Nonlethal Sex Determination of the Greater Amberjack, with Direct Application to Sex Ratio Analysis of the Gulf of Mexico Stock

Abstract

Stock assessments for the Gulf of Mexico (Gulf) Greater Amberjack Seriola dumerili continue to designate the stock as overfished and undergoing overfishing, despite increasing regulatory measures. Knowledge of sex‐specific spatial distribution and fishing mortality may contribute to our understanding of the stock's overexploitation, especially since Greater Amberjacks may be subject to sex‐specific mortality resulting from minimum size regulations. Currently, the sex ratio of the stock is assumed to be 1:1. An average and range of sex ratios were estimated for the Gulf stock based on sampling of fish landed in the recreational and commercial fisheries and based on released fish that were nonlethally sexed during a tagging study of sex‐specific movement patterns and release mortality. The nonlethal method of sex determination was developed based on external features of the urogenital region; urogenital catheterization was used to validate the external sexing and to collect oocyte samples for determining maturity stages of females during the spawning season. Of the 238 fish (108 males and 130 females) for which sex was verified, only one smaller female was incorrectly sexed. Urogenital catheterization identified females that were spawning or that would likely spawn in the upcoming spawning season, but no differentiation could be made between immature and resting females. Analysis of published data sets suggested that the Gulf stock has an overall female‐skewed sex ratio, with estimates ranging from 1:1.7 to 1:2.5 (male : female), while the nonlethal sexing data from the current study suggest that the overall sex ratio is slightly male skewed (1:0.8) in some regions. All studies report a female‐skewed sex ratio of 1:2.3, on average, for 1,000‐mm FL and larger fish in the Gulf. Sex ratios that deviate from the assumed 1:1 ratio should be incorporated into future assessments of the Gulf Greater Amberjack stock to investigate potential consequences for stock status and management.

Received December 30, 2013; accepted May 8, 2014