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Trophic Ecology of Juvenile Weakfish (cynoscion Regalis) in The Delaware Bay Using Stomach Content and Stable Isotope Analyses

Show simple item record Galvez, Brian 2019-08-28T13:12:32Z 2019-08-28T13:12:32Z
dc.description.abstract The Weakfish (Cynoscion regalis) is a commercially and recreationally valuable fish species that inhabits the Atlantic Coast of North America, where it primarily occurs from North Carolina to New York. The Weakfish fishery is depleted and has not rebounded despite fishing mortality decreases since 2011. The failure of the fishery to recover is currently attributed to elevated rates of natural mortality, the causes of which are not fully understood. Natural mortality is known to occur at higher rates in juvenile fish compared to adults. Because the diet of juvenile fishes is a factor of natural mortality and recruitment success, I studied the foraging habits of juvenile Weakfish in one of their primary spawning and nursery habitats, the Delaware Bay, in order to increase and update the understanding of their feeding habits prior to their emigration to offshore overwintering grounds. To answer questions regarding sources of organic matter, prey availability, prey importance, and prey origin (marsh or pelagic derived), I applied stomach content and stable isotope analyses to juvenile Weakfish in 2017 and 2018 throughout most of their Delaware Bay residency (July through October), along both coasts of the Delaware Bay (Delaware and New Jersey nearshore areas), and along the salinity gradient (lower, middle, and upper bay). These methods were used to elucidate the mechanisms and factors controlling stable isotope values of juvenile Weakfish and to determine the prey species responsible for their tissue production and growth. Juvenile Weakfish were caught from nearshore sampling stations sampled by Delaware and New Jersey Fish and Wildlife state agencies. The fish were separated into three size classes that represent a known ontogenetic shift in diet. Small Weakfish (0-60 mm SL) are mysid specialists, medium Weakfish (61-100 mm SL) begin to incorporate more forage fish into their diet, and large Weakfish (101-137 mm SL) are known to eat roughly equal proportions of forage fish and crustaceans by weight. Commonly consumed prey items, including mysid shrimp, gammarid amphipods, polychaete worms, isopods, sand shrimp, and Bay Anchovy, were also collected in 2018 for stable isotope analysis use in a Bayesian mixing model to infer the proportional contributions of individual prey items to juvenile Weakfish productivity. Generalized linear models (GLMs) were employed to analyze the relationships between carbon, nitrogen, and sulfur (CNS) stable isotopes and the factors of state (DE and NJ), bay location (lower, middle, and upper), and season (spring, summer, and fall). Results from stomach content analysis (SCA) were used as a measure of short-term diet and were also used as prior distributions in the Bayesian isotope mixing models using C and N stable isotopes. Generalized linear models illuminated that CNS isotopic values varied between states, bay locations, seasons, and some sampling stations. Contrarily, GLMs also found that certain sample groups of Weakfish appeared to exhibit a degree of site fidelity, as isotopic values of Weakfish collected in individual sampling stations occasionally remained stable from summer to fall, demonstrated by slow and rapid turnover tissues of muscle and liver, respectively. SCA and mixing model results revealed seasonal, locational, and state relationships for both individual prey species and groups of prey species. During the summer, Weakfish of all size classes from the middle bay consistently consumed higher frequencies of the same prey items (using percent frequency as the SCA method) compared to Weakfish from the lower and upper bays as revealed by stomach content analysis and stable isotope mixing models. Mixing model results also found that Weakfish consistently relied on gammarid amphipods (mainly Ampelisca sp.) as the most important prey item (>60% assimilation in many cases) throughout their estuarine residency, contradictory to the findings of previous studies that found mysid shrimp to be the most important prey item of small and medium size classes (by weight). The stomach content and stable isotope analyses applied to juvenile Weakfish indicate the overall variability in diet and organic matter sources over different spatiotemporal scales throughout the nearshore areas of the Delaware Bay. This variability highlights the importance of small-scale and locational (lower, middle, upper bay) primary and secondary productivity contributing to Weakfish production. Based on the findings presented in this study, future land-management decisions should consider the importance of small-scale and locational variability of optimal nursery habitats of juvenile Weakfish in the Delaware Bay.
dc.title Trophic Ecology of Juvenile Weakfish (cynoscion Regalis) in The Delaware Bay Using Stomach Content and Stable Isotope Analyses 2019-05-30T19:05:57Z
dc.language.rfc3066 en

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