Retention of White Perch and Striped Bass Larvae: Biological- Physical Interactions in Chesapeake Bay Estuarine Turbidity Maximum

Transcription

1 Estuaries Vol. 24, No. 5, p October 2001 Retention of White Perch and Striped Bass Larvae: Biological- Physical Interactions in Chesapeake Bay Estuarine Turbidity Maximum E. W. NORTH* and E. D. HOUDE University of Maryland Center for Environmental Science, Chesapeake Biological Laboratory, P. O. Box 38, Solomons, Maryland ABSTRACT: Physical and biological properties of the Chesapeake Bay estuarine turbidity maximum (ETM) region may influence retention and survival of anadromous white perch (Morone americana) and striped bass larvae (Morone saxatilis). To evaluate this hypothesis we collected data in five cruises, three during May 1998 and two during May 1999, in upper Chesapeake Bay. Time series of freshwater discharge, water temperature, wind, and water level explain differences in ETM location and properties between cruises and years. During high flows in 1998, a two-layer response to wind forcing shifted the ETM up-estuary, while a high discharge event resulted in a down-estuary shift in the salt front and ETM location. In 1999, extremely low discharge rates shifted the salt front 15 km up-estuary of its position in During 1999, the ETM was less intense and apparently topographically fixed. Gradients in depth-specific abundance of ichthyoplankton were compared with salinity and TSS concentrations along the channel axis of the upper Bay. During 1998, the high flow year, most striped bass eggs (75%) and most early-stage white perch larvae (80%) were located up-estuary of the salt front. In addition, most striped bass (91%) and white perch (67%) post-yolk-sac larvae were located within 10 km of maximum turbidity readings. Total abundance of white perch larvae was lower in 1999, a low freshwater flow year, than in 1998, a high flow year. In 1999, striped bass larvae were virtually absent. White perch ( ) and striped bass ( ) juvenile abundances were positively correlated with spring Susquehanna River discharge. The ETM region is an important nursery area for white perch and striped bass larvae and life-history strategies of these species appear to insure transport to and within the ETM. We hypothesize that episodic wind and discharge events may modulate larval survival within years. Between years, differences in freshwater flow may influence striped bass and white perch survival and recruitment by controlling retention of egg and early-stage larvae in the ETM region and by affecting the overlap of temperature/salinity zones preferred by later-stage larvae with elevated productivity in the ETM. Introduction In upper Chesapeake Bay, year-to-year recruitment of white perch and striped bass juveniles can vary 70 and 20 fold, respectively. Survival and recruitment of striped bass and white perch larvae may be controlled by density-independent environmental factors (Ulanowicz and Polgar 1980) such as changes in temperature and salinity (Margulies 1989; Van Den Avyle and Maynard 1994; Rutherford and Houde 1995; Secor and Houde 1995; Mc- Govern and Olney 1996). Rainfall, riverflow, and corresponding changes in toxic contaminants and ph also may be important (Hall et al. 1985; Uphoff 1989). In addition, changes in prey availability and predation rates also may affect survival and recruitment (Marguiles 1989, 1990; McGovern and Olney 1996; Limburg et al. 1997; Rutherford et al. 1997). Although each factor is supported by evidence, * Corresponding author; present address: University of Maryland Center for Environmental Science, Horn Point Laboratory, P. O. Box 775, Cambridge, Maryland 21613; tele: 410/ ; fax: 410/ ; enorth@hpl.umces.edu. none links recruitment to a major physical feature at the head of estuaries: the estuarine turbidity maximum (ETM). ETMs, characterized by elevated turbidity and suspended sediment concentrations compared to those up-estuary and down-estuary, have been found in coastal plain estuaries throughout the world (Schubel 1968). This study is a component of research that investigates the linkage of larval survival and recruitment to physics of the ETM region in Chesapeake Bay and its tributaries. The low salinity (0 to 11 psu) area of the upper Chesapeake Bay and tributaries has long been hypothesized to be a nursery area, a critical zone for early developmental stages of freshwater, estuarine, and marine spawning fish (Dovel 1971). Secor and Houde (1995) suggested that the salt front could serve as a physical barrier that retained striped bass eggs and larvae and limited their down-estuary transport in the Patuxent River. Secor et al. (1996) hypothesized that increased freshwater flow expanded the larval nursery area and increased the extent of the ETM and salt front, the region of favorable larval growth in 2001 Estuarine Research Federation 756

