DISTRIBUTION AND RELATIVE ABUNDANCE OF BLUE CRAB CALLINECTES SAPIDUS IN THE UPPER BARATARIA ESTUARY, LOUISIANA. A Thesis

Transcription

1 DISTRIBUTION AND RELATIVE ABUNDANCE OF BLUE CRAB CALLINECTES SAPIDUS IN THE UPPER BARATARIA ESTUARY, LOUISIANA A Thesis Submitted to the Graduate Faculty of Nicholls State University In partial fulfillment of the requirements for the degree of Master of Science in Marine and Environmental Biology by MattiLynn D. Dantin B.S., Nicholls State University, 25 Spring 27

2 CERTIFICATE This is to certify that the thesis entitled Distribution and Relative Abundance of blue crab Callinectes sapidus in the Upper Barataria Estuary, Louisiana submitted for the award of Master of Science to Nicholls State University is a record of authentic, original research conducted by Mrs. MattiLynn D. Dantin under our supervision and guidance and that no part of this thesis has been submitted for the award of any other degree, diploma, fellowship, or other similar titles. APPROVED SIGNATURE DATE Quenton Fontenot, Ph.D. Assistant Professor of Biological Sciences Committee Member Allyse Ferrara, Ph.D. Assistant Professor of Biological Sciences Committee Member Earl Melancon, Ph.D. Professor of Biological Sciences Committee Member i

3 ABSTRACT Blue crabs Callinectes sapidus are marine organisms that seasonally migrate within an estuary and contribute to energy transfer throughout the system. Because blue crab is a commercially and recreationally important species within Louisiana estuaries, it is important to understand factors that may affect blue crab distribution and abundance. The Barataria Estuary is bordered by the Mississippi River to the east, Bayou Lafourche to the west, and the Gulf of Mexico to the south. The upper-most reaches of the Barataria Estuary are comprised of approximately 41% of forested freshwater wetlands including the Lac Des Allemands/Bayou Chevreuil area. Blue crabs were sampled weekly between 11 July and 6 December 26, with modified commercial crab traps at seven fixed sites in Bayou Chevreuil. Traps were baited with fish carcasses or chicken pieces, and remained deployed for approximately 24 hours. Surface and bottom water temperature ( C), salinity (ppt), dissolved oxygen (DO; mg/l) and specific conductance (µs) were measured at each site when traps were deployed. Blue crab catch per unit effort (CPUE) was determined as the mean number of crabs collected per trap per day. Crabs were enumerated and transported to the Bayousphere Research Laboratory to be sexed, reproductive state determined, and measured for carapace width (mm), carapace length (mm), cheliped-free body weight (g), and individual cheliped weight (g). Individual trap CPUE ranged from -24 crabs/trap/day. Of the 649 blue crabs collected from Bayou Chevreuil, there were 24 immature females, 34 mature females, and 591 males. Overall, females were wider than males, but males were heavier than females of similar width (P <.1). Temperature, dissolved oxygen, salinity, and specific conductance were positively correlated (P <.5) to blue crab abundance. Distribution and abundance were ii

4 highest in July and August and lowest in November and December. Blue crabs are a seasonally abundant species in Bayou Chevreuil. iii

5 ACKNOWLEDGEMENTS I would like to thank my committee members, Dr. Earl Melancon and Dr. Allyse Ferrara for their continued support, kindness, and pool of knowledge. Special thanks is regarded for my major professor, Dr. Quenton Fontenot. He has been my mentor and friend throughout my graduate experience. The never ending guidance, wisdom, and patience of my graduate committee has kept me motivated in the pursuit of this degree. I would like to thank the Nicholls State University Department of Biological Sciences and the Nicholls State University Bayousphere Research Laboratory for the use of their vehicles, vessels, and equipment during this endeavor. I would also like to thank Mr. Joey Toups for donating the crab traps that were used in this study. Special thanks are held for my family and friends. None of this would have been possible without the constant love and push by my parents to do better for myself. They have always supported my decisions for further education and have made themselves available for whatever tasks that entailed. I thank my siblings and their spouses. They too were always willing to assist in this undertaking with physical labor and moral support. I cannot continue without recognition of my graduate professors and fellow graduate students. They are truly a wonderful group of peers whom every one of them has helped with the completion of this project. I only wish I could thank everyone by name. As for my office mates, Olivia Smith and Heather Dyer, I hold great appreciation. I could always count on these two women, no matter the situation. The greatest appreciation is held for my husband. He believed in me and my success when I no longer did. His love and encouragement was my drive and confidence to accomplish this goal. iv

6 TABLE OF CONTENTS Certificate..i Abstract ii Acknowledgements. iv Table of Contents. v List of Figures.vi List of Tables..ix List of Scientific Names...x Introduction..1 Methods.. 14 Results 19 Discussion..47 Recommendations..54 Literature Cited..55 Appendix I.6 Appendix II 76 Appendix III...83 Biographical Sketch...86 Curriculum Vitae...87 v

7 LIST OF FIGURES Figure 1. Location of the Barataria Estuary (gray area) in southeastern Louisiana... 2 Figure 2. Approximate salinity gradient within the Barataria Estuary based on data obtained from Braud et al. (26), LDWLF (25), and Jaworski (1972)...3 Figure 3. Geographic distribution of blue crab. Populations around Europe and Japan have been introduced and are not native to those areas.7 Figure 4. Sexually dimorphic characteristics of male and female blue crabs. Illustrated above is the abdominal apron of the male (a), immature female (b), and mature female (c) blue crab 9 Figure 5. Approximate inland most regions occupied by blue crabs in the Barataria Estuary for each stage of the blue crab life cycle..13 Figure 6. Bayou Chevreuil and Lac Des Allemands in the Barataria Estuary (earth.google.com). Location of seven fixed study sites..15 Figure 7. Modified commercial crab trap with closed escape rings.16 Figure 8. Mean (±SD) water temperature in Bayou Chevreuil for all sites combined for each sample date...21 Figure 9. Mean (±SD) water temperature for each site in Bayou Chevreuil for all sample dates combined from 11 July 26 to 6 December Figure 1. Mean (±SD) dissolved oxygen levels in Bayou Chevreuil for all sites combined for each sample date. The dashed line represents DO levels at 2. mg/l.23 Figure 11. Mean (±SD) overall dissolved oxygen for each site in Bayou Chevreuil for all sample dates combined from 11 July 26 to 6 December 26. Means with similar letters are not different...24 Figure 12. Mean (±SD) salinity in Bayou Chevreuil for all sites combined for each sample date.25 Figure 13. Mean (±SD) specific conductance in Bayou Chevreuil for all sites combined for each sample date..26 Figure 14. Mean (±SD) specific conductance for each site in Bayou Chevreuil for all sample dates combined from 11 July 26 to 6 December 26. Means with a similar letters are not different 27 vi

