in concentration during afternoon and night. Ctenophore biovolumes increased from the first to the fourth period. Maximum biovolumes were ml/m 2 during the fourth period. Mean biovolumes at the fixed station ( ml m -2 ), were comparable to mid-bay biovolumes (mean: ml m -2 +/ s.e.) found in June 1993 by Rilling and Houde (1999a). Combined axial and fixed station biovolumes were not significantly lower than 1993 June bay-wide mean biovolumes (560.7 ml m -2 +/ s.e., t-test, t = 0.99, P = 0.32, df = 75). Mortality Rates. Anchovy egg mortality rates were calculated for four peaks in egg abundance during the first fixed-station occupation, the first through third periods of different physical conditions (Fig. 1.12). Mortality rates calculated for egg abundances during the first (at 4-11 hrs) and third (at hrs) periods were comparable (first period: Z = 0.16 hr -1, third period: Z = 0.17 hr -1 ). The mortality rates calculated during the second period were much higher (Z = 0.46 hr -1 at hrs, Z = 0.48 hr -1 at hrs). The second-period mortality rates probably did not represent actual mortality. These rates could have resulted from differences in egg abundance between two water masses that were advected past the fixed station, as can be seen by the increasing trend in mean upper-level displacement (Fig. 1.12). Mean upper-layer displacement records indicate that the mortality rate calculated during the third period was based on samples from the same water mass (at 73 and 82 hrs) (Fig. 1.12). Because these samples were potentially < 0.5 km apart, the decrease in anchovy eggs 29

56 during the third period likely resulted from biological processes as opposed to physical advection. The mortality rate of anchovy eggs during the third period (Z = 0.17 hr -1 ) was higher than mean anchovy mortality rates (Z = hr -1 +/ s.e) found by Dorsey et al. (1996) in July 1991 in mid to lower Bay. Correlation Analysis. Pearson correlation coefficients summarize trends in larvae, prey and predator abundances in relation to each other (Table 1.3). Bay anchovy earlylife stage abundances were positively correlated and most highly correlated with abundances in adjacent length classes. Abundances of bay anchovy larvae < 13 mm were positively correlated with prey abundance (copepod nauplii and copepodites). Abundances of larvae 3-9 mm were negatively correlated with biovolume of ctenophores, a major predator on small larvae. Copepod nauplii and A. tonsa copepodite abundances also were negatively correlated with ctenophore biovolumes suggesting a predator-prey relation. Although negative correlations between ctenophore biovolume and anchovy larvae and copepod abundances could be the result of predation mortality, these general trends are more likely a result of water masses with different physical characteristics and community structures moving past the fixed station. Temperature-salinity plots (Fig. 1.7b-c) show organism concentrations in relation to temperature and salinity characteristics of the changing water masses. Highest concentrations of copepod nauplii (Fig. 1.7c) occurred in warm waters > 11 psu associated with the first period (Fig. 1.7a, black lines) while most ctenophores (Fig. 1.7d) occurred in cooler waters < 12 psu associated with the third and fourth periods (Fig. 1.7a, gray lines). 30

57 Mean depth of occurrence. Mean depths of bay anchovy eggs were deepest just before sunset whereas sciaenid egg mean depths were highest in the water column at sunset (Fig. 1.13a), probably a result of spawning times and depths, and possible increases in density before hatching (Rulifson and Tull 1999). The deep observed mean depths of anchovy eggs just before nighttime spawning probably represented unhatched eggs that were sinking since their previous night s spawn. The relatively shallow mean depths of sciaenid eggs just before sunset correspond to peaks in their concentrations, suggesting that they were newly spawned eggs. As the time series of mean depths indicates, most sciaenid eggs (75.6 %) were found below the 3.0 mg L -1 oxycline, while most anchovy eggs (95.2 %) were above it. Egg-stage mortality for bay anchovy eggs (M = 5.14, percent mortality = 99.41%) was lower than that of sciaenid eggs (M = 7.72, percent mortality = 99.96%). Potential differences in net extrusion rates between anchovy and sciaenid yolk-sac larvae and the non-stationarity of the lower-layer possibly confound this analysis, but the difference between egg-stage mortalities does suggest that hatching success and survival of sciaenid yolk-sac larvae could have been impaired by sub-pycnocline low dissolved oxygen conditions. Mean depths of ctenophores, copepod nauplii, and A. tonsa copepodites and adults overlapped throughout fixed-station sampling. However, mean depths of nauplii tended to be shallower, and those of A. tonsa adults deeper, than ctenophore and copepodite mean depths during the first occupation (Fig. 1.13c). Mean depths of nauplii, copepodites and ctenophores appeared to track pycnocline depth during the 31