2 Retention of Fish Larvae in Chesapeake ETM 757 the Nanticoke River. Boynton et al. (1997) reported elevated abundance of white perch larvae, striped bass larvae, and their potential prey in and near the upper Chesapeake Bay ETM. They suggested that the ETM region could be an important nursery area for larval fish where biological conditions structured by the physics of the region could enhance recruitment potential. The ETM region has been found to be an important nursery area for larval fish in the St. Lawrence River estuary (Dodson et al. 1989; Dauvin and Dodson 1990). Correlative evidence suggests that the ETM region also may be a larval fish nursery area in San Francisco Bay/Delta estuary ( Jassby et al. 1995). Retention within the ETM region may place larvae in a zone of increased zooplankton biomass and production (Simenstad et al. 1994; Boynton et al. 1997), create a predation refuge due to high turbidity (Chesney 1989), maintain larvae in optimal temperature or salinity conditions (Strathmann 1982), and/or keep them from entering osmotically stressful, high salinity waters (Winger and Lasier 1994). Since the net flow of water is down-estuary, larval fish that use the ETM region as a nursery area must have mechanisms of dispersal to the ETM and retention within it. Larvae of white perch and striped bass that are spawned in the freshwater reaches of the upper Chesapeake Bay could be retained in the ETM region by the convergence zone associated with the landward margin of the salt intrusion, the area in which the ETM occurs (Schubel 1968). To use the convergence zone for retention, fish larvae could remain in lower-layer, net landward-flowing water just downstream of the salt front. Retention by this mechanism could be the result of active behavior or passive transport. Fish larvae also could use tidally-timed vertical migration for retention by moving up into surface waters during flood and down into bottom waters during ebb tide (Laprise and Dodson 1989; Dauvin and Dodson 1990; Rowe and Epifanio 1994; Bennett 1998). Since a larva s repertoire of behavior, swimming ability, and buoyancy regulation improves with development, late-stage larvae have better migratory abilities than early-stage larvae that can result in differing spatial distributions (Fortier and Leggett 1983; Laprise and Dodson 1989). In addition to synchrony with hydrodynamics, larval distributions could be related to other factors such as prey abundance, predator density, or physiological preferences related to temperature or salinity. If larval prey makes tidally-timed vertical migrations, larvae that track prey populations may be retained in the ETM. Together with differences in larval behavior, variations in life history strategies may affect larval use of the ETM region as a nursery area. Spawning of white perch and striped bass peaks in April and May in the headwaters of Chesapeake Bay and its tributaries (Mansueti 1964; Dovel 1971). White perch spawn eggs that adhere to the bottom in tidal fresh and brackish waters. Depending upon temperature, yolk-sac larvae hatch at a length of 2.6 mm in about 2 d. Larvae absorb the yolk-sac and develop a swimbladder within 3 to 5 d posthatch at a length of 3.8 mm (Mansueti 1964) suggesting that they have an ability to control their position in the water column at an early age. In contrast, striped bass spawn pelagic eggs in tidal freshwater above the salt front (Dovel 1971; Secor and Houde 1995) where the slightly heavy eggs (specific gravity to ) are suspended by currents greater than 0.3 m s 1 (Albrecht 1964; Rulifson and Tull 1999). Striped bass larvae hatch in about 2 d at a larger size ( 3.1 mm) than white perch and do not develop a swimbladder and absorb their yolk-sac until they are 5 d old and 5 mm in length (Mansueti 1958, 1964; Doroshev 1970). Both striped bass and white perch yolk-sac larvae may have the ability to swim weakly toward surface waters during the day since both exhibit positive phototaxis upon hatching in the laboratory (Mansueti 1958, 1964). Although the ETM is a well-established feature of the upper Chesapeake Bay, its location and intensity are variable. The ETM ranges from km in extent and is generally associated with the tip of the 1 psu isohaline, but can be displaced from it by as much as 10 km due to tidal excursion. In addition, the longitudinal position of the Chesapeake Bay ETM varies among hours, days, seasons, and years depending upon tidal excursion, wind forcing, and the amount of freshwater flow (Boynton et al. 1997; Sanford et al. 2001). Since the extent of the ETM nursery area and larval retention mechanisms are implicitly linked to the variable physics of the region, seasonal and annual changes in ETM properties may have consequences for larval survival and recruitment. The overall goal of our research is to determine how the physical and biological properties of the Chesapeake Bay ETM region influence retention and survival of white perch and striped bass larvae. Based on results from two years of field research, combined with environmental monitoring and juvenile fish survey data, we sought to determine whether larvae are concentrated in the ETM region, describe the vertical distribution of larvae in relation to changing physical conditions, identify potential retention mechanisms, and formulate a hypothesis linking ETM physics to white perch and striped bass recruitment.

