Developing an ecosystem-based approach to management of the Gulf menhaden fishery using Ecopath with Ecosim
Geers, Tess Misak
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The Gulf of Mexico (GoM) is a valuable ecosystem both socially and economically, and fisheries contribute substantially to this value. Gulf menhaden, Brevoortia patronus, support the largest fishery in the Gulf (by weight) and provide forage for marine mammals, seabirds and commercially and recreationally important fish species. Understanding the complex interactions among multiple fisheries and myriad unfished species requires tools different from those used in traditional single-species management. Ecopath with Ecosim (EwE) is increasingly being used to construct food web models of aquatic ecosystems and to evaluate fisheries management options within a broader, ecosystem context. An EwE model was developed to examine the impact of the menhaden fishery on both fished and unfished species in the GoM. This model builds on previously published EwE models of the GoM, and is tailored to the range and habitat of Gulf menhaden. Several management scenarios were run for commercially and recreationally important fisheries. Results indicated that recreational fishing levels for some species may be unsustainable, but that recovery is not currently inhibited by a lack of prey. Increased fishing for menhaden to target or limit levels resulted in a decrease in ecosystem maturity and ascendency as well as decreases in predator biomasses including seabirds, sharks, red drum and red snapper. Additionally, a number of different harvest strategies for menhaden and other forage fish were modeled under both deterministic and stochastic conditions and the results were evaluated in terms of impacts on other fisheries and unfished predator populations. The results of the stochastic simulations indicated that harvesting forage fish groups at 100% FMSY levels using a constant fishing mortality rate is likely to result in population collapse, lower long-term average yields and substantial declines in predator populations. Fishing at 75% FMSY appears to cause much lower declines in predator populations, while producing as high or higher yields than fishing at 100% FMSY. Pressure on both commercial and recreational fisheries will likely continue to increase in the future and menhaden fisheries are no exception. The results of this analysis stress the need for a precautionary, ecosystem-based approach to management of Gulf menhaden in order to maintain ecosystem structure and prevent declines of both fished and unfished predator species.