58 first occupation and were significantly correlated with pycnocline depth for the entire fixed-station sampling period (Table 1.4). During the first occupation, mean depths of A. tonsa adults were mostly located between the pycnocline and 3.0 mg L -1 oxycline with some evidence that mean depths were deeper during day. Ctenophores were more highly correlated with 3.0 mg L -1 oxycline depth than pycnocline depth. During the beginning of the second occupation, mean depths of copepods and ctenophores did not overlap pycnocline depth, perhaps because the 2.0 mg L -1 oxycline was close to the base of the pycnocline. The shallow mean depths of organisms relative to pycnocline depth during the second occupation corresponded to even distributions of organisms throughout the upper-layer, probably a result of wind mixing just prior to this sampling period. In contrast to mean depths of their predators and prey, mean depths of bay anchovy larvae were often located near, but did not always track, pycnocline depth (Fig. 1.13b). The difference in mean depths between large and small larvae indicated that other factors influenced the vertical distribution of larvae. During the first 30 hrs of fixedstation sampling, the average mean depths of larger larvae (6-13 mm) were shallower than smaller larvae (< 6 mm) during the night (average mean depths: yolk-sac larvae = 5.7 m, larvae 3-6 mm = 6.7 m, 6-9 mm = 4.6 m, and 9-13 mm = 4.3 m) but deeper during day (average mean depths: yolk-sac larvae = 4.2 m, larvae 3-6 mm = 4.6 m, 6-9 mm = 6.4 m, and 9-13 mm = 6.3 m). Larger larvae may have tracked high prey concentrations (i.e., A. tonsa adults) that were located deeper during the day than at night. The differences in distribution between small and large larvae also could have 32

59 been a result of net evasion. Anchovy larvae located in well-lit surface waters could have seen and reacted to the trawl more quickly than larvae located deeper in dimly-lit waters. Because larval vision improves during development, catches of larvae in welllit surface waters may decrease with development. Decreasing near-surface catches of increasingly older larvae would bias mean depth calculations so that larger larvae appeared to be deeper in the water column than smaller larvae during the day. Peaks in larval mean depths during the early evening may be related to diel periodicity in swimbladder inflation and the physiological requirement of clupeoid larvae to gulp air at the surface to inflate their swimbladders (Blaxter and Hunter 1982). When collections were made at least forty-five minutes after sunset, the mean depths of anchovy larvae > 6 mm were shallowest in early evening (hrs 20, 45, 70) and deepened through the night (Fig. 1.13b). During the entire fixed-station occupation, the highest proportion of larvae with inflated swimbladders consistently occurred during the night (Fig. 1.14). The clear diel periodicity in swimbladder inflation increased with larval development: 50% of larvae 6-7 mm and > 80% of larvae > 11 mm had inflated swimbladders at night. Inflated swimbladders were observed in larvae as small as 4-5 mm. Results of the multiple regression analysis (Table 1.5) indicated that displacement, day-night, and/or periods of different physical conditions explained a significant amount of the variability in organism abundances. The day-night effect accounted for most of the variability in abundances of larvae > 9 mm and was a significant factor for larvae > 6 mm. This probably reflects net evasion by larvae > 6 mm during the day. 33

60 The fact that the day-night effect was also significant for A. tonsa copepodites and adults could indicate that these stages are able to evade capture in Niskin bottles during the day. The significant day-night effect for sciaenid eggs almost certainly represents evening spawning activity. However, this effect was not significant for bay anchovy eggs, perhaps because some spawning may have taken place just before sunset (Fig. 1.12). Periods of different physical conditions explained a significant amount of the variability in anchovy eggs, yolk-sac larvae, larvae 6-9 mm, ctenophore, and copepod nauplii abundances. This suggests that the different upper-layer water masses sampled between periods one, three and four contained different biological communities. Although displacement and periods of different physical conditions were related, displacement included a measure of spatial association within water masses. Smallscale variability might have accounted for variations in abundance: displacement accounted for more variability than periods of different physical conditions in the models for larvae 3-6, 6-9, 9-13 mm, and sciaenid eggs. Results of the hypothetical transport analysis demonstrate the strong effect of changing vertical distributions on the potential transport of anchovy larvae (Table 1.6). A 1-m difference in daytime mean depth could have resulted in 2 km difference in potential transport after 3.5 days. If bay anchovy larvae made diurnal vertical migrations that became deeper during the day as larvae developed, then potential transport of larger larvae would have been up-estuary while transport of younger larvae would have been down-estuary. Alternatively, Table 1.6 demonstrates the error 34