3 758 E. W. North and E. D. Houde Fig. 1. Chesapeake Bay, USA. Axial CTD survey stations in 1998 and 1999 (filled circles) as well as additional 1999 stations (open circles). Also pictured are the locations of environmental monitoring stations (stars). Methods Five research cruises were conducted in the upper Chesapeake Bay (Fig. 1) on the 52-ft R/V Orion during the spawning seasons of striped bass and white perch. Three cruises were in May 1998 (May 2 5, 11 14, and 19 22) and two were in May 1999 (May 4 6 and 17 19). Each cruise consisted of a CTD survey along the axis of the Bay, collections to map gradients in ichthyoplankton distribution, and a fixed station occupation within the ETM to document changes in ichthyoplankton abundance over a tidal cycle. Although limited sampling was conducted in shoal areas, only results of the axial survey and gradient mapping in the shipping channel are presented here. The location of the ETM and landward margin of salt intrusion (defined as the intersection of the 1 psu isohaline with bottom) were determined in the axial CTD survey. During each cruise, the axial CTD survey (Fig. 1) was conducted from south to north and was completed in 4 to 6 h. A Seabird CTD equipped with a SeaTech 5-cm pathlength transmissometer was used to measure temperature, salinity, and turbidity every 7.4 km at 9 stations upestuary of N. One (1998) or two (1999) extra stations were added to the axial survey to better resolve the location of the salt front. Prior to the 1998 and 1999 cruises, transmissometer voltage was calibrated to NTU units with a Formazin NTU standard. In the field, 100-ml water samples were collected with a pump attached to the CTD frame to calibrate turbidity measurements with total suspended solids (TSS) concentrations. These samples were refrigerated, filtered within 72 h, and brought to the lab for TSS analysis. In 1998, the NTU to TSS calibration for TSS values 60 mg l 1 was weak. The highest TSS values in 1998 are somewhat uncertain but comprise only 4.3% of the data. During the 1998 and 1999 axial surveys, contour plots of transmissometer voltage and salinity were used to locate the ETM and the intersection of the 1 psu isohaline with the bottom. This information and predicted tides from Tides and Currents Pro 2.5 software package were used to designate sampling locations for subsequent mapping of ichthyoplankton abundance. On each cruise, 6 gradient mapping stations were designated, 1 within the ETM, and 2 up-estuary and 3 down-estuary from the ETM station at intervals equal to one-half the tidal excursion (first cruise) or two-thirds the tidal excursion (remaining cruises) at the ETM location. Sampling at the gradient mapping stations was conducted from north to south at night and completed in 7 to 8 h (Fig. 1). At each station, ichthyoplankton was collected in depth-stratified tows of a Tucker trawl in 2 or 3 depth intervals depending on water depth. The 1-m 2 opening-closing Tucker trawl was fitted with 280- m mesh nets, a General Oceanics flow meter, and a Mini-Log temperature and depth recorder. Because the average depth of the upper Bay channel was about 12 m, most Tucker trawl tows were taken in 0 3.5, 3.5 7, and 7 11 m depth intervals. Ichthyoplankton samples were preserved in ethanol, transferred to fresh ethanol within one day, and taken to the laboratory for enumeration and identification. Three samples, one from each cruise in 1998, were lost (broken sample jars). White perch and striped bass postyolk-sac larvae (larvae that have absorbed the yolk sac) were identified using criteria based on external morphological features (Waldman et al. 1999). Results from cruises were mapped with contour plots of physical factors and ichthyoplankton concentrations (Surfer software). The gridding method was kriging with an isotropic linear variogram model. Grid-line geometry was about half the average distance between measurements in the X (distance) and Y (depth) directions. In the contour plots, station locations were expressed in kilometers from the mouth of the Susquehanna River (Fig. 1). Freshwater discharge, water level, wind, and water temperature monitoring data were used to identify large-scale environmental forcing events that produced physical conditions observed during the cruises (Fig 1). Since the source of most freshwater in the upper Bay is the Susquehanna River (Schubel and Pritchard 1986), daily mean discharge data (U.S. Geological Survey) for the Sus-

4 Retention of Fish Larvae in Chesapeake ETM 759 Fig. 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. quehanna River at Conowingo, Maryland was used as a measure of freshwater input. Wind forcing events were identified from hourly water-level height measurements at Baltimore, Maryland (National Ocean Service), and from wind speed and direction observations at Baltimore-Washington International Airport (B.W.I.), Maryland (National Oceanic and Atmospheric Administration National Climatic Data Center). The hourly wind data was vector averaged in 6-h intervals. Water temperature, measured every 10 min at a depth of 2.4 m and near bottom, was obtained from a Chesapeake Bay Observing System (CBOS) buoy deployed by the University of Maryland Center for Environmental Science. To explore possible relationships between river flow and recruitment, Pearson correlation coefficients were calculated between spring Susquehanna River discharge rates and indices of white perch and striped bass young-of-the-year (YOY) abundance in the upper Chesapeake Bay. Spring Susquehanna River flow rates were means of the daily discharge data from March through May. The white perch ( ) and striped bass ( ) YOY abundance indices in the upper Bay were obtained from the Striped Bass Juvenile Index Seine Survey (Maryland Department of Natural Resources). Abundance indices (no. haul 1 ) were derived from replicate hauls of a 30.5 m beach seine with 6.44 mm bar mesh at seven stations in the upper Chesapeake Bay sampled three times between July and September each year. YOY indices were square-root transformed to satisfy assumptions of correlation significance tests. Although comparison of the relationship between freshwater flow and white perch and striped bass Fig. 