Gag grouper, marine reserves, and density-dependent sex change in the Gulf of Mexico

The use of marine reserves for fishery management remains a controversial tool despite evidence of their success. In the Gulf of Mexico (GOM), two marine reserves were put in place in order to protect spawning aggregations of gag grouper, which has experienced a steep decline in the male to female sex ratio since the 1970s. Reserve success is ultimately a function of the total amount of spawning aggregations protected, and fisher response to reserve implementation may have significant influence on the ultimate effectiveness of the reserves. We modeled the GOM gag grouper population under a range of potential reserve sizes and fisher responses. In general, larger reserves resulted in higher adult populations and sex ratio. Fisher response to reserves also had a large impact on reserve success. When we included density-dependent sex change in the model, the results showed a reduction in the sex ratio under high fishing pressure when compared to model simulations when sex change was fixed. While our model suggests that the reserves currently in place should result in a small increase in adult sex ratio given the current level of fishing mortality, such benefits might not be achieved in practice given possible future changes in transition rates or fisher behavior.

► Marine reserves remain a controversial tool for fishery management.
► We model the population level sex ratio response of gag grouper to spawning reserves.
► Response in adult sex ratio was dependent on reserve size: large reserves result in higher sex ratio.
► Adult sex ratio was also highly dependent on changes in fishing effort post-reserve execution.
► Inclusion of density-dependence in the sex change function altered expected benefits of reserves.