Grouper Ecology

There are about 159 species of grouper (Family Serranidae) world-wide and they are extremely important ecologically and economically wherever they occur. They include many of the top-level predators in warm-temperate & tropical ecosystems, associated with deep-water and shallow coral reefs, following pleistocene shorelines. Their relationships with the places they live are so striking in some cases that they appear to be acting as keystone species-that is, species which by their very presence enhance the complexity of the habitat & thus the diversity of the communities within which they live.

Each grouper species has a suite of traits like no other. Even individuals have their own idiosyncrasies. But there are a number of traits they all share. One of the ways our group has approached studying these fish is to examine them species by species, evaluating each stage in their life cycle separately, for instance linking each stage with the habitat on which it depends. This approach works well for reef fish because the tendency is for each life stage to be associated with different habitats, with very different requirements for being successful, occupying different niches relative to their size (whether they are large or small) and their position in a food web, whether they eat plankton, bottom-dwelling crustaceans, or other fish.

Groupers, like many reef fish, spawn offshore (typically mid winter for warm temperate species like gag, scamp, and red grouper), late summer for sub-tropical species like goliath grouper) on shelf (30 m) and shelf-edge reefs (70 m), They have pelagic larvae that are transported from spawning sites over 40-60 days to arrive in inshore nursery grounds-almost exclusively seagrass beds on the Gulf coast for gag, hard bottom for red grouper, and mangroves for goliath grouper. There, they settle out as small juveniles, remain in their nursery habitat for various lengths of time (5-6 months for gag; 4-5 years for red grouper, 5 to 6 years for goliath grouper). They then move offshore to join adult populations. As they move from habitat to habitat, they have size- & age-related shifts in the ecological role of an individual, with the same individual dependent upon different foods and different habitats at different life stages.

As an overlay of this life cycle are life histories and behaviors that they also share. They are slow to mature, have a high degree of site fidelity, exhibit complex social systems that provide cues for sex change, and aggregate to spawn, so they are easy to capture, particularly with the advent of navigational and positioning gears, including Loran and now Global Positioning Systems for targeting specific spawning locations.

In grouper, spawning aggregations vary in number, size, and location , along a more or less continuous spectrum. Compare, for instance, Nassau grouper, which has spectacularly large aggregations that occur at specific sites typically around the full moons of December and January, with gag and scamp, which form smaller aggregations over a broader area for about two or more months, and red grouper, which does not appear to aggregate, but rather forms small, widely dispersed spawning groups over a longer (4 months or more) period of time.

Groupers on the Edge

Groupers, the dominant predatory fish on many coral reefs, are particularly vulnerable to rapid overfishing because of their reproductive behaviors. First, they have complex social systems that lead them to change sex from female to male (a condition known as protogyny), and second, they form large groups called "aggregations" to spawn for relatively short periods of time. While together in aggregations, fish are easily caught, fact not lost on fishermen.

Sex change in groupers is a one-way street, from female to male. It occurs in a social context when fish form spawning groups offshore. For gag (Mycteroperca microlepis)-- one of the more important species fished in the eastern Gulf of Mexico, taken more by recreational than by commercial fisheries--this only occurs for a relatively brief period of time during the late winter or early spring. At other times of the year, males and females occur in separate locations, with males staying offshore on spawning sites while females move to shallower water. Virtually all of the reproduction in the population takes place in the brief time the sexes are together. So do all the cues for sex change. Social interactions among individuals in the group allow some sort of assessment to occur of the relative numbers of males and females. If there are two few males, then dominant females will change sex so that by the following spawning season, more males are available.

When these characteristics and fishing collide is during gag's spawning, when fishers target locations within which fish reproduce. The fisher's advantage of getting larger catches over a shorter period of time--like fishing in a barrel-results in the fish's rapid decline. Species with a few large aggregations occurring over a short period of time are more susceptible to fishing that targets aggregations than are those species with a greater number of smaller spawning groups that occur over a relatively lon reproductive season. Not only are the largest fish removed from the population, but the social cues that trigger sex change are interrupted. Thus, fishing that concentrates on aggregations results in smaller fish with fewer eggs, and ultimately leads to loss of males. This is compounded by loss of fish that are changing sex because they act more like males than like females and are thus more vulnerable to capture. Gag in the Gulf of Mexico and throughout the South Atlantic, for instance, have 90% fewer males now than they did thirty years ago. No males, no sex, no babies, no fish. Noone wants that. And yet resistance to protecting males and grouper populations in general is profound because protecting males means protecting specific habitats where males reside, and because protecting populations involves protecting spawning sites. It turns out that these sites often form the very heart of the fishery.

How widespread is this phenomenon? A quick review of the status of economically important groupers in the southeastern United States reveals that the most important reef fish species that we know anything about (those for which the National Marine Fisheries Service has conducted stock assessments) are on the verge of or are being overfished. Many of them are grouper, and all of them change sex from female to male. At least two of those suffer from low proportions of males in the population and all of them aggregate to spawn. Two of them (Nassau grouper and Goliath Grouper-the fish formerly known as jewfish) are already protected from fishing. Two are on the verge of being protected (Warsaw grouper and speckled hind). And finally, twenty-six grouper species worldwide are being considered for listing on the IUCN list as vulnerable to extinction.

There are currently no management plans in effect to preserve either the social structure or the natural proportion of males in these fishes. Most management approaches, in fact, fail to address these overwhelmingly important aspects of their reproduction. The most logical tool for protecting these fish is marine reserves, areas of the ocean closed to fishing. This is not a radical or unprecedented idea. All anglers understand--and accept the need to protect juveniles, for instance. They throw fingerlings back in streams or hunters know they don't shoot young deer. The advantage of a closed area, is that it protects spawning fish. It would also specifically offer protection for male gag, which tend to stay on these reproductive sites year-round. Perhaps the strongest two arguments for closing areas essential to protecting these species is that it allows investigation into the natural range of complex behaviors that are otherwise intractable, and it provides insurance to protect us from our ignorance about natural systems.