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. abundances was conducted on the entire time series of juvenile abundance, additional correlation analyses were limited to the period because the striped bass spawning population was severely depleted during the late 1970s to mid 1980s. Results and Discussion PHYSICS Freshwater flows to the upper Bay differed in May 1998 and When compared to the 30-yr mean Susquehanna monthly discharge, discharge values in 1998 were generally above average while those from 1999 were below average (Fig. 2). May 1999 was the driest May in the past 30 yr. Discharge in spring 1998 was characterized by several large peaks, one occurring in May during the second research cruise (Fig. 3). During this event, water temperature at the CBOS northern Bay buoy declined by about 1 C. The peak in flow was preceded by strong northern winds that caused a decrease in water level in the upper Bay. After the wind-forcing and high freshwater-flow events, discharge declined steadily prior to the third research cruise. The most significant feature of environmental conditions in May 1999 was the very low discharge

5 760 E. W. North and E. D. Houde Fig. 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. rate (Fig. 4). Water temperature at the CBOS buoy was about 2 C colder than in 1998 and there were no major peaks in discharge. Prior to the first 1999 research cruise, a northern wind event caused a decrease in water level in the upper Bay. This resulted in the intrusion of colder, lower-layer water into the upper Bay as indicated by the deviation between surface and bottom temperature in the CBOS record. Salinity and TSS profiles from the axial CTD surveys reflect the differences in environmental conditions between cruises and years. During the first axial survey in 1998, the salt front was located near km 30 and salinity in the upper Bay ranged from 1 to 11 psu (Fig. 5). Maximum TSS concentrations were located near the foot of the salt front. On May 11, after the strong northern wind event but before the peak in discharge, the pycnocline had intensified and the salt front moved up-estuary. Salinity in the lower layer increased substantially as indicated by the location of the intersection of the 11 psu isohaline with the bottom, which had shifted 20 km up-estuary between the first and second cruise. Highest TSS concentrations on May 11 were found within a tidal excursion (8 10 km) up-estuary of the salt front and had decreased by about Fig. 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 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). 80% from those observed during the May 2 survey. On May 19, after the peak in freshwater discharge, the salt front had moved down-estuary to km 35 and stratification had weakened (Fig. 5). Highest concentrations of TSS were associated with the salt front. Results from 1998 are consistent with other findings (Schubel 1968; Boynton et al. 1997; Sanford et al. 2001) that suggest the upper Chesapeake Bay ETM is associated with the salt front and maintained by gravitational circulation with tidal asymmetry. The results also demonstrate the influence of wind-forcing and discharge events on the properties of the ETM region. The intrusion of salt and increase in stratification between the May 2 and May 11 survey in 1998 suggests a two-layer response to wind forcing. The increase in stratification associated with the wind event could have resulted in reduced TSS concentrations during the May 11 survey since increased stratification can suppress turbulence and tidal resuspension of sediment (Geyer 1993). The location of the salt front and ETM during the May 19 survey suggests that freshwater discharge can push the salt front and ETM down-estuary. This situation occurs in the Tamar

6 Retention of Fish Larvae in Chesapeake ETM 761 and San Francisco Bay/Delta estuaries where the bottom location of the 1 and 2 psu isohalines, respectively, can be predicted by freshwater flow (Uncles and Stephens 1993; Jassby et al. 1995). In May 1999, TSS concentrations in the upper Bay were generally lower than those in 1998 while salinity was higher, increasing from a maximum of 13 psu in 1998 to a maximum of 17 psu in 1999 (Fig. 5). Stratification near the salt front was less intense and the intersections of the 1 psu isohalines with the bottom were about 15 km further up-estuary than in Temperature in the upper Bay in 1999 was generally lower than in 1998, but was 12 C where salinity was 11 psu in 1998 and The salinity structure on the May 4 cruise was more stratified than on May 17, perhaps due to the north winds just prior to the first cruise. During both cruises, peak TSS concentrations were found in higher salinity water than in 1998 and were located near km 25, just downstream from a shoal area about 15 km south of the Susquehanna River mouth. The low TSS concentrations within and near the ETM in 1999 may be due to decreased suspended sediment input during low runoff conditions as well as a weaker convergence zone. The ETM in 1999 may have resulted from gravitational circulation just down-estuary of a topographic high because it appeared to be fixed near shoaling topography (Burau et al. 1998). Salinity structure on May 17 probably characterized the upper Bay in May 1999 because there were no major wind forcing or discharge events after May 3. BIOLOGY During all cruises, white perch yolk-sac larvae were found in greatest concentration within and up-estuary of the salt front and ETM (Fig. 6). In fact, 93% of the yolk-sac larvae were found in waters of salinity 5 psu in both 1998 and 1999 (Table 1, add percentage in salinity 1 psu to that in salinities 1 and 5 psu). Although collected throughout the water column, 70% of white perch yolk-sac larvae were found in mid-depth and bottom waters during both years (Table 1). In contrast to yolk-sac larvae, white perch postyolk-sac larvae were more prevalent near and within the salt front and ETM in 1998 (Fig. 7a c). More post-yolk-sac larvae were found within 10 km of maximum TSS concentrations (67% compared with 40% of yolk-sac larvae) and in salinities 1 and 5 psu (47% compared with 13% of yolk-sac larvae; Table 1). In 1999, post-yolk-sac larval concentrations peaked within the salt front and upestuary of the ETM (Fig. 7). During the first cruise, most white perch post-yolk-sac larvae (60%) were found within 10 km of maximum TSS concentrations and 85% were in salinities 6 psu. During Fig. 