61 in potential transport estimates that could be induced by net evasion. For example, if the majority of mm larvae were located at 10 m during the day but most larvae were captured at 14 m due to net evasion, then the estimate of potential transport (-6.9 km) would be of similar magnitude but opposite in direction from the actual transport (6.1 km). DISCUSSION Results of this research demonstrated the dynamic nature of wind-forced circulation patterns that can have significant consequences for organism distributions and potential transport. Research results illustrated the major influence that physical conditions have on structuring the plankton community in Chesapeake Bay via variability in pycnocline depth, distribution of dissolved oxygen concentrations, and the light/dark cycle. Twelve to 24-hr wind events had both direct and indirect effects on organism distributions. Direct wind mixing deepened the upper-layer of the water column, apparently dispersed organisms throughout it, and could have resulted in a decline of anchovy egg abundances if the wind-mixing event reduced adult spawning. Moser and Pommeranz (1999) found fewer eggs and larvae of the northern anchovy E. mordax after the passage of a storm in the Southern California Bight. Lasker (1975) suggested that destratification associated with wind-mixing events could have negative impacts on survival of first-feeding larvae by reducing prey concentrations. Although the lack of egg and larval collections before and during the wind event prior to the second 35

62 station occupation during this study precludes assessment of Lasker s hypothesis, the wind event could have diminished first-feeding larval survival by increasing the depth of the upper layer and dispersing prey organisms throughout it. Wind events also may have an indirect, but perhaps more significant, impact on the distribution of organisms in estuaries by influencing the direction and strength of gravitational circulation. Enhanced gravitational circulation that began about 20 hrs after first occupation of the fixed station quickly provided the plankton community above the pycnocline with access to a different water mass below it. Such changes potentially can expand or reduce suitable habitat and may decrease or expand predator populations that could migrate into the surface layer. In addition, differences in residual current velocities throughout the water column (i.e., shear) may have restructured biological communities above and below the pycnocline. As an example of potential effects, compare cumulative displacement of water at 5-m and 7-m depths between 30 hrs (the start of the northern wind event during the first fixed-station occupation) and 50 hrs (Fig. 1.6). Organisms that were in waters separated in depth by 2 m potentially would have been separated horizontally by 3 km within 20 hrs after the start of the northern wind event. Wind-related restructuring of the biological community could transport bay anchovy early-life stages into waters where predators were abundant. High ctenophore biovolumes may have caused decreased abundances of anchovy eggs by influencing adults choice of spawning areas (Dorsey et al. 1996) or by direct predation mortality on eggs. The mortality rates of anchovy eggs during the first and third period (first 36

63 period: Z = 0.16 hr -1, third period: Z = 0.17 hr -1 ) were higher than mean anchovy mortality rates (Z = hr -1 +/ s.e) found by Dorsey et al. (1996) in July 1991 in the mid to lower Bay. The Dorsey et al. (1996) estimates included predation mortality by both ctenophores and the scyphomedusan C. quinquecirrha. Although gelatinous zooplankton may only account for 21 +/- 17 % of total egg mortality (Purcell et al. 1994), higher egg mortality rates in this study than in Dorsey et al. (1996) suggested that higher egg predation rates could occur in the presence of large concentrations of ctenophores ( ml m -2 mean biovolume during period three, Table 1.2) than in the presence of a smaller mixed community containing both ctenophores and scyphomedusae (612.8 ml m -2 maximum combined biovolume from Dorsey et al. 1996). Variations in the abundances of organisms were associated with changing water masses that had differing physical characteristics. Results clearly demonstrated that the pycnocline was an important frontal boundary (Largier 1993) that structured the biological community and controlled the overlap of bay anchovy larvae, their prey and predators in the water column. Other studies also have demonstrated that the pycnocline influences the distribution of organisms. As in the present research, Purcell et al. (1994) found ctenophores to be most abundant above the pycnocline in mid- Chesapeake Bay. Also in mid Bay, bay anchovy larvae were found in greatest abundance above the pycnocline (MacGregor and Houde 1996). In lower Chesapeake Bay, Govoni and Olney (1991) found that bay anchovy eggs and ctenophores were separated by the pycnocline at one well-stratified station but co-occurred at another 37

64 well-mixed station, indicating different potentials for predator-prey and competitive interactions at the two stations. The observed low concentrations of organisms (except sciaenid eggs) below the pycnocline indicated that dissolved oxygen concentrations could have had an important effect on plankton distributions. As in this study, Roman et al. (1993) found that copepod abundances were highest in the pycnocline when dissolved oxygen concentrations were < 1 mg L -1 in bottom waters in Chesapeake Bay. Keister et al. (2000) demonstrated that naked goby (Gobiosoma bosc) and bay anchovy larvae and copepod concentrations declined when dissolved oxygen levels were < 2.0 mg L -1 in the Patuxent River, a tributary of Chesapeake Bay. The fact that most ctenophores, copepods, and fish larvae co-occurred in waters > 3.0 mg L -1 (above the pycnocline) in this study supports the Keister et al. (2000) suggestion that low dissolved oxygen concentrations could be an important factor influencing predator-prey interactions. Results of the axial survey suggest that physical factors structured the longitudinal gradient in abundance of organisms in the planktonic community. Sciaenid eggs were the second most abundant eggs (after bay anchovy) in the axial and fixed-station collections, similar to results reported for ichthyoplankton surveys in lower Chesapeake Bay in (Olney 1983). Spawning at the fixed station during the present study apparently was at salinities near the lower end of the range for sciaenid reproduction. Olney (1983) found a distinct polyhaline peak in the distribution of sciaenid eggs: most were at stations with mean salinities > 26 psu. The shift from presence of sciaenid eggs throughout the water column at axial stations near the Bay 38

65 mouth to presence only in the lower-layer at stations 230 km (Fig. 1.3) indicates that sciaenid spawning may be restricted to a narrow salinity range which limited spawning to near-pycnocline and sub-pycnocline waters at up-estuary locations. If sciaenid spawning is limited to sub-pycnocline depths up-estuary of Rappahannock Shoals (~ 225 km, Fig. 1.2a), then low dissolved oxygen conditions potentially may be a bottleneck to egg and larval survival, and could reduce reproductive success of sciaenids in Chesapeake Bay in years of severe hypoxia. The light/dark cycle (day versus night) is another physical cue that affected the distribution and abundance of bay anchovy eggs, larvae, prey and predators. Peak concentrations of bay anchovy eggs occurred at night. Highest anchovy egg abundances at night have been reported in Peconic Bay, New York (Ferraro 1980) and Chesapeake Bay (Luo and Musick 1991; Zastrow et al. 1991). Peak concentrations of sciaenid eggs also were found in the evening when they were highest in the water column indicating that spawning occurs at this time, as was reported for C. regalis in Peconic Bays, New York (Ferraro 1980). These observations support the general supposition that dusk spawning is a characteristic of sciaenid fishes (Holt et al. 1985). The light/dark cycle also was a cue for inflation and deflation of swimbladders by bay anchovy larvae. Diel periodicity in swimbladder inflation (inflation at night) has been reported for other clupeoid species such as the European anchovy Engraulis encrasicolus (Ré 1987, 1990), Japanese anchovy E. japonicus (Uotani et al. 2000), northern anchovy E. mordax (Hunter and Sanchez 1976), gulf menhaden Brevoortia tyrannus (Hoss and Phonlor 1984) and Atlantic menhaden Brevoortia tyrannus 39

66 (Forward et al. 1993). The diel cycle of swimbladder inflation and deflation was previously unreported for bay anchovy larvae. Inflation of the swimbladder at night could reduce the energetic costs of activity at night when larvae are not feeding (i.e. less swimming to prevent sinking) (Hunter and Sanchez 1976; Uotani et al. 2000). It has been suggested that deflation of the swimbladder during the day may reduce predation by visual predators (inflated swimbladders are highly visible) and enhance prey-capture ability due to better body mechanics (Forward et al. 1994). Bay anchovy larvae as small as 4-5 mm were observed to have inflated swimbladders. Fifty-percent of 6-7 mm larvae had inflated swimbladders at night. The observations indicate that diel periodicity in swimbladder inflation may begin at a smaller size in bay anchovy than in other marine clupeoids (7 10 mm for those listed above). The small length of bay anchovy larvae at first inflation of the swimbladder is an adaptation that allows its early-stage larvae to more easily maintain themselves in upper layers of estuarine water columns where low salinities provide less buoyancy. In Chesapeake Bay, early development of a swimbladder may help bay anchovy larvae maintain their position above the pycnocline where copepod nauplii prey are abundant and prevent sinking into subpycnocline depths where dissolved oxygen conditions may be stressful. There is evidence for up-estuary transport of bay anchovy larvae. Bay anchovy larvae occur up-estuary from spawning locations in the Patuxent River, Chesapeake Bay (Dovel 1971), and Hudson River (Dovel 1981; Schultz et al. 2000), possibly indicating up-estuary dispersal. In addition, larger larvae were found further upstream 40

