Modeling gag grouper (Mycteroperca microlepis) in the Gulf of Mexico: exploring the impact of marine reserves on the population dynamics of a protogynous grouper

Abstract

The gag grouper (Mycteroperca microlepis) population in the Gulf of Mexico supports both a commercial and recreational fishery but has experienced a decrease in the male to female sex ratio over the past thirty years. Protogynous fish populations naturally have a smaller male to female ratio than gonochoristic fish populations; however the decline in the gag population is such that sperm limitation may be occurring. In an effort to correct the decline in sex ratio, fishery managers have recently implemented two marine reserves designed specifically to protect gag spawning aggregations. Results from two population models (an age-structured model and an individual based model) suggest that utilizing marine reserves in the management of gag (a female-first protogynous grouper) may be an effective method to increase the male to female sex ratio in the population. Both models show that marine reserves can have a positive impact on sex ratio, but the effectiveness of marine reserves is a function of the magnitude and pattern of fishing effort. The inclusion of density-dependence to the sex change function of the models, in an effort to model socially-induced sex change, showed that the mechanism controlling sex change in the population will impact the population response to fishing. Socially-induced sex change may act as a buffer to high fishing mortality and increase the expected benefits of marine reserves. Both models successfully simulated a protogynous fish population and these methods may improve upon existing stock assessment models.