Trophic dynamics of spotted seatrout (Cynoscion nebulosus ) in a river-dominated coastal estuary, Apalachicola Bay, Florida

Estuaries are one of the most ecologically and economically valuable habitats on earth in part due to high nutrient inputs from their associated watersheds that supports high productivity of fish populations. While numerous studies have investigated the spatial and trophic dynamics of estuarine fish communities, few of these studies have included estuarine piscivores that typically occupy the highest trophic level in the system. The overall objective of this research is to investigate the trophic dynamics of top-level consumers in relation to habitat (e.g., marine seagrass, saltmarsh) and environmental (e.g., river flow) factors using spotted seatrout (Cynoscion nebulosus ) as a focal species. My primary objectives are to: (1) determine the role of alternative nutrient sources underlying the productivity of seatrout, and (2) investigate how diet, nutritional condition, and trophic level of spotted seatrout vary spatially and temporally in relation to habitat type and variable river flow. Dietary analysis of spotted seatrout yielded 1338 prey items from 28 different prey categories representing 14 families of fishes, three families of shrimp, one family of crab, two classes of mollusk and two orders of crustacean. Teleost fishes and penaeid shrimp were identified as the two most important prey types supporting spotted seatrout in Apalachicola Bay. Within the large category of teleost fishes, species belonging to the families Engraulidae and Sciaenidae were the most numerically abundant and frequently occurring in the diets of spotted seatrout. Juvenile spotted seatrout fed more frequently on decapod shrimp than teleosts while adult trout fed roughly equally on teleost fishes and decapod shrimp. Regional and seasonal differences within the diet community of spotted seatrout were primarily driven by the seasonal and regional differences in penaeid shrimp distribution and abundance. Results of stable isotope analysis revealed significant regional and seasonal structuring in the delta13C, delta 15N, and delta34S ratios of spotted seatrout in Apalachicola Bay. Regional variation in 13C and mixing model analysis suggested fish living in the upper regions of the bay are primarily supported by in situ primary production in the form of phytoplankton and terrestrial detritus while secondary production in the outer regions of the bay is most heavily supported by seagrasses. There was little evidence to suggest large changes in the primary source of productivity in each region with seasons. The lower regions of the bay were the only regions to exhibit seasonal changes in primary production seagrass epiphytes played a larger contribution to seatrout production during the fall months. Nitrogen varied regionally and was characterized by elevated levels in the head regions of the by reflecting the dependence of the food web in that area on N enriched sources such as phytoplankton while fish collected in the lower regions of the bay possessed lower nitrogen levels indicative of benthically derived carbon sources. This project provides insight on how various estuarine habitat types and nutrient sources support the productivity of estuarine consumers that form the basis of important recreational fisheries in the region. Such information can be used by managers to help prioritize habitat conservation measures and more effectively target limited resources to the habitats and life stages that contribute most to population productivity.