Distributions of coastal sharks in the northern Gulf of Mexico: Consequences for trophic transfer and foodweb dynamics

Sharks are considered to be apex predators in marine ecosystems, yet relatively few studies have quantified the nature of trophic interactions between sharks and their prey, much less the degree to which this role varies spatially. As traditional fisheries management advances towards ecosystem based approaches, identifying the role of sharks on ecosystem specific bases will be of paramount importance.;This study identified discrete patterns of depth use by coastal shark species. The Atlantic sharpnose (Rhizoprionodon terraenovae) and blacknose (Carcharhinus acronotus) sharks were shown to use waters deeper than 30 meters, while the blacktip shark (C. limbatus) was shown to preferentially occupy shallow waters. Examination of sex ratios and length frequency indicates these patterns may be reproductively motivated.;Intensive monthly longline sampling revealed a distinct along-shelf gradient in shark community structure across an area previously hypothesized to be a faunal break. Examination of primary productivity (represented by chl a) and secondary productivity (represented by trawl data) indicated that regionally, smaller species of shark occupy waters with higher primary productivity and higher biomass of forage fish. Shark distributions across the Gulf of Mexico displayed a similar correspondence to primary productivity, but not secondary productivity. On both scales, the distributional gradient seen in smaller sharks was not seen for several larger species. These results offer an explanation for the gradient in shark distribution seen between the eastern and western Gulf of Mexico, particularly on a regional scale.;Stable isotope and stomach content analysis of the Atlantic sharpnose shark revealed a trophic role intermediate between secondary and tertiary consumers. Both methods suggested ontogenetic shifts in diet, in accordance with previous studies. Seasonal and regional shifts in trophic position and carbon source were highly significant, and statistically dependent. My analysis suggests that stable isotope data from liver tissue, while offering the potential for increased temporal resolution, should be interpreted with caution. Future studies should follow the dual method approach taken in this study, and would benefit from isotope analysis of additional elements in addition to compound specific approaches.