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) May 4 5, 1998, b) May 12 13, 1998, c) May 21 22, 1998, d) May 4 5, 1999, and e) May 17 18, 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). the second cruise, when maximum TSS concentrations were 15 km down-estuary of the 1 psu isohaline, most post-yolk-sac larvae (98%) were in salinities 5 psu but only 25% were within 10 km of maximum TSS concentrations. During both years, the depth of peak post-yolk-sac larval concentrations varied from surface to bottom waters (Fig. 7). Peak concentrations of striped bass eggs ranged from near surface (Fig. 8c) to near bottom (Fig. 8a). The differences in vertical distribution of eggs may depend upon depth of spawning, physical advection and mixing processes, and/or developmental stage (egg density increases with embryo development, Rulifson and Tull 1999). In 1998, most striped bass eggs (75%) were located up-estuary of the ETM in salinities 1 psu (Fig. 8; Table 1), except during the high-discharge-event cruise (May 12 13, 1998) when eggs were found near bottom within and near the salt front. This is consistent with distributions observed in the Patuxent (Secor and Houde 1995) and Pamunkey Rivers (McGovern and Olney 1996). In 1999, peak concentrations of eggs were up-estuary of the ETM

7 762 E. W. North and E. D. Houde TABLE 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 in parentheses 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 Percent YSL a) 1998 Within 10 km of max TSS 39.8 (11.7) In salinity 1 psu 79.9 (10.6) In salinity 1 and 5 psu 13.3 (6.4) In bottom waters 53.2 (8.2) In bottom and mid-depth waters 77.6 (6.8) In temperature 15 C and 20 C 59.4 (29.7) White Perch Post-yolk-sac Larvae 66.9 (16.7) 42.7 (22.1) 46.5 (15.6) 57.1 (9.8) 81.1 (8.0) 57.9 (20.5) Eggs 32.4 (20.3) 75.3 (21.8) 1.9 (1.8) 57.1 (13.7) 85.6 (8.4) 44.0 (29.3) Striped Bass YSL 90.1 (3.8) 51.4 (24.5) 41.7 (24.5) 66.2 (11.5) 85.2 (7.6) 56.5 (26.8) Post-yolk-sac Larvae 90.8 (3.2) 46.7 (15.8) 48.7 (17.1) 55.4 (12.6) 74.5 (13.2) 81.1 (14.6) b) 1999 Within 10 km of max TSS (12.5) (17.9) (35.1) (0.0) In salinity 1 psu In salinity 1 and 5 psu 53.9 (38.8) 65.6 (32.3) 84.8 (15.2) 50.0 (50.0) In bottom waters 37.7 (5.6) 25.7 (11.0) 4.0 (4.0) 0.0 (0.0) In bottom and mid-depth waters (0.6) In temperature 15 C and 20 C 73.3 (25.9) (3.0) 68.8 (29.1) (0.1) (0.0) (0.0) 50.0 (50.0) (Fig. 8d,e) and most ( 70%) were found in salinities 5 psu. In contrast to its eggs, significant numbers of striped bass yolk-sac and post-yolk-sac larvae were collected only in No post-yolk-sac larvae and only 2 yolk-sac larvae were collected in In 1998, both striped bass yolk-sac (Fig. 9) and postyolk-sac larvae (Fig. 10) were concentrated within and just up-estuary of the salt front. Nearly all striped bass yolk-sac larvae (90%) and post-yolk-sac larvae (91%) were collected within 10 km of maximum TSS concentrations (Table 1). Like white perch, striped bass post-yolk-sac larvae had peak concentrations near surface and at depth. Peaks in surface concentrations occurred during predicted flood tide (Figs. 10b and 7a), while those near bottom often occurred during predicted ebb tide (Figs. 10c and 7c). Although both striped bass and white perch post-yolk-sac larvae were found in greatest numbers in and near the salt front and ETM in 1998, the location of their peak concentrations did not always coincide (compare Figs. 7a and 10a). Striped bass apparently are more strongly associated with the convergence zone at the tip of the salt front than white perch. Boynton et al. (1997, upper Bay) and Shay (1997, Potomac River) also found that striped bass and white perch larvae were concentrated in the ETM region, but striped bass were found in greatest numbers down-estuary from white perch. Striped bass larvae were approximately one-seventh as abundant as white perch larvae (Fig. 11). The abundances of white perch and striped bass yolk-sac and post-yolk-sac larvae (number under 1 m 2 ) were lower in 1999 than in 1998 (Fig. 11). The lower white perch abundances and virtual absence of striped bass larvae in 1999 collections could be due to decreased spawning by adult fish, a shift in primary spawning area from the Susquehanna channel to the Elk River channel, an increase in larval mortality, and/or a consequence of the 1999 sampling time and design. There were only two research cruises in 1999 and sampling during the second cruise was not conducted up-estuary of the 1 psu isohaline where larvae could have been present. Despite these drawbacks, it seems likely that elevated mortality influenced larval abundance in 1999 because both white perch yolk-sac larvae and striped bass eggs were present but white perch post-yolk-sac larvae were scarce and striped bass post-yolk-sac larvae were virtually nonexistent. The

8 Retention of Fish Larvae in Chesapeake ETM 763 Fig. 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) May 4 5, 1998, b) May 12 13, 1998, c) May 21 22, 1998, d) May 4 5, 1999, and e) May 17 18, 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). Fig. 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) May 4 5, 1998, b) May 12 13, 1998, c) May 21 22, 1998, d) May 4 5, 1999, and e) May 17 18, 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). fact that abundances of white perch and striped bass YOY were much lower in 1999 than in 1998 in the upper Chesapeake Bay (Fig. 12) also supports the assertion that the decline in sampled larvae represented a real decrease in abundance rather than sampling error. The decline in juvenile abundance coincides with the decrease in freshwater flow between 1998 and A correlation analysis indicates that this relationship is robust. The correlation between white perch YOY and freshwater discharge was not significant for the period (r 0.54, n 11, p 0.09), but was significant for the entire period (r 0.49, n 23, p 0.02). There was a strong and significant correlation (r 0.78, n 11, p 0.005) between freshwater discharge and striped bass YOY abundance from (Fig. 12). Despite the stock collapse in the late 1970s and early 1980s, spring freshwater discharge and striped bass YOY abundance for the entire period also were significantly correlated (r 0.43, n 32, p 0.01). Kernehan et al. (1981) did not find a significant correlation Fig. 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) May 4 5, 1998, b) May 12 13, 1998, and c) May 21 22, 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).