8 Figure 15. Size distribution based on carapace width of male and female blue crabs collected in Bayou Chevreuil from 11 July 26 to 6 December Figure 16. Percentage of male, mature female, and immature female blue crabs collected from Bayou Chevreuil on each sample date from 11 July 26 to 6 December Figure 17. Mean (±SD) width (mm), length (mm), and body weight (g) for male and female blue crabs collected in Bayou Chevreuil from 11 July 26 to 6 December 26. Means within each group that share a common letter are not different 31 Figure 18. Carapace length (a.) and width (b.) as a predictor of cheliped free weight for male and female blue crabs in Bayou Chevreuil. There is no difference between males and females based on length-weight relationship. Males weighed more than females of similar width (P <.1).32 Figure 19. Carapace width as a predictor of left (a.) and right (b.) cheliped weights for male female blue crabs in Bayou Chevreuil. Males had larger chelipeds than females of similar width (P <.1)...33 Figure 2. Mean (±SD) condition (K) of male and female blue crabs in Bayou Chevreuil from 11 July 26 to 6 December Figure 21. Mean (±SD) condition (K) of male and female blue crabs for all sites combined in Bayou Chevreuil for each sample date 35 Figure 22. Mean (±SD) CPUE for each site in Bayou Chevreuil for all sample dates combined from 11 July 26 to 6 December 26. Means with similar letters are not different...36 Figure 23. Mean (±SD) CPUE of blue crabs in Bayou Chevreuil for all sites combined and the mean (±SD) water temperature for all sites combined for each sample date. Critical temperature (15 C) is the water temperature that blue crabs have been documented to migrate down estuary for the winter months (Jaworski 1972)..37 Figure 24. Water temperature ( C) and blue crab CPUE at sites 1 7 in Bayou Chevreuil by sampling date...39 Figure 25. Mean (±SD) CPUE of blue crabs in Bayou Chevreuil for all sites combined and the mean (±SD) DO for all sites combined for each sample date.4 Figure 26. Mean (±SD) CPUE of blue crabs in Bayou Chevreuil for all sample vii

9 dates when DO > 2. mg/l was higher than mean CPUE of blue crabs in Bayou Chevreuil for all sample dates when DO 2. mg/l (P <.39) 41 Figure 27. Dissolved oxygen (mg/l) and blue crab CPUE at sites 1-7 in Bayou Chevreuil by sampling date...42 Figure 28. Mean (±SD) CPUE of blue crabs in Bayou Chevreuil for all sites combined and the mean (±SD) salinity for all sites combined for each sample date.43 Figure 29. Mean (±SD) CPUE of blue crabs in Bayou Chevreuil for all sites combined and the mean (±SD) specific conductance for all sites combined for each sample date...44 Figure 3. Specific conductance (µs) and blue crab CPUE at sites 1-7 in Bayou Chevreuil by sampling date...45 Figure 31. Mean (±SD) condition (K) of all blue crabs collected in Bayou Chevreuil and the mean (±SD) condition of all blue crabs collected in Fourchon/Grand Isle for each sample date. Circles group saltwater samples with the closest freshwater samples before and after each saltwater sample. Means with a similar letter in each group are not different (P <.5) 46 viii

10 LIST OF TABLES Table 1. Total number of species collected in Bayou Chevreuil from 11 July 26 to 6 December 26, using modified commercial crab traps. 2 ix

11 LIST OF SCIENTIFIC NAMES Bald cypress Tupelo gum Blue crab Lesser blue crab Black drum Red drum Atlantic croaker American eel Alligator gar Spotted gar Channel catfish Blue catfish Gizzard shad Chicken Spotted gar Bluegill White crappie Yellow bullhead Redear sunfish Bowfin Taxodium distichum Nyssa aquatica Callinectes sapidus Callinectes similis Pogonias cromis Sciaenops ocellatus Micropogonias undulatus Anguilla rostrata Lepisosteus spatula Lepisosteus oculatus Ictalurus punctatus Ictalurus furcatus Dorosoma cepedianum Gallus domesticus Lepisosteus oculatus Lepomis macrochirus Pomoxis annularis Ameiurus natalis Lepomis microlophis Amia calva x

12 INTRODUCTION Formed approximately 3,5 4, years ago, the Barataria Estuary is the most recently abandoned Mississippi River deltaic lobe (LDWLF 25, Barr and Hebrard 1976). Bordered by the Mississippi River on the east and Bayou Lafourche on the west, this interconnected hydrologic network extends inland from the Gulf of Mexico for 12 km (Swenson et al. 26; Jaworski 1972; Figure 1). The Barataria Estuary was historically connected to the Mississippi River by a series of distributaries and interdistributaries. The predictable annual spring floods of the Mississippi River would inundate low-lying areas within the Barataria Estuary with nutrient and sediment rich water. Many organisms within the Barataria Estuary have adapted to the historic high water levels associated with the annual Mississippi River spring floods for spawning and foraging. High water levels coupled with increasing temperature may be an important cue for many organisms to move onto the floodplain for spawning (Snedden et al. 1999; Sparks 1995). The Barataria Estuary is characterized by forested wetlands (11.7%), fresh marsh (1.2%), intermediate marsh (4.2%), brackish marsh (3.9%), saline marsh (7.2%), and open saline waters of the Gulf of Mexico along a continuous hydrologic and salinity gradient (Braud et al. 26; Figure 2). Approximately 42.5% of the Barataria Estuary is water (Braud et al. 26). Local flora and fauna of these regions have adapted to periodic flooding. The upper-most reaches of the Barataria Estuary (east of Lac Des Allemands) are approximately 41% forested wetlands (swamps), 38% agricultural lands, and include a number of bayous and canals (Braud et al. 26). Salinities in these waters rarely exceed 1. ppt. Dominated by alluvial clay soils, woody vegetation, and high levels of 1

