Groupers (Family Epinephelidae) on the Edge
The 159 species of grouper that occur world-wide are extremely important ecologically and many of them are important economically wherever they occur. They include many of the top-level predators in warm-temperate & tropical ecosystems, associated with deep-water and shallow hardbottom reefs, following pleistocene shorelines of the continental shelf and shelf edge. Their relationships with the places they live are so striking in some cases that they appear to be acting as keystone species and ecosystem engineers-- species which by their very presence or behaviors enhance the complexity of the habitat and thus the diversity of the communities within which they live.
While each grouper species has a specific suite of traits—and individuals their own idiosyncrasies--there are a number of traits that they all share. Groupers, like many reef fish, spawn offshore on shelf and shelf-edge reefs. Their pelagic larvae remain the open ocean for 40-60 days before reaching inshore nursery grounds. Once there, they transform into small juveniles, and remain in their nursery habitat for periods that vary from 5-6 months (Gag) to 5 to 6 years (Goliath Grouper). They then move offshore to join adult populations. As they move from habitat to habitat, each life stage has very different requirements for being successful, occupying different niches relative to their size and their position in a food web, whether they eat plankton, bottom-dwelling crustaceans, or other fish.
Life Cycle of Red Grouper
Juveniles live in shallow-water nearshore hardbottom reefs where they remain for 4 to 5 years. Adults occur in rock formations and crevices of limestone reef as deep as 300 feet. They grow slowly, reaching a maximum weight of about 50 pounds in their 29 years of life. Females reach sexual maturity between the ages of 4 and 6 years when they start to pair spawn with males. They release egg and sperm into the water column and the resultant larvae are pelagic, floating in the plankton for up to 45 d before settling out in hardbottom habitat inshore.
As an overlay on this life cycle are life histories and behaviors that they also share. They are slow to mature and have complex social systems that provide cues for sex change. They also exhibit a high degree of site fidelity within their home ranges and to spawning aggregation sites where they are easy to capture, particularly with the remarkable improvements in navigational gear that allows targeting specific spawning locations.
Sex change in groupers is a one-way street, from female to male. For Gag (Mycteroperca microlepis)-- one of the more important species fished in the eastern Gulf of Mexico—the period in which sex change is initiated is brief, occurring only during the late winter or early spring. At other times, males and females are separated, with males staying offshore on spawning sites while females move to shallower water. All of the reproduction in the population takes place in the brief time the sexes co-occur. So do all the cues for sex change. If there are two few males, then dominant females will change sex so that by the following spawning season, more males are available.
This combination of traits make them highly vulnerable to exploitation and habitat loss. There are currently no management plans in effect to adequately protect either their social structure or their nursery habitat. While marine reserves have proved an effective tool for protecting offshore spawning grounds, they have not been applied to nursery habitat which remains vulnerable to the effects of eutrophication, development, and industrial contamination.