Modeling the Role of Sharks in the Trophic Dynamics of Apalachicola Bay, Florida
John K. Carlson
Abstract.—A dynamic mass-balance ecosystem model (Ecopath with Ecosim) was used to investigate how relative changes in fishing mortality on sharks can affect the structure and function of Apalachicola Bay, Florida, a coastal marine ecosystem. Simulations were run for 25 years, wherein fishing mortality rates from recreational and trawl fisheries were doubled for 10 years and then decreased to initial levels. Effect of time/area closures on ecosystem components were also tested by eliminating recreational fishing mortality on juvenile blacktip sharks Carcharhinus limbatus. Simulations were run assuming mixed control and top-down control. In the mixed control, biomass of juvenile coastal sharks (finetooth shark C. isodon, spinner shark C. brevipinna, sandbar shark C. plumbeus), juvenile blacktip sharks, and bull sharks C. leucas declined up to 57% when recreational fishing mortality was doubled. Increases in biomass were also observed for the Atlantic sharpnose shark Rhizoprionodon terraenovae and, to a lesser extent, skates and rays. Increasing the fishing mortality imposed by trawl fisheries affected only a few elasmobranch groups, primarily skates and rays. Increases and decreases in biomass lasted only as long as fishing mortality was elevated, although a lag time was observed for some groups to recover to initial biomass. Simulating a time/ area closure for juvenile blacktip sharks caused increases in their biomass but decreases in juvenile coastal shark biomass, a competing multispecies assemblage that is the apparent competitor. Top-down control scenarios resulted in greater variation and magnitude of response than those elicited under mixed control, although the direction of the response was similar. In general, reduction of targeted sharks did not cause strong top-down cascades.