13 Figure 1. Location of the Barataria Estuary (gray area) in southeastern Louisiana. 2

14 Figure 2. Approximate salinity gradient within the Barataria Estuary based on data obtained from Braud et al. (26), LDWLF (25), and Jaworski (1972). 3

15 primary production, the swamp forest connects with fresh marsh south of Lac Des Allemands (Barr and Hebrard 1976). Fresh marshes are characterized by salinities less than 2. ppt (LDWLF 25) and non-woody vegetation that is adapted to saturated soils. High levels of terrestrial primary production are because of fertile soils comprised of partially decomposed organic matter. The fresh marsh of the Barataria Estuary has greater wildlife diversity as compared to other marsh habitats (LDWLF 25). The fresh marsh connects with intermediate marsh south of Lake Salvador. Intermediate marsh is characterized by an irregular tidal and salinity regime with a salinity range of ppt (Braud et al. 26; LDWLF 25). The diversity of species in intermediate marsh derives from an overlap of organisms common to surrounding fresh and brackish marshes. Brackish marshes connect intermediate marsh with saline environments and are the inland most units that are strongly influenced by tidal actions (Barr and Hebrard 1976). Salinities in brackish marsh range from ppt (Swenson and Turner 1998; Jaworski 1972). The brackish marsh of the Barataria Estuary exhibits high biodiversity, especially in larval forms of marine organisms. The saline marsh of the Barataria Estuary extends inland from the Gulf of Mexico about 3 km and is the habitat most influenced by diurnal tidal variation (Jones et al. 22). Soils are predominately sand and yield the lowest number of plant species; however, saline marshes are primary nursery grounds for numerous marine organisms (Heck et al. 21). Saline marshes end where the open ocean begins and can have salinities up to 3 ppt, whereas the coastal waters of the Gulf of Mexico can reach 4 ppt (Jaworski 1972). Weather fronts and storms affect the flux of salinity in the Barataria system as heavy rainfall pushes the salinity gradient southward and periods of drought drive the salinity gradient northward 4

16 (Melancon et al. 1998; Swenson and Turner 1998). The relative area of land mass of the Barataria system decreases from the upper estuary to the open waters of the Gulf of Mexico, as the salinity increases. Bayou Lafourche and other distributaries were cut off from the Mississippi River and the annual Mississippi River flood pulse in order to prevent flooding in the Barataria Estuary. Without the connection to the annual spring floods of the Mississippi River, freshwater input into the estuary is primarily through local precipitation. Yet, with the escalating problem of coastal erosion and saltwater intrusion, Louisiana authorities have implemented several freshwater diversion projects, one of which is located in the Barataria Estuary. The Davis Pond freshwater diversion structure was designed to convey water through the Mississippi River s west containment levee (Swenson et al. 26). Nutrient and sediment rich Mississippi River water is diverted through a holding pond, which drains into Lake Cataouache, and then south to the Gulf of Mexico through the mid and lower Barataria Estuary (Swenson et al. 26). The main open water body in the upper Barataria Estuary is Lac Des Allemands. This 486 ha flat-bottom lake lies west of New Orleans, Louisiana. With its southeasterly flow, Bayou Chevreuil weaves across the upper Barataria Estuary and empties into Lac Des Allemands. Ninety percent of the low-lying cypress-tupelo (Taxodium distichum, Nyssa aquatica) swamps that surround Bayou Chevreuil drain directly into the bayou during rains and high water periods (Day et al. 1976) carrying along leaf litter and other organic materials that serve as an important food source for many aquatic organisms, including blue crab Callinectes sapidus. 5

17 The blue crab is a member of the decapod family Portunidae, the swimming crabs, which contain 3 extant species (Guillory et al. 21). Swimming crabs are identified by their most posterior pair of walking legs that have evolved to form swimming paddles for better mobility throughout the water column. Blue crab is one of only two species of swimming crabs found in Louisiana and is identified by four carapace ridges between the eyes. The other swimming crab C. similis has six carapace ridges between the eyes (Jaworski 1972). Blue crabs are blue to gray in color with a somewhat convex carapace that is approximately 2.5 times wider than it is long (Meinkoth 1981). Blue crabs are classified as detritivores, omnivores, and cannibals; eating everything from decaying fish flesh, to clams and submerged aquatic vegetation, and even smaller individuals of their own kind, making them an important organism for nutrient cycling and energy transfer within an ecosystem (Fitz and Weigert 1991; Laughlin 1982; Darnell 1961). Blue crabs have a large geographic distribution. Blue crabs are abundant throughout the Gulf of Mexico and along the Atlantic coast as far north as Nova Scotia and south as far as northern Argentina (Van Engel 1958). Blue crabs have also been introduced into coastal waters of Europe, the Mediterranean, and Japan (Van Engel 1958; Figure 3). Blue crabs support a large commercial and recreational fishery and are an economically important organism within the Barataria Estuary (Guillory et al. 21). A study by Tagatz (1969) indicated that blue crabs can increase their tolerance to temperature changes with an increased acclimation period. Tagatz (1969) tested blue crabs at C with different acclimation times ranging from 3 21 days, and found that blue crab survival increased with increased acclimation time. However, significant 6

18 Figure 3. Geographic distribution of blue crab. Populations around Europe and Japan have been introduced and are not native to those areas. 7