67 than smaller larvae in the Patuxent River (Loos and Perry 1991) and Hudson River (Schultz et al. 2000). In the Bay and near the mouth of the Patuxent River, larger larvae were found inshore (MacGregor and Houde 1996), suggesting directional transport from the Bay s offshore regions towards shore and mouths of tributaries. In contrast, there also is evidence for transport down-estuary and out of Chesapeake Bay. Most anchovy larvae in this study were found in the upper-layer where net transport is expected to be down-estuary. In a study at the mouth of the Bay, Olney (1996) found that cumulative net flux of bay anchovy larvae was seaward, although net flux of postflexion larvae (> 6.2 mm) was less than half that of preflexion larvae (< 6.2 mm). The fact that larvae, but not eggs, were found at a station sampled outside the mouth of the bay during this study supports Olney s conclusions. Otolith microchemistry research provides further evidence that net transport may be down-estuary during larval stages, at least in the Chesapeake Bay mainstem (Kimura et al. 2000). Trends in Sr:Ca ratios in otoliths of young-of-the-year collected at low salinity sites (5 psu) started low near the otolith core, increased at mid-regions of the otolith, and then decreased near the otolith edge, suggesting initial down-estuary transport to polyhaline regions followed by up-estuary migration to oligohaline regions (Fig. 4a in Kimura et al. 2000). Otoliths of YOY collected at polyhaline (>18 psu) regions did not demonstrate either up- or down-estuary dispersal. Kimura et al. (2000) concluded that bay anchovy do not begin directed up-estuary movement in the Bay mainstem until near metamorphosis at lengths > 25 mm. Although it is an important study, the Sr:Ca ratio technique was not sensitive enough to detect differences in 41

68 salinities that could be indicative of up-estuary dispersal by larvae within the polyhaline lower Bay region. There are three explanations that account for the observations that larger bay anchovy larvae are found up-estuary of smaller larvae, implying up-estuary dispersal. One is derived from catchability information combined with turbidity; a second is based on gradients in adult spawning or larval mortality; and the third is related to ontogentic shifts in larval diets concurrent with ontogentic shifts in prey depth distributions. Perhaps the simplest explanation for relatively big catches of larger larvae upestuary of smaller larvae is artifactual, resulting from difference in catchability related to regional differences in turbidity. In this study, net evasion apparently affected day versus night abundances of larvae > 5 mm. Olney (1996) also concluded that higher nighttime abundances of postflexion bay anchovy larvae (> 6.2 mm) were likely a result of gear avoidance. Avoidance of plankton nets during the day is a well-known phenomenon for many larval fish (Morse 1989), including other clupeoids (McGurk 1992). Because turbidity tends to increase in the up-estuary direction, light levels suitable for larval vision may be diminished up-estuary compared to down-estuary. Hence, net avoidance related to visual cues may be reduced up-estuary, accounting for bigger catches of larger larvae. Gradients in larval mortality or shifts in adult spawning locations may account for observations of larger larvae up-estuary of smaller larvae. Gradients in larval mortality may account for these observations if spawning were uniform throughout the estuary 42

69 but larval survival is higher up-estuary (Schultz et al. 2000). Rilling and Houde (1999b) did find that mortality rates were higher in the lower Chesapeake Bay than in the upper Bay, but pointed out that selective up-estuary transport of larger larvae could bias estimated survival rates. In addition, a down-estuary shift in spawning areas through the course of the spawning season could account for larger larvae being found up-estuary of areas where peak egg and small larvae production occur: larger larvae could be survivors from earlier spawning when adults were located further up-estuary (Schultz et al. 2000). Jung and Houde (2000) demonstrated that regions of peak bay anchovy adult biomass could shift from spring to summer, although not always in the down-estuary direction. A third explanation for the observation of larger larvae being found up-estuary of smaller larvae is that no net transport or up-estuary transport of larvae actually occurs. A developmental shift in bay anchovy larval diet concurrent with developmental shifts in prey depth-distributions may place larger larvae at greater depths than smaller larvae, potentially resulting in no net or up-estuary transport. During the first period of this study, larvae < 6 mm were found higher in the water column than larvae > 6 mm during the day. During the first fixed-station occupation, A. tonsa copepodite mean depths were deeper than nauplii mean depths, and A. tonsa adults were deeper than copepodites. Larger bay anchovy larvae were more common at depths where larger prey were located. In the North Sea, the vertical distribution of herring larvae, Clupea harengus, coincided with prey distributions (Munk et al. 1989). Munk et al. suggested that the daytime vertical distribution of herring larvae was determined by prey 43