9 764 E. W. North and E. D. Houde Fig. 10. 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) May 4 5, 1998, b) May 12 13, 1998, and c) May 21 22, 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). between abundance of upper Chesapeake Bay striped bass YOY and river flow before the stock collapsed in They may have been unable to detect a significant correlation because of small sample size or because freshwater flow during (CV 24%) was less variable than during (CV 42%). Fig. 12. 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. Fig. 11. Mean abundance (no. m 2 ) of white perch a) yolksac and b) post-yolk-sac 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. SYNTHESIS The ETM region is an important nursery area for white perch and striped bass based upon descriptive analysis of spatial distributions. Unlike spatial representation of physical factors, the distribution of organisms may be misrepresented by contour plotting because organisms can have patchy distributions. The persistence in the pattern of larval fish distributions between cruises supports the conclusion that the concentration of fish larvae in the ETM region is not an artifact of sampling patchy distributions. Data from the fixed-station component of each cruise (North 2001) do not indicate extensive patchiness within the ETM on smaller spatial scales (1 4 km) than those resolved by the gradient mapping (5 10 km). Most early-stage larvae of both white perch and striped bass were in a position to be passively transported to and retained within the convergence zone that creates the ETM. Although differences in the vertical distribution of post-yolk-sac larvae

10 Retention of Fish Larvae in Chesapeake ETM 765 could be interpreted as tidally-timed vertical migration, it is likely that these patterns resulted from differences in the developmental stage of post-yolksac larvae and/or aggregation of larvae in response to prey with varying vertical distributions (North 2001). White perch and striped bass larvae could use either of two mechanisms for retention once within the salt front and ETM region: tidallytimed vertical migration or consistent maintenance in lower-layer bottom waters. Although evidence suggests striped bass larvae use tidally-timed vertical migration in the San Francisco Bay/Delta estuary (Bennett 1998), results presented in North (2001) and previous research in upper Chesapeake Bay support the latter mechanism (Boynton et al. 1997; Kernehan et al. 1981). Dispersal of eggs to the salt front and retention of larvae within low salinity waters in the convergence zone may be an important striped bass lifehistory strategy that promotes early-stage survival. Results of 12-d and 10-d salinity toxicity studies with California and Georgia striped bass eggs and larvae (48-h posthatch) indicate that both 0 psu and salinities 6 psu result in higher mortalities than low salinities ( 6 psu) (Lal et al. 1977; Winger and Lasier 1994). Winger and Lasier (1994) determined that the LC50 for 48-h posthatch larvae of Georgia striped bass was 10 psu (10-d exposure). Furthermore, they reported that live eggs floated at salinities 9 psu, and that 15% to nearly 100% of eggs ruptured in salinities from 12 to 18 psu. In upper Chesapeake Bay, high salinities in the ETM region in 1999 may have prevented a significant portion of striped bass eggs from sinking into the entrapment zone, resulting in transport downestuary into high salinity waters where mortality was likely. Egg and yolk-sac larvae that were trapped in bottom waters of the ETM region in 1999 could have been subjected to salinities as high as 10 psu, the LC50 for Georgia striped bass 48-d posthatch larvae. Although salinity tolerance may vary with genetic strain, evidence from toxicity studies suggests that high salinities in the ETM region during low-flow conditions in 1999 could have been detrimental to egg retention and larval striped bass survival, and may help account for the virtual absence of larvae. In contrast to the life history strategy of striped bass, white perch hatch from demersal eggs in tidal freshwater, swim up or are resuspended into the water column for transport downstream as yolk-sac larvae, and accumulate in the ETM region as postyolk-sac larvae. This scenario supports Mansueti s (1964) suggestion of rapid downstream dispersal of white perch larvae based on observations in the Patuxent River. Survival of white perch larvae may not be as dependent upon the characteristics of the ETM as striped bass because transport downestuary to the nursery area occurs during the yolksac stage. This, coupled with early development of a swimbladder, may enable white perch larvae to make movements in relation to physical cues and/ or prey concentrations that result in retention soon after transport to the ETM region. Episodic events may influence larval survival by altering conditions favorable for retention. During the 1998 spawning season, wind and freshwaterflow events significantly affected location of the salt front, ETM, and larval distributions. Down-estuary current enhanced by discharge or wind forcing could swiftly transport eggs and early-stage larvae out of the ETM region into areas of low prey abundance, high predation potential, and osmotically stressful high salinity. Mortality is highly probable for striped bass larvae transported down-estuary of the salt front as Secor et al. (1995) demonstrated in a mark-recapture study. No larvae released below the salt front and presumed ETM in the Patuxent River were recaptured, while larvae released above it were retained and recaptured near or upstream of their release site. Research in other spawning systems suggests that episodic riverflow events can result in mortality of larval striped bass. In the Potomac