19 mortality occurs if blue crabs are exposed to extended periods ( 15 days) of water temperatures below 3 C (Rome et al. 25). Blue crabs have very little tolerance for low temperatures and practice autotomy (sacrificing limbs for survival) at water temperatures below 5 C to conserve energy (Rome et al. 25). Most blue crab activity occurs from late spring to early fall, but remain dormant and buried in the marsh sediment during the winter months. Blue crabs vacate lower saline waters of the upper estuary when temperatures drop to 15 C to seek out warmer waters near the coast (Jaworski 1972). With smaller size classes (carapace width (CW) 3 mm) caught in the winter (15.7 ±.19 C) and the largest size class (CW 1 mm) caught in the summer (3.1 ±.13 C), Jones et al. (22) have suggested that juveniles are more tolerant to low temperatures than are adults. Blue crabs have sexually dimorphic external physical characteristics (Figure 4). Mature females have a wide and rounded abdominal apron, while the abdominal apron of immature females is less rounded and is more triangular in shape (Guillory et al. 21; Jaworski 1972; Tagatz 1968; Van Engel 1958). Females also have bright red coloration on their chelae, or claws. Mature and immature males have the same shape abdominal aprons, which makes it difficult to determine maturity. Male abdominal aprons are very slender and tower shaped, and mature males have blue chelae (Guillory et al. 21; Jaworski 1972; Tagatz 1968; Van Engel 1958). Immature males will often have a slight touch of red coloration on the tips of the chelae; however, this is not always a reliable determination for maturity. Blue crabs undergo several morphological changes and use different areas of estuarine systems during their life expectancy of 2-4 years. Blue crabs are characterized 8

20 Figure 4. Sexually dimorphic characteristics of male and female blue crabs. Illustrated above is the abdominal apron of the male (a), immature female (b), and mature female (c) blue crab. 9

21 by discontinuous growth that only occurs during ecdysis (molting; Miller and Smith 23). The blue crab sheds its exoskeleton during ecdysis to make room for somatic growth and then takes in water to puff up the new shell before it hardens. Juveniles can increase approximately 14 mm in carapace width per month, and adults can increase 15-2 mm in carapace width per month (Adkins 1972). However, adults molt less frequently than juveniles due to the greater amount of energy needed by adults for ecdysis. Blue crabs are often arranged in the following size classes: sub-juveniles (zoeae and megalops); less than 2 mm CW: juveniles; 2-8 mm CW: and adults; greater than 8 mm CW (McClintock et al. 1993; Fitz and Wiegert 1991). Male blue crabs molt throughout their lifetime, but females experience a limited number of molts. The final molt of the female blue crab is called the pubertal molt. Female blue crabs become sexually mature during their pubertal molt (Tagatz 1968; Van Engel 1958; Churchill 1919). Prior to the pubertal molt, the female blue crab releases pheromones into the water column to attract a mate (Gleeson 198). The pheromones signal that the female has reached maturity, is about to enter her final molt, and is ready to mate. The attracted male blue crab stays with the female until she molts, at which time mating occurs during the female s soft-shell state (Van Engel 1987). The male continues to guard the female until her shell has hardened and she is less vulnerable to predation. Because the female only mates during her pubertal molt, the spermatozoa from the single mating are held for multiple spawnings (Van Engel 1958). Blue crab mating occurs in the brackish waters of an estuary. After mating, females migrate down estuary to spawn in higher salinities (Hines 23; McClintock 1993; Van Engel 1987). A gravid female bearing eggs is commonly referred to as a 1

22 sponge crab. Her first spawning occurs within 2-6 months after mating and she produces approximately 2 million eggs per sponge (Churchill 1919; Guillory et al. 21). The female blue crab carries the eggs for about two weeks and then releases the larvae in the warm, more saline coastal waters. Water temperatures greater than 19 C and salinity greater than 2 ppt are optimal for spawning (Sulkin et al. 1976). Blue crab larvae go through two stages of development. The first stage is the zoeal stage (Heck et al. 21). Starting at.25 mm in width, zoeae have very little physical resemblance to the adult blue crab, and either stay in the coastal bays or move into the open ocean for further growth and development. They are planktonic consumers that remain in surface waters for feeding. Like all other stages of the blue crab life cycle, zoeal growth only occurs during molting. Zoeae undergo 4-7 molts over a 3-5 day period (Van Engel 1958). The final molt of the zoeal stage occurs when the zoea are approximately 1. mm wide and transforms from the zoeae into the megalops stage. Megalopae stay in the nearshore, mesohaline estuarine waters for development, where they swim freely about feeding near the water column bottom (Tagatz 1968). After 6-2 days and one transforming molt, megalopae enter the juvenile stage (Van Engel 1958). Early juveniles are approximately 2.5 mm CW and physically resemble adult blue crabs. Juvenile blue crabs migrate up estuary into lower saline and even fresh waters of the system, where they continue to grow and mature through a number of molts. Juveniles reach maturity in the lower salinity estuarine waters after 18-2 post-larval molts at a carapace width greater than 1 mm (Miller and Smith 23). However, some juveniles remain in the lower estuary. Maturity is reached months after larval release, at which time mating can occur and the females will then migrate down estuary 11

23 to saline waters for spawning (McClintock et al. 1993). The entire blue crab life cycle can occur in one estuarine system (Figure 5). Blue crabs are prey for many organism in saltwater and freshwater habitats including black drum Pogonias cromis, red drum Sciaenops ocellatus, Atlantic croaker Micropogonias undulatus, American eel Anguilla rostrata, alligator gar Lepisosteus spatula, spotted gar Lepisosteus oculatus, channel catfish Ictalurus punctatus, and blue catfish Ictalurus furcatus (Darnell 1961). In both environments, blue crabs serve as a connection delivering energy and nutrients from terrestrial sources to aquatic organisms through the consumption of detrital material. Because blue crabs can be found in fresh water areas of the upper Barataria Estuary and are an important commercial and ecological species, the goal of this project was to determine the relative abundance and distribution of blue crabs in the upper Barataria Estuary, and to describe the size structure of that population. Water quality parameters were measured to determine the relationship between water quality and the abundance and distribution of the blue crab population. Finally, the blue crab population collected from the upper Barataria Estuary was compared to a population collected from saline waters to determine any sex-based segregation and differences in condition of the organisms between the two populations. 12

24 13 Figure 5. Approximate inland most regions occupied by blue crabs in the Barataria Estuary for each stage of the blue crab life cycle.