70 concentrations within the region where light was sufficient for larval feeding. Although catchability problems associated with Niskin bottle sampling in the present study may bias estimates of mean depths of copepod occurrence, Roman et al. (1988) found that A. tonsa copepods were deeper in the water column during day than at night based on submersible pump collections calibrated with net tows. Because the diet of bay anchovy larvae shifts from nauplii to later-stage copepods as the larvae develop, the depth distributions of larvae that track prey during the day may deepen with larval development. Even if the depth distributions of both small and larger bay anchovy larvae result in down-estuary transport, smaller larvae would be displaced further down-estuary than larger larvae as long as larger larvae remain deeper in the water column (Table 1.6). Kimura et al. (2000) found no net up- or down-estuary dispersal had been experienced by small juveniles collected in polyhaline regions, suggesting that there could be some mechanism for retention within the lower Bay region and/or return to the estuary by larvae transported out of the mouth of the Bay. Progressive increases in daytime depths as larvae develop may begin to counteract the certain down-estuary transport of eggs and early-stage larvae before strongly directed up-estuary migration begins during metamorphosis (Kimura et al. 2000). This research and other studies suggest that some clupeoid larvae move deeper in the water column during the day (Govoni and Pietrafesa 1994; Olivar and Sabatés 1997), that clupeoid larvae move deeper in the water column as they develop (Matsuura and Kitahara 1995), and that bay anchovy postfexion larvae can show no 44

71 net or weak up-estuary transport (Schultz et al. 2000). But, the significant confounding factor of reduced daytime catchability makes it difficult to determine the actual depthdistribution of larger larvae during the day and associated potential transport. Until the depth distribution of large larvae can be accurately measured or estimated in the field during day, the question of ontogentic trends in larval transport will remain unresolved. Although problematic, understanding potential size-dependent larval transport of bay anchovy is important for addressing regional differences in mortality and growth (Rilling and Houde 1999b). The interactive effect of freshwater flow on lower-layer hypoxia and the residence time of water in the estuary could have implications for transport and survival of bay anchovy larvae during years of high freshwater flow. High freshwater flow rates in spring and summer of 1996 coincided with low abundances of eggs and larvae, low sub-pycnocline dissolved oxygen conditions (Fig. 1.2), as well as the lowest recruitment of bay anchovy young-of-the-year in a five-year time series (Jung and Houde 2000). High flow rates could have reduced the residence time of water in the estuary (Geyer et al. 2000) and caused swift transport of bay anchovy larvae out of Chesapeake Bay. In addition to controlling residence time, high freshwater flow could have affected fish early-stage survival by increasing lower-layer hypoxia. High stratification during high flow conditions can result in larger volumes of hypoxic subpycnocline waters by reducing vertical exchange across the pycnocline (Boicourt 1992). Low subpycnocline dissolved oxygen concentrations may have caused direct mortality of sciaenid eggs. In addition, dissolved oxygen concentrations < 3.0 mg L -1 45

72 may limit potential up-estuary transport or retention of anchovy larvae by preventing access to lower-layer, up-estuary flowing water. Low subpycnocline dissolved oxygen concentrations coupled with reduced residence times during high-flow years could result in transport of a significant portion of larvae out of Chesapeake Bay. Although recruitment dynamics of bay anchovy larvae are certainly influenced by many factors, including temperature and the biomass and spatial distribution of spawners (Jung and Houde 2000), the interactive effect of freshwater flow on dissolved oxygen concentrations and residence time could have important implications for potential transport and survival of fish early-life stages. 46

