California Sea Grant College Program

In this project, researchers investigated possible causes for the observed decline in the average sizes of both male and female sheephead in Southern California. The leading theory, borne out in this study, is that sportfishing, because it selectively removes large territorial males, is probably the main reason individuals in heavily fished areas are smaller than their counterparts in more remote regions. Because the fish are socially cued protogynous hermaphrodites, the removal of large territorial males triggers sex change in the largest females. As a result, paradoxically, the removal of large males has the effect of dramatically reducing the number of eggs produced, and hence the total reproductive output of a population. The findings suggest that the usual fisheries management techniques (size limits) don’t work for a species that changes sex during its life. In particular, scientists report that, at popular sportfishing destinations such as Catalina Island, the entire male population could be legally fished out. The scientists are recommending that state biologists implement slot limits, which would establish upper and lower bounds on legal-size sheephead, to ensure that both males and females are present to reproduce.