25 METHODS Field Data Collection Blue crabs were sampled weekly from 11 July 26 through 6 December 26, from six fixed sites in Bayou Chevreuil and one fixed site in Lac Des Allemands (Figure 6). Although blue crabs were sampled in Lac Des Allemands, the entire sample population was designated as the Bayou Chevreuil population. Surface and bottom water temperature ( C), dissolved oxygen (DO; mg/l), salinity (ppt), and specific conductance (us) was measured with a hand-held oxygen-conductivity-salinity-temperature meter at each site for each sample date (Yellow Springs Instruments, Yellow Springs, Ohio). The mean value for surface and bottom measurements taken for each sample was used for analysis. Blue crabs were sampled weekly with modified commercial crab traps (6.9 cm x 6.9 cm x 43.2 cm). Each trap was constructed of vinyl-coated 3.8 cm mesh wire and two escape rings (5.9 cm inner diameter), which were closed with plastic zip-ties to prevent escapement of smaller ( 127 mm CW) individuals (Figure 7). A polystyrene buoy, painted red with black lettering for identification, was attached to each trap by 3.66 m of rope. Traps were baited either with gizzard shad Dorosoma cepedianum or chicken Gallus domesticus pieces. At each of the six sites in Bayou Chevreuil, two traps were set on each side of the channel, just close enough to the bank to not obstruct boat traffic. Traps remained deployed for approximately 24 hours. Blue crabs were harvested on the following day, and the number of blue crabs caught per trap per site was recorded. Once the blue crabs were removed from the trap and enumerated, they were pooled together by 14

26 Figure 6. Bayou Chevreuil and Lac Des Allemands in the Barataria Estuary (earth.google.com). Seven fixed study sites were located by GPS coordinates. Site 1: N, W; Site 2: N, W; Site 3: N, W; Site 4: N, W; Site 5: N, W; Site 6: N, W; Site 7: N, W. 15

27 Figure 7. Modified commercial crab trap with closed escape rings. 16

28 site and placed in an ice bath for transport to the Bayousphere Research Laboratory at Nicholls State University. Site specific blue crab catch per unit effort (CPUE) was determined as the mean number of crabs collected per trap per day. Laboratory Data Collection Each crab was sexed and females were designated as mature or immature based on the shape of the abdomen apron (Figure 4). Carapace width (mm) was determined as the distance between the two outermost lateral spines. Carapace length (mm) was determined as the distance from the anterior of the carapace to the posterior of the carapace centered between the two outermost lateral spines. Total weight (g) of each crab was taken before detaching the chelipeds. Then the weight (g) of both the left and right cheliped and the cheliped-free body weight (g) was measured separately. Saltwater Comparison Blue crabs were sampled from Fourchon and Grand Isle, Louisiana, then sexed, measured, and weighed with the same methods as used for the Bayou Chevreuil population on three separate sampling events. These sampling dates included one 24- hour duration on 18 July 26, 19 August 26, and 17 November 26. Condition of the saltwater population was compared to the Bayou Chevreuil population from the closest dates of Bayou Chevreuil sampling before and after each saltwater sampling. Statistical Analysis Water quality was assessed temporally and spatially. First, mean water quality values for all sites combined for each sample date were calculated to describe water quality over time. Second, site specific mean values were calculated for all sample dates combined to compare water quality among sites using analysis of variance. 17

29 The relationship between blue crab CPUE for all sites combined for each sample date and mean temperature, DO, salinity, and specific conductance for all sites combined for each sample date in Bayou Chevreuil was determined with regression analysis. Analysis of variance was used to compare the mean length, width, and chelipedfree body weight for male and female blue crabs. Regression analysis was used to determine sex-specific width-weight and length-weight relationships, and analysis of covariance was used to compare growth between male and female blue crabs in Bayou Chevreuil. Analysis of covariance was used to compare cheliped weight between male and female blue crabs collected from Bayou Chevreuil. Because blue crabs can regenerate lost chelipeds, we did not include chelipeds that were obviously being regenerated (based on size) when we compared cheliped size between males and females with regression analysis. Condition for the freshwater (Bayou Chevreuil) and saltwater (Fourchon/Grand Isle) blue crabs were calculated using the cheliped-free body weight and carapace length as: Weight/Length 3 x 1 Analysis of variance was used to compare differences in condition indices of the freshwater and saltwater blue crabs. All inferences were made based on alpha =.5 and all analyses were performed on log-transformed data. 18

30 RESULTS Field Data A total of 649 blue crabs were collected from 11 July 26 to 7 November 26, in Bayou Chevreuil. In addition to blue crab, 6 fish species were collected in the crab traps in Bayou Chevreuil (Table 1). Individual water temperature measurements ranged from C and averaged 25.7 ± 6.8 C. Mean water temperature for all sites combined declined as the sampling period progressed (Figure 8). Mean water temperature did not vary among sample sites (Figure 9). Individual DO measurements ranged from mg/l and averaged 3.6 ± 1.21 mg/l. There was no trend in DO levels among the sampling dates for all sites combined in Bayou Chevreuil (Figure 1). Mean DO was greatest at site 1 compared to all other sites for all sample dates combined (Figure 11). Individual salinity measurements ranged from.1-1. ppt and averaged.3 ±.2 ppt. With the exception of 11 October 26 and 18 October 26, salinity remained fairly constant among sample dates (Figure 12). Individual specific conductance measurements ranged from µs and averaged 63.1 ± µs. With the exception of 11 October 26 and 18 October 26, specific conductance remained fairly constant throughout the sampling period (Figure 13). Mean specific conductance for all sites combined was greatest at site 1 (Figure 14). A total of 275 blue crabs were collected from saltwater areas on the sampling dates of 18 July 26, 19 August 26, and 17 November 26, in Fourchon and Grand Isle. Water temperature for the three sample dates averaged 27.4 ± 6.8 C and salinity averaged 28.3 ± 2.6 ppt. Dissolved oxygen was not measured at the saltwater sites. 19

31 Table 1. Total number of individuals of each species collected in Bayou Chevreuil from 11 July 26 to 6 December 26, using modified commercial crab traps. Species Common Name Number Callinectes sapidus Blue Crab 649 Lepisosteus oculatus Spotted Gar 1 Lepomis macrochirus Bluegill 1 Pomoxis annularis White Crappie 5 Ameiurus natalis Yellow Bullhead 4 Lepomis microlophis Redear Sunfish 4 Amia calva Bowfin 1 Total 683 2

32 Mean Tempe rat ure ( C) Jul 3-Jul 19-Aug 8-Sep 28-Sep 18-Oct 7-Nov 27-Nov Date Figure 8. Mean (±SD) water temperature in Bayou Chevreuil for all sites combined for each sample date. 21