73 Table 1.1. Fixed-station organism occurrences. Percent organisms above the pycnocline during the entire fixed-station sampling (total), as well as the first and second occupation. Mean percentages were calculated for each set of depth-stratified samples using organism concentrations (no. m -3 for eggs and larvae, no. L -1 for copepod stages) and biovolume (ml m -3 for gelatinous zooplankton). Standard error (+/- one stderr) and sample number (n) are reported in parentheses below mean percentages. Sets of depth-stratified samples in which no organisms were captured were excluded. Bay Anchovy Total First Station Occupation Second Station Occupation Eggs (+/- 3.1, n=38) (+/- 2.2, n=28) (+/- 7.2, n=10) Yolk-sac larvae (+/- 4.6, n=22) (+/- 4.9, n=21) (n=1) Larvae 3-6 mm (+/- 4.9, n=33) (+/- 5.8, n=26) (+/- 8.3, n=7) Larvae 6-9 mm (+/- 4.6, n=36) (+/- 5.3, n=28) (+/- 9.4, n=8) Larvae 9-13 mm (+/- 5.0, n=36) (+/- 5.6, n=28) (+/- 12.1, n=8) Larvae > 13mm (+/- 4.5, n=28) (+/- 6.3, n=19) (+/- 0.7, n=9) Other Organisms Sciaenid Eggs (+/- 2.4 n=38) (+/- 2.8, n=28) (+/- 4.8, n=10) Gelatinous Zooplankton (+/- 2.8, n=38) (+/- 2.8, n=28) (+/- 7.4, n=10) Copepod Nauplii (+/- 1.9, n=38) (+/- 2.2, n=28) (+/- 3.9, n=10) A. tonsa copepodites (+/- 2.3, n=38) (+/- 2.2, n=28) (+/- 4.0, n=10) A. tonsa adult copepods (+/- 4.3, n=38) (+/- 4.1, n=28) (+/- 6.5, n=10) 47

74 Table 1.2. Mean abundance (no. m -2 for fish early-life stages, ml m -2 for ctenophores, no. x 10 3 m -2 for copepods) of organisms for the entire fixed-station sampling (total) and for four time periods with different physical characteristics. Standard errors (+/- 1 stderr) are presented in small text below the mean. Range of sample sizes (n) for means in each time period are located at the bottom of the table. Bay Anchovy Total 2-20 hrs hrs hrs hrs Eggs Yolk-sac larvae Larvae 3-6 mm Larvae 6-9 mm Larvae 9-13 mm Larvae > 13mm Other Organisms Sciaenid Eggs Gelatinous Zooplankton Copepod Nauplii A. tonsa copepodites A. tonsa adult copepods Sample Size (n)

75 Table 1.3. Results of correlation analysis of fixed-station organism abudances (no. m -2 or ml m -2 ). Significant Pearson correlation coeficients (r ) are presented with stars to indicate probability levels (*P<0.05, **P<0.01, ***P<0.001). Sample sizes ranged from n = 33 to n = 38. Anchvoy eggs Anchovy YSL 3-6 mm 6-9 mm 9-13 mm Larvae >13 mm Sciaenid Eggs Ctenophors Copepod Nauplii Acartia copepodites Acartia adults Anchvoy eggs *** 0.45** 0.56*** 0.45** 0.36* Anchovy YSL *** 0.56*** 0.50** Larvae 3-6 mm *** 0.41* -0.38* 0.70*** 0.52** Larvae 6-9 mm *** 0.42** -0.35* 0.50** 0.54*** Larvae 9-13 mm *** 0.34* Larvae >13 mm *** Sciaenid Eggs ** 0.59*** 0.74*** Ctenophores ** -0.36* Copepod Nauplii *** 0.40* Acartia copepodites *** Acartia adults 1 49

76 Table 1.4. Results of correlation analysis of mean depths of organisms, pycnocline depth, 3.0 mg L -1 oxycline depth, and time of day. Significant Pearson correlation coefficients (r ) are presented with stars to indicate significance test probability levels (*P<0.05, **P<0.01, ***P<0.001). Due to potential confounding related to net evasion during the day by anchovy larvae > 6 mm, only mean depths of larvae > 6 mm caught during the night were included in this analysis. Sample sizes ranged from n = 6 to 13 for bay anchovy larvae > 6 mm, and from n = 22 to 38 for other organisms and size classes. Anchvoy eggs Anchovy YSL 3-6 mm 6-9 mm 9-13 mm Larvae >13 mm Sciaenid Eggs Ctenophors Copepod Nauplii Anchvoy eggs * 0.67* 0.69* 0.40* 0.51** 0.36* 0.75*** 0.67*** Anchovy YSL 1 Larvae 3-6 mm 1 Larvae 6-9 mm 1 Larvae 9-13 mm ** 0.57* 0.67* 0.58* Larvae >13 mm * 0.78** 0.92*** 0.81** Sciaenid Eggs ** 0.34* Ctenophores * 0.40* 0.47** 0.53*** Copepod Nauplii * 0.67*** 0.58*** Acartia copepodites * 0.43** 0.42** Acartia adults 1 Pycnocline Depth *** Oxycline Depth 1 Acartia copepodites Acartia adults Pycnocline Depth Oxycline Depth 50

