Modeling the Trophic Dynamics Between Sharks and Fisheries within the Gulf of Mexico

Understanding ecosystem structure and trophic linkages often requires ecosystem models structured specifically to model the unique dynamics of a system. To assess the ecosystem impacts of alternative fishing policies on coastal sharks in the Gulf of Mexico (GOM), a published GOM Ecopath with Ecosim model was reparameterized to incorporate information on federally (i.e., NMFS) and internationally (ICCAT) managed species, including sharks, billfishes, tunas, dolphins, and seabirds. The GOMEXSI database of dietary links enabled a comprehensive assessment of region-wide trophic dynamics. Model application is discussed with respect to the ecosystem impacts of alternative fishing policies on large coastal sharks. Four alternative patterns of variation in natural and fishing mortality were modeled to test hypotheses regarding the ecological role of sharks in the GOM. Alterations to fishing patterns for the commercial shark and menhaden fisheries led to substantial changes in biomass for both higher trophic level predators and lower trophic level organisms. Results suggest that altering fishing mortality on sharks, whether directly or indirectly, clearly impacts ecosystem structure. Further, increased shark biomass tended to reduce biodiversity.  Ultimately, model results will be compared to ecosystem models of the West Florida Shelf and will feed directly into federal fisheries management.