33 35 Temperature ( C) Site Figure 9. Mean (±SD) water temperature for each site in Bayou Chevreuil for all sample dates combined from 11 July 26 to 6 December

34 DO ( mg/l ) Jul 3-Jul 19-Aug 8-Sep 28-Sep 18-Oct 7-Nov 27-Nov Date Figure 1. Mean (±SD) dissolved oxygen levels in Bayou Chevreuil for all sites combined for each sample date. The dashed line represents DO levels at 2. mg/l. 23

35 8 Mean DO (mg/l) a b b b b b b Site Figure 11. Mean (±SD) overall dissolved oxygen for each site in Bayou Chevreuil for all sample dates combined from 11 July 26 to 6 December 26. Means with similar letters are not different. 24

36 Salinity (pp t ) Jul 3-Jul 19-Aug 8-Sep 28-Sep 18-Oct 7-Nov 27-Nov Date Figure 12. Mean (±SD) salinity in Bayou Chevreuil for all sites combined for each sample date. 25

37 Mean Conductance (us) Jul 3-Jul 19-Aug 8-Sep 28-Sep 18-Oct 7-Nov 27-Nov Date Figure 13. Mean (±SD) specific conductance in Bayou Chevreuil for all sites combined for each sample date. 26

38 2 Mean Condu ctanc e (u S ) a b b b b b b Site Figure 14. Mean (±SD) specific conductance for each site in Bayou Chevreuil for all sample dates combined from 11 July 26 to 6 December 26. Means with a similar letters are not different. 27

39 Data Collection Bayou Chevreuil blue crabs ranged from 8-29 mm in carapace width (Figure 15). More males (N=591) were collected than females (mature N = 34; immature N = 24) in Bayou Chevreuil (Figure 16). The overall sex ratio of males to females (mature and immature combined) was 1.2:1. Basing all analyses on log-transformed data, there was no difference in mean length or mean cheliped-free body weight between male and female blue crabs (Figure 17). However, mean width for females was greater than mean width for male blue crabs (Figure 17). Length and width were accurate measurements for predicting cheliped-free weights of both male and female blue crabs in Bayou Chevreuil (Figure 18). Based on the length-weight relationship, there was no difference in weight for similar sized male and female blue crabs. Based on the width-weight relationship, males weighed more than similar sized females (P <.1; Figure 18). Males also had larger chelipeds than females based on width-cheliped weight relationships (P <.1; Figure 19). There was no difference in mean condition between male and female blue crabs (Figure 2). There was also no apparent trend in condition index throughout the study period for Bayou Chevreuil blue crabs (Figure 21). Overall, blue crab CPUE was greatest in July and August and decreased from September through November. Blue crab CPUE was greater at downstream sites than at upstream sites (Figure 22). Blue crab CPUE was positively correlated (P =.9) to temperature, with a peak in July and August and a steady decline through the cooler autumn months (Figure 23). Blue crabs were collected from site 1 on more sampling dates (N = 13) than from the other sites and were collected the fewest times (N = 3) from 28

40 12 1 Male Female Frequency Width (mm) Figure 15. Size distribution based on carapace width of male and female blue crabs collected in Bayou Chevreuil from 11 July 26 to 6 December

41 Male Female Immature Female 1% 8% Percent 6% 4% 2% % 1-Jul 3-Jul 19-Aug 8-Sep 28-Sep 18-Oct 7-Nov Date Figure 16. Percentage of male, mature female, and immature female blue crabs collected from Bayou Chevreuil on each sample date from 11 July 26 to 6 December 26. 3

42 Male Female b a mm or g a a a a 3 Width Length Body wt Figure 17. Mean (±SD) width (mm), length (mm), and body weight (g) for male and female blue crabs collected in Bayou Chevreuil from 11 July 26 to 6 December 26. Means within each group that share a common letter are not different. 31

43 Male Female 6 Male R 2 =.923 Female R 2 = ln Weight ln Length Male Female 6 Male R 2 =.9284 Female R 2 = ln Weight ln Width Figure 18. Carapace length (a.) and width (b.) as a predictor of cheliped free weight for male and female blue crabs in Bayou Chevreuil. There is no difference between males and females based on length-weight relationship. Males weighed more than females of similar width (P <.1). 32

44 Male Female 4.5 Male R 2 =.7987 Female R 2 = ln Left Cheliped WT ln Length Male Female 4.5 Male R 2 =.795 Female R 2 =.7546 ln Right Cheliped WT ln Length Figure 19. Carapace width as a predictor of left (a.) and right (b.) cheliped weights for male female blue crabs in Bayou Chevreuil. Males had larger chelipeds than females of similar width (P <.1). 33

45 Mean Condition Male Female Figure 2. Mean (±SD) condition of male and female blue crabs in Bayou Chevreuil from 11 July 26 to 6 December

46 Mean Male K Mean Female K.55 Mean Condition (K) Jul 22-Jul 11-Aug 31-Aug 2-Sep 1-Oct 3-Oct 19-Nov Date Figure 21. Mean (±SD) condition (K) of male and female blue crabs for all sites combined in Bayou Chevreuil for each sample date. 35

47 6 5 Mean CPUE a ab abc Site abc abc bc c Figure 22. Mean (±SD) CPUE for each site in Bayou Chevreuil for all sample dates combined from 11 July 26 to 6 December 26. Means with similar letters are not different. 36

48 CPUE Temp Temp = 15 C 6 35 Mean CPUE MeanTemperature ( C) 1-Jul 3-Jul 19-Aug 8-Sep 28-Sep 18-Oct 7-Nov 27-Nov 1 Date Figure 23. Mean (±SD) CPUE of blue crabs in Bayou Chevreuil for all sites combined and the mean (±SD) water temperature for all sites combined for each sample date. Critical temperature (15 C) is the water temperature that blue crabs have been documented to migrate down estuary for the winter months (Jaworski 1972). 37