River, Rutherford and Houde (1995) observed complete mortality of striped bass eggs and newly hatched larvae when temperature dropped suddenly in relation to the passage of a storm front. Riverflow peaked during this period (Rutherford et al. 1997) and was a major cause of the drop in water temperature. Uphoff (1989) also detected effects of episodic riverflow events on Choptank River striped bass larvae in During short time periods (7 18 d), minimum water temperatures were associated with low abundance of 6-mm larvae and high rainfall and riverflow coincided with high larval mortality. In addition to episodic events, annual differences in levels of freshwater flow dramatically altered the character of the salt front and ETM in Chesapeake Bay. The decline in abundance of white perch and striped bass larvae between 1998 and 1999 suggests that annual variation in freshwater input may influence larval survival. The close association of striped bass larvae with the ETM and salt front in 1998, their virtual absence in 1999, and the strong correlation between striped bass YOY abundance and river flow from suggests that larval striped bass survival may be strongly connected to physics of the ETM convergence zone. In contrast, white perch survival may not be as closely linked. White perch larvae were broadly distributed in the ETM region in both

11 766 E. W. North and E. D. Houde Fig. 13. 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 and 1999, and YOY abundance was weakly related to river flow. HYPOTHESIS Considering high-flow and low-flow scenarios, we developed a hypothesis linking variation in biological and physical properties of the ETM region to white perch and striped bass recruitment (Fig. 13). The hypothesis has two components: retention of early-stage larvae within the ETM and laterstage larval preferences overlapping ETM productivity. The first component is related to freshwater flow and physical conditions favorable for retention. Changes in freshwater flow are likely to alter the strength of gravitational circulation within the salt front. In the upper Bay, enhanced gravitational circulation during a high-flow year may create a strong entrapment zone where eggs and early-stage larvae could be passively retained (Fig. 13a). Lowflow conditions could lead to increased egg and larval mortality from transport down-estuary of the ETM region due to a weakened entrapment zone associated with decreased gravitational circulation (Fig. 13b). Although both tidal asymmetry and gravitational circulation are required for particle accumulation in the ETM ( Jay and Musiak 1994; Burchard and Baumert 1998), gravitational circulation alone may be sufficient for larval retention since larvae, unlike sediment, do not require resuspension. This component of the hypothesis is speculative because the relationship between freshwater input and gravitational circulation has not been established for the upper Bay and theoretical studies show that gravitational circulation is not linearly related to freshwater flow (Li 1999). Although gravitational circulation can increase with increasing flow, very high flows may transform a partially mixed estuary to a salt-wedge estuary in which lower-layer gravitational circulation weakens and stratification intensifies. Because depth, width, and tidal amplitude control estuarine characteristics, this transition will likely occur at different flow rates from one estuary to another. In addition to having increased temperature variability (Limburg et al. 1999), narrow tidal rivers may have a greater tendency to resemble salt-wedge estuaries during high flow years, resulting in decreased retention and, ultimately, poor recruitments during both high and low flow conditions. In the Roanoke River, Rulifson and Manooch (1990) found that striped bass recruitment was best at moderate flows and poor during high and low flow conditions. In contrast, Turner and Chadwick (1972) found that striped bass juvenile abundance and freshwater flow in the Sacramento-San Joaquin estuary were positively related, though not linearly as found in the upper Chesapeake Bay (this study, Fig. 12). Further research on this topic may help explain differences in ETM properties between years and estuaries, and may help explain dissimilarities in anadromous fish recruitment levels between river systems. The second component of the hypothesis emphasizes the overlap of later-stage larval salinity/ temperature preference zones with biological production in the ETM region. During a high flow year (Fig. 13a), organic matter may be elevated in the ETM due to increased input from runoff and intensified particle trapping. Elevated organic matter enhances the productivity of the detritus-based food web. Due to the intense salt front in high flow years, the preferred salinity/temperature zone for larvae remains close to the ETM, a region of observed high zooplankton prey abundance (Kimmerer et al. 1998; Roman et al. 2001) and hypothesized low predation pressure due to high turbidity refuge. Abundant food and refuge from predation may lead to high larval growth rates and low mortality resulting in enhanced recruitment. In low-flow years (Fig. 13b), decreased input of organic matter may reduce the productivity of the detritus-based food web, leading to declines in productivity and zooplankton prey abundance. The diffuse salt-front may shift the larval preferred salinity/temperature zone away from the ETM where highest prey abundance and lowest predation pressure are expected. Such sub-optimum conditions in low-flow years may lead to slower growth, in-