77 Table 1.5. Fixed station regression table: results of repeated measures multiple regression analysis on log e -transformed organism abundances (no. m -2 or ml m -2 for ctenophores). The regression analysis was conducted to determine whether advection (displacement), photoperiod (day-night), or time periods of differing physical conditions explained a significant amount of variability in organism abundance. Parameter estimates and their standard errors are reported for significant effects ( = 0.05). Mean displacement in the lower-layer was used for sciaenid eggs; upper-layer mean displacement was used for other organisms. The day-night variable was coded day = 0 night = 1 (night: sunrise to sunset). Period represents the four time periods when physical conditions differed (1 = 2-20 hrs, 2 = hrs, 3= hrs, 4= hrs). Displacement Day-Night Period Estimate F P Estimate F P Estimate F P Bay anchovy Anchvoy eggs 0.42 n.s n.s / Anchovy YSL / n.s / Larvae 3-6 mm / n.s n.s. Larvae 6-9 mm / / / Larvae 9-13 mm / / n.s. Larvae >13 mm 2.85 n.s / n.s. Other Organisms Sciaenid Eggs / / n.s. Ctenophores 0.07 n.s n.s / Copepod Nauplii 0.17 n.s n.s / A. tonsa copepodites 3.86 P < / n.s. A. tonsa adults 0.47 n.s / n.s. 51

78 Table 1.6. Potential transport of bay anchovy larvae by size class during the first station occupation (0 to 83 hrs) under the hypothesis that larvae made diurnal vertical migrations that became deeper during the day as larvae developed. In this hypothetical analysis, potential transport was calculated by summing the cumulative displacement (km) at the 5-m depth interval during night (sunset to sunrise) with the cumulative displacement at depths specific to each length class during the day (sunrise to sunset). Positive potential transport indicated hypothetical transport down-estuary. Length Class (mm) Nightime Depth (m) Daytime Depth (m) Potential Transport (km)

79 39.25 Baltimore CBOS Latitude Sampling Station CBBT Longitude Figure 1.1. Chesapeake Bay, USA. Location of fixed station sampling ( ), current velocity measurements at Chesapeake Bay Observing System (CBOS) buoy ( ), and water level records ( ) at Baltimore, MD, and Chesapeake Bay Bridge Tunnel (CBBT), VA. Open circles ( ) represent axial sampling stations on June

80 -1 a) June 11-12, 1996 salinity (psu) contour lines and dissolved oxygen (mg L ) shaded contours 0 Depth (m) Depth (m) b) Mean June salinity (psu) contour lines and dissolved oxygen (mg L ) shaded contours DO -1 (mg L ) Distance from Head of Chesapeake Bay (km) Fig Salinity (psu) contour lines and dissolved oxygen (mg L -1 ) shaded contours along the axis of Chesapeake Bay for a) June 10-11, 1996, and b) mean June measurements from Data from Chesapeake Bay Monitoring Program. Black dots indicate depths of CTD measurements. Dissolved oxygen data up-estuary of 50 km were not available in June 1996 (panel a). The location of the fixed sampling station in this study is marked by an arrow in the upper panel. 54

81 a) Axial suvey station locations N Potomac River Abundance (no. m -2 ) Biovolume (ml m -2 ) Abundance (no. m -2 ) Abundance (no. m -2 ) b) Bay anchovy eggs upper-layer lower-layer c) Bay anchovy larvae d) Gelatinous zooplankton e) Sciaenid eggs Distance from Head of Bay (km) Figure 1.3. a) Chesapeake Bay sampling stations for the June 1996 axial survey. Bar graphs depict b) bay anchovy egg and c) bay anchovy larvae abundances (no m -2 ), d) gelatinous zooplankton biovolume (ml m -2 ) and e) sciaenid egg abundances (no. m - 2 ) above (upper-layer) and below (lower-layer) the pycnocline. Fixed-station sampling occurred near 201 km. Stations from km were sampled at night. 55