Male color phase in gag: implications for monitoring sex ratio via visual underwater surveys and port sample observations

Fishing of gag (Mycteroperca microlepis Goode and Bean, 1879) spawning aggregations in the northern Gulf of Mexico has reduced the percentage of males in the population from approximately 17% to 2% since the 1970s. This is critical as gag are monandric, protogynous hermaphrodites, and accurate estimation of size-at-transition and sex ratio is necessary for proper management. Presence of darkened pigmentation on the abdomen was thought to indicate transition from female to male, and thus, video observations of gag with dark pigmentation were used to estimate sex ratios and indices of abundance. Recent video observations of gag demonstrate that they can change pigmentation patterns within seconds, which could be leading to misidentification of males in those surveys. Results of our investigation showed that males observed in the video survey, as determined by pigmentation only, are significantly shorter than males observed in the commercial fishery, which were determined using both pigmentation and histological examination. Significant differences were detected between pigmented and non-pigmented gag in the commercial data, but not in the video survey data. Additionally, size at which 50% of females had transitioned to male was significantly larger when estimated using commercial fishery data vs the video survey data. Due to significant differences in length distributions and estimates of size-at-transition, and evidence of rapid changes in pigmentation patterns, it is inadvisable to estimate sex ratios or relative indices of abundance by sex for gag from video observations. To address potential shifts in size- and age-at-transition, continued monitoring of sex ratios using histological techniques is recommended.