12 Retention of Fish Larvae in Chesapeake ETM 767 creased mortality, and eventually to poor recruitment. This component of the hypothesis integrates the results of previous studies linking the survival and recruitment of striped bass and white perch larvae to temperature, salinity, water quality, and prey availability and incorporates hypotheses on causes of larval mortality based on predation rates (Secor and Houde 1995; Limburg et al. 1999). Temperature (Houde et al. 1997), salinity (Winger and Lasier 1994), and poor water quality (Hall et al. 1985) can affect striped bass larval growth and survival. Compared to wet years, low flow years have been associated with lower larval abundance in the Pamunkey River (McGovern and Olney 1996) and with decreased growth, survival, and recruitment in the Nanticoke River (Secor et al. 1996). White perch larval growth (Limburg et al. 1999) and striped bass late-stage larval abundance (Rutherford et al. 1997) have been positively correlated with zooplankton abundance. Annual differences in freshwater flow may be the mechanism promoting these relationships. The idea that enhanced delivery of organics during a high flow year could stimulate plankton productivity and improve larval fish survival is not new (Turner and Chadwick 1972; Boynton et al. 1976). In the San Francisco Bay/Delta estuary, changes in ETM location in response to freshwater flow are significantly related to annual measures of estuarine organism abundance and larval striped bass survival ( Jassby et al. 1995). Other research suggests that ETMs contain productive heterotrophic communities (Boynton et al. 1997) with enhanced microbial components and bacterial growth rates (Baross et al. 1994). Although current research suggests that variation in organic input may be important to ETM food webs, the influence of changes in freshwater flow on the productivity of the ETM community has not been established and is an important avenue for future research. This hypothesis highlights freshwater flow as the mechanism controlling the physical properties of the ETM region, which in turn controls retention of eggs and early-stage larvae and promotes biological conditions favorable to growth and survival of post-yolk-sac larvae. Change in the retention capacity of the ETM in response to freshwater flow is a likely controller of recruitment because most mortality occurs during the earliest larval stages in response to density-independent environmental factors (Ulanowicz and Polgar 1980; Secor and Houde 1995). While low flow years may not cause larval starvation (Bennett et al. 1995), survival of later-stage larvae could be controlled by flow-dependent variations in the physical and biological properties of the ETM. Within years, episodic wind and river flow events could modulate larval survival by modifying ETM properties and result in variability in the relationship between freshwater flow and YOY abundance. Further investigation of the link between ETM physics and the biological community in the ETM region of different estuaries will enhance our understanding of anadromous fish recruitment. ACKNOWLEDGMENTS We are indebted to W. B. Boicourt, L. P. Sanford, and S. E. Suttles for valuable guidance, manuscript review, and willingness to share knowledge of physics. We thank the crew of RV Orion and cruise participants for capable field support, S. M. Jones for assistance with ichthyoplankton sorting, and E. Durrell of Maryland Department of Natural Resources for supplying young-of-the-year data. D. H. Secor, E. Russek-Cohen, W. R. Boynton, and two anonymous reviewers provided valuable comments on the manuscript that are much appreciated. This research was supported by the National Science Foundation Biological Oceanography Program (Grant No. NSF OCE ) and the Environmental Protection Agency Science-To-Achieve- Results Fellowship Program (Fellowship No. U ). LITERATURE CITED ALBRECHT, A. B Some observations on factors associated with survival of striped bass eggs and larvae. California Fish and Game 50: BAROSS, J. A., B. CRUMP, AND C. A. SIMENSTAD Elevated microbial loop activities in the Columbia River estuarine turbidity maximum, p In K. R. Dyer and R. J. Orth (eds.), Changes in Fluxes in Estuaries: Implications from Science to Management. Olsen and Olsen, Fredensborg, Denmark. BENNETT, W. A Vertical migration and retention of native and exotic larval fish within the entrapment zone of a tidally dominated estuary, p In W. Kimmerer (ed.), Report of the 1994 Entrapment Zone Study. Technical Report 56. Interagency Ecological Program for the San Francisco Bay/ Delta Estuary, Sacramento, California. BENNETT, W. A., D. J. OSTRACH, AND D. E. HINTON Larval striped bass condition in a drought-stricken estuary: Evaluating pelagic food-web limitation. Ecological Applications 5: BOYNTON, W. R., W. BOICOURT, S.BRANT, J.HAGY, L.HARDING, E. HOUDE, D. V. HOLLIDAY, M. JECH, W. M. KEMP, C. LASCARA, S. D. LEACH, A. P. MADDEN, M. ROMAN, L. SANFORD, AND E. M. SMITH Interactions between physics and biology in the estuarine turbidity maximum (ETM) of Chesapeake Bay, USA. CM 1997/S:11. International Council for the Exploration of the Sea, Copenhagen, Denmark. BOYNTON, W. R., E. M. SETZLER, K.V.WOOD, H.H.ZION, M. HOMER, AND J. A. MIHURSKY Potomac River fisheries program ichthyoplankton and juvenile investigations. Final report to Maryland Department of Natural Resources. University of Maryland, Center for Environmental Science, Chesapeake Biological Laboratory Reference No Solomons, Maryland. BURAU, J. R., J. W. GARTNER, AND M. STACEY Results from the hydrodynamic element of the 1994 entrapment zone study in Suisun Bay, p In W. Kimmerer (ed.), Report of the 1994 Entrapment Zone Study. Technical Report 56. Interagency Ecological Program for the San Francisco Bay/ Delta Estuary, Sacramento, California. BURCHARD, H. AND H. BAUMERT The formation of estuarine turbidity maxima due to density effects in the salt wedge.