Determining the Trophic Role of Gulf Menhaden (\u3ci\u3eBrevoortia patronus\u3c/i\u3e) Using Carbon and Nitrogen Stable Isotopes

Gulf menhaden (Brevoortia patronus) play multiple roles in the Gulf of Mexico coastal ecosystem as filter feeders and as an important food source for many recreationally and economically important finfish. Gulf menhaden also support a large commercial fishery, and little is known about the impact of fishing on the role of this species as a filter feeder in the food web. This study examines the spatial, temporal and ontogenetic dynamics of food selectivity and trophic role observed in Gulf menhaden. The most important dietary item for juvenile fish was found to be phytoplankton (75.7% dietary composition) while that of subadults and adults was found to be zooplankton (77.6% for sub-adults and 63.0% for adults). Juvenile fish were also found to utilize detritus when present in the water column and their diet was varied between individuals more so than observed in sub-adults and adults. In addition, juveniles were found to occupy a trophic level approximately one step lower (2.65±0.31) than that of sub-adults (3.45±0.27) and adults (3.52±0.28). Spatial dietary variation was found to be related to that of ontogenetic variation while temporal variation was found to be minimal especially in sub-adults and adults. While the fishery largely targets age 1 + fish (sub-adults and adults), these results suggest that if overfishing occurs to the extent that it impacts recruitment, it may decrease the resiliency of the inshore Gulf of Mexico ecosystem to eutrophication by decreasing the abundance of juvenile fish seasonally present in this environment