Age and Stock Analysis Using Otolith Shape in Gags from the Southern Gulf of Mexico

On Campeche Bank (CB) in the southern Gulf of Mexico, Gags Mycteroperca microlepis are captured as part of a sequential multispecies fishery. Limited scientific data suggest that the Gag is the third most important commercial species in the region, contributing 21% (total weight) of the overall grouper catch. Neither age nor stock data are currently available for Gags in this region, making it impossible to determine how this species is affected by current fishing practices. Such information is vital for the CB stock since it acts as a genetic source for a mixed Gag population on the west Florida shelf through a constant unidirectional flow of larvae. The goal of this study was to generate baseline age and stock structure data for Gags on CB. Age was quantified from otoliths by counting the number of annuli (indicated by the number of opaque zones from the otolith core to the margin) in thin sections. Otolith shape was analyzed by using discrete wavelet transforms, linear otolith morphometrics, and shape indices. In each of 198 specimens, age was determined based on examination of the right sagittal otolith, and shape analysis was conducted on the left sagittal otolith. Individual age ranged from 4 to 17 years, and otolith opaque band deposition occurred annually between December and February. Using principal components analysis and linear discriminant analysis, two distinct groups were identified (discrimination success = 98%), indicating the presence of substructure in the CB stock of Gags. Otolith analysis clearly assisted in achieving a better understanding of the CB Gag stock and in identifying the complexity within the stock, demonstrating that otolith analysis is a useful tool in characterizing fish populations. The study results highlight the urgent need for Gag stock assessment in this region and for the application of a cautious approach to fishery management given the immense gaps in biological data for this species.