49 site 7 (Figure 24). There is weak positive correlation (P =.5) between blue crab CPUE and DO in Bayou Chevreuil (Figure 25). Blue crab CPUE was greater for samples taken when DO > 2. mg/l than for samples taken when DO 2. mg/l (P <.39; Figure 26). Blue crab CPUE was affected by the increase in DO at the upstream sites in October. Site 1 never had a recorded DO 2. mg/l and was the site with the greatest number of blue crabs caught, while all other sites experienced several hypoxic events (Figure 27). Salinity never exceeded 1. ppt and was not related to blue crab CPUE in Bayou Chevreuil (Figure 28). There was no detectable relationship between specific conductance and blue crab CPUE (Figure 29). Specific conductance fluctuated most at the downstream sites (sites 1-4) and except for one event, did not vary much at the upstream sites (Figure 3). Upstream blue crab CPUE was affected by the October increase in specific conductance (Figure 3). Saltwater Comparison More females (N=159) were collected than males (N=116) in Fourchon and Grand Isle. The overall sex ratio of males to females was 1:1.4. Blue crabs sampled from Fourchon and Grand Isle had an approximate 12% higher condition than those sampled from Bayou Chevreuil during July and August (Figure 31). However, there was no difference between the condition of the freshwater and saltwater blue crabs for the November sampling period (Figure 31). 38

50 Site 1 Temp Temp = 15 CPUE Site 2 Temp Temp = 15 CPUE 4 7 Te mperature ( C) CPUE Te mpe rature ( C) CPUE 1 -Jul 3 -Jul 19 -Aug 8- Sep 28 -Sep 18 -Oct 7- Nov 27 -Nov 19-A 8-Se 1-Ju l 3-Ju l ug p 28-Se p 18-O ct 7-No v 27-Nov Date Date Site 3 Temp Temp = 15 CPUE Site 4 Temp Temp = 15 CPUE Temper atu re ( C) Jul 3 -Jul 1 9-Aug 8-Sep 28-Sep 18-Oct 7-Nov 27-Nov CP U E Temperat ur e ( C) Jul 3-Jul 19-Aug 8-Sep 28-Sep 18-Oct 7-Nov 27-Nov CPUE Date Date Site 5 Temp Temp = 15 CPUE Site 6 Temp Temp = 15 CPUE Temp erature ( C) Jul 3-J 19-A 8-Se 28-S 7-N CPUE mperature ( C) Te ul ug p ep Oct ov Nov 1-Jul 3 -Jul 19-Aug 8 -Sep 2 8-Sep 1 8-Oct 7-Nov 27-Nov CPUE Date Date Site 7 Temp Temp = 15 CPUE Tempe rat ure ( C) Jul 3-Jul 19-Aug 8-Sep 28-Sep 18-Oct 7-Nov 27-Nov CPU E Date Figure 24. Water temperature ( C) and blue crab CPUE at sites 1 7 in Bayou Chevreuil by sampling date. 39

51 CPUE DO DO = 2. mg/l Mean CPUE Mean DO (mg/l) Jul 3-Jul 19-Aug 8-Sep 28-Sep 18-Oct 7-Nov 27-Nov Date Figure 25. Mean (±SD) CPUE of blue crabs in Bayou Chevreuil for all sites combined and the mean (±SD) DO for all sites combined for each sample date. 4

52 Mean CPUE a 1 b.5 DO > 2. mg/l DO = 2. mg/l Figure 26. Mean (±SD) CPUE of blue crabs in Bayou Chevreuil for all sample dates when DO > 2. mg/l was higher than mean CPUE of blue crabs in Bayou Chevreuil for all sample dates when DO 2. mg/l (P <.39). 41

53 Site 1 DO DO = 2. mg/l CPUE Site 2 DO DO = 2. mg/l CPUE DO (mg/l) Jul 3-Jul 19-Aug 8-Sep 28-Sep 18-Oct 7-Nov 27-Nov CPUE DO (mg/l) Oct 7-Nov 27-Nov CPUE 1-Jul 3-Jul 19-Aug 8-Sep 28-Sep Date Date Site 3 DO DO = 2. mg/l CPUE Site 4 DO DO = 2. mg/l CPUE DO (mg/l) Jul 3-Jul 19-Aug 8-Sep 28-Sep 18-Oct 7-Nov 27-Nov CPUE DO (mg/l) Jul 3-Jul 19-Aug 8-Sep 28-Sep 18-Oct 7-Nov 27-Nov CPUE Date Date Site 5 DO DO = 2. mg/l CPUE Site 6 DO DO = 2. mg/l CPUE DO (mg/l) Jul 3-Jul 19-Aug 8-Sep 28-Sep 18-Oct 7-Nov 27-Nov CPUE DO (mg/l) Jul 3-Jul 19-Aug 8-Sep 28-Sep 18-Oct 7-Nov 27-Nov CPUE Date Date Site 7 DO DO = 2. mg/l CPUE DO (mg/l) CPUE 1-Jul 3-Jul 19-Aug 8-Sep 28-Sep 18-Oct 7-Nov 27-Nov Date Figure 27. Dissolved oxygen (mg/l) and blue crab CPUE at sites 1-7 in Bayou Chevreuil by sampling date. 42

54 CPUE salinity 7 1. Mean CPUE Mean Salinity (ppt) 1-Jul 3-Jul 19-Aug 8-Sep 28-Sep 18-Oct 7-Nov 27-Nov. Date Figure 28. Mean (±SD) CPUE of blue crabs in Bayou Chevreuil for all sites combined and the mean (±SD) salinity for all sites combined for each sample date. 43

55 CPUE COND Mean CPUE Jul 3-Jul 19-Aug 8-Sep 28-Sep 18-Oct 7-Nov 27-Nov Mean Conductance (us) Date Figure 29. Mean (±SD) CPUE of blue crabs in Bayou Chevreuil for all sites combined and the mean (±SD) specific conductance for all sites combined for each sample date. 44

56 ov Site 1 COND CPUE Site 2 COND CPUE 25 Specific Conductance (us) Jul 3-Jul 19-Aug 8-Sep 28-Sep 18-Oct 7-Nov 27-Nov CPUE Specific Conductance (us) Jul 3-Jul 19-Aug 8-Sep 28-Sep 18-Oct 7-Nov 27-Nov CPUE Date Date Site 3 COND CPUE Site 4 COND CPUE 25 Specific Conductance (us) Jul 3-Jul 19-Aug 8-Sep 28-Sep 18-Oct 7-Nov 27-Nov CPUE Specific Conductance (us) Jul 3-Jul 19-Aug 8-Sep 28-Sep 18-Oct 7-Nov CPUE 27-N Date Date Site 5 COND CPUE Site 6 COND CPUE Specific Conductance (us) Jul 3-Jul 19-Aug 8-Sep 28-Sep 18-Oct 7-Nov 27-Nov CPUE (us) Specific Conductance Jul 3-Jul 19-Aug 8-Sep 28-Sep 18-Oct 7-Nov 27-Nov CPUE Date Date Site 7 COND CPUE Specific Conductance (us) Jul 3-Jul 19-Aug 8-Sep 28-Sep 18-Oct 7-Nov 27-Nov CPUE Date Figure 3. Specific conductance (µs) and blue crab CPUE at sites 1-7 in Bayou Chevreuil by sampling date. 45

57 .6 Freshwater K Saltwater K a Mean Condition.5.4 b b b a b a a.3 1-Jul 3-Jul 19-Aug 8-Sep 28-Sep 18-Oct 7-Nov Date Figure 31. Mean (±SD) condition (K) of all blue crabs collected in Bayou Chevreuil and the mean (±SD) condition of all blue crabs collected in Fourchon/Grand Isle for each sample date. Circles group saltwater samples with the closest freshwater samples before and after each saltwater sample. Means with a similar letter in each group are not different (P <.5). 46

58 DISCUSSION The annual floodpulse of unregulated large river systems is predictable, but varies among river systems according to local precipitation and discharge. During times of high discharge, well confined channels within large river floodplains overflow onto their floodplains (Junk et al. 1989). The occasional inundation of the Bayou Chevreuil floodplain is directly related to local precipitation and is highly unpredictable (Day et al. 1976; Davis 26). Vegetation and faunal organisms on the floodplain floor that are not adapted to high waters perish during times of flooding and contribute large amounts of decaying organic matter to the ecosystem (Vannote et al. 198). The floodplain provides a detrital food supply, spawning ground, and shelter for many organisms living in the main channel that have evolved to use periodically available floodplain habitats (Junk et al. 1989). Bowfin Amia calva are dependent on the Bayou Chevreuil floodplain for spawning habitat. In 25-26, bowfin in Bayou Chevreuil had a weak spawning event due to unusually low water during the spawning period (Davis 26). Water levels remained low in Bayou Chevreuil, never inundating the surrounding floodplain, and the bowfin population was unable to reach its preferred spawning ground. Although, blue crabs have not been documented leaving the main channel and entering the floodplain during high water, blue crabs may benefit from floodplain detrital food sources and large woody debris shelters during ecdysis. Blue crabs may be seasonally abundant in Bayou Chevreuil as they migrate up estuary during development (Van Engel 1987). Smaller megalops and juveniles are found in the more saline waters of the coast and adults are found throughout the estuary as far inland as the swamps. The size distribution of blue crabs in Bayou Chevreuil 47

59 ranged from 8 21 mm CW. Blue crabs less than 8 mm CW may be present in Bayou Chevreuil but were not collected possibly because the gear used in this study was size limiting and the mesh size of the traps allowed for the escapement of smaller individuals (< 8 mm CW). Fewer numbers of large ( 18 mm CW) blue crabs may be due to the large blue crab commercial fishery in the Barataria Estuary. Larger individuals are more susceptible to being harvested in commercial crab traps and blue crab fisherman run traps throughout the entire Barataria Estuary. Therefore, this research cannot be considered a fishery-independent study. Based on length-weight relationships, similar sized males and females have similar weights, but based on width-weight relationships, males are heavier than similar sized females. Olmi and Bishop (1983) found that intermolt males were heavier than intermolt females of similar size. Millikin et al. (198) found that both male and female juveniles from the same gravid female attained similar width and weight at the same rate, suggesting that only after maturity do males reach greater sizes than females. Cessation of ecdysis by female blue crabs prohibits females from reaching the same maximum size as male blue crabs. The greatest differences in weight between male and female blue crabs collected for this study occurred in the larger individuals. For example, when comparing blue crabs less than 127 mm CW, males had a mean cheliped-free body weight of 67.9 ± 14.3 g whereas females had a mean cheliped-free body weight of 69.2 ± 8. g; a difference of 1.3 g. When comparing blue crabs of 17 mm CW or greater, males had a mean cheliped-free body weight of ± 26.5 whereas females had a mean cheliped-free body weight of ± 19.3 g; a difference of 35.7 g. Similar to other crustaceans, male blue crabs have larger chelipeds than similar size females. This may 48

60 indicate resource competition among males, or the allocation of energy for cheliped growth rather than egg development. Size at maturity varies within blue crab populations. The largest immature female collected by Tagatz (1968) had a carapace width of 177 mm and the smallest mature female had a carapace width of 99 mm. The largest immature female found in this study was 159 mm CW whereas the smallest mature female was 117 mm CW (Appendices I and II). There was no difference in condition factor between male and female blue crabs collected from Bayou Chevreuil. Atar and Secer (23) also found condition factor of male and female blue crabs to be similar. Blue crabs collected from saltwater areas for this study had a higher condition than blue crabs collected in freshwater during July and August. This may indicate that blue crabs in freshwater expend more energy for osmoregulation resulting in lower somatic growth rates than blue crabs from saline water. Water quality appears to affect the distribution and abundance of the Bayou Chevreuil blue crab population. Blue crab larvae are released in early spring in lower estuarine waters. Larvae develop and migrate up estuary. During warm summer months there is a gradual increase in size towards larger juveniles and adults as the total population abundance decreases (Fitz and Wiegert 1992). Blue crabs do not acclimate well to low temperatures with tolerance to low temperatures further reduced at low salinities (Rome et al. 25), such as the low salinities found in Bayou Chevreuil. The blue crab CPUE was correlated to temperature with the highest levels of CPUE in July and August and a steady decline in CPUE towards the winter months. Surviving blue crabs seek out warmer waters of the lower estuary to overwinter at which time somatic growth is halted (Miller and Smith 23). Once water temperatures dropped to 15 C in 49