Stephen Szedlmayer

Alabama’s artificial reef program began in 1953 and since then has grown into the largest in the nation, with more than 14,000 reefs providing relief on the flat, sandy floor of the Gulf of Mexico.

The reefs off coastal Alabama have been a boon to the commercial and sport fishing industries, as catches of popular reef fish, such as red snapper and gray triggerfish, have increased dramatically through the years.

The most widely held theory as to why artificial reefs attract certain species of fish relates to the food chain. Over time, corals, sponges, and other encrusting organisms take up residence on the reef. Small fishes come to feed on these organisms, and larger fishes are drawn to feed on the smaller. A complete reef food web is created.

To date, however, the food chain theory has not been scientifically proved. Some studies of the diets of reef fishes, in fact, have indicated that these fishes are foraging on animals found, not on the reef, but on the surrounding sand substrate.

To determine whether artificial reefs actually produce food and cause the food chain effect, AAES researchers at the Gulf Coast Research and Extension Center in Fairhope studied the diets of the two reef fishes, red snapper (Lutjanus campechanus) and gray triggerfish (Balistes capriscus).

For the red snapper, fish were collected from two habitats: open substrate, which is basically flat, and artificial reefs. The open substrate fish were taken from July through December 1994, while the reef fish were collected from June 1993 through December 1994. All fish were collected in the Gulf of Mexico, 10 to 20 miles south of Dauphin Island. Specimens ranged in size from 1 to 12 inches standard length (SL).

The stomach contents of the red snapper were evaluated for each item of prey, with percent volume, percent number, and percent occurrence estimated and then used to calculate the Index of Relative Importance (IRI).

Researchers found that, in the open habitat, red snapper under 2 inches SL fed primarily on minute marine crustaceans known as copepods, planktonic wormlike marine organisms called chaetognaths, crabs (Reptantia), and shrimps (Natantia), while those red snapper that ranged from 2 to 3 inches SL fed primarily on shrimps. The diets of red snapper larger than 3 inches SL were dominated by teleosts, or bony fishes.

Researchers work up red snapper samples for diet, age, and growth studies.

Reef-associated red snapper, meanwhile, fed on squid and fish species that were identified as reef-associated prey items.

Red snapper between 3 and 6 inches SL were collected from both open and reef habitats. The Bray-Curtis dissimilarity index for the IRIs was calculated for these overlapping size classes. This index was then used in a multi-dimensional scaling analysis (MDS). The MDS showed apparent differences between the diets of red snapper in open habitat and those in reef habitats, independent of the size of the snapper. This suggests differential habitat values for red snapper. Thus, this information provides evidence that the food chain theory does function on artificial reefs off coastal Alabama.

For the gray triggerfish component of the study, the fish were collected from artificial reefs off coastal Alabama by two scuba divers using a drop net. Each gray triggerfish captured was measured and weighed, and the contents of the entire gastrointestinal tract were removed for study. The lengths of gray triggerfish collected from artificial reefs were found to vary by season and by reef age.

Percent composition by weight, number, and frequency of occurrence, and the percent composition of an index of relative importance were used to examine the diets of gray triggerfish of various sizes. Diet comparisons were also made for fish collected from artificial reefs that were of different ages (three months, 14-19 months, and 29-31 months). Three prey levels were examined: (1) broad prey type, which pooled prey items into higher taxonomic categories, (2) narrow prey type, which separated prey to the lowest possible taxonomic level, and (3) habitat type prey, which categorized prey according to their ecological living space, e.g., pelagic (open sea), reef, or open sand.

For the broad prey level, pelagic mollusks, crustacean larvae, and decapod crustaceans such as shrimps, lobster, and crab dominated triggerfish diets. At all prey levels, reef ages, and fish sizes, open-sea or pelagic animals were important in gray triggerfish diets. The primary pelagic prey items were crustacean larvae (mostly Brachyuran crab zoea), cavolines, mollusks, and squid. Decapod crustaceans, miscellaneous encrusting species, and other crustaceans were also important diet items.

In habitat comparisons, all prey types were used by gray triggerfish. However, the reef-associated prey species were an important component for this particular fish species, again providing further evidence for the food chain theory.

In conclusion, gray triggerfish in the northeast Gulf of Mexico feed from a variety of organisms that occur in different habitats. Pelagic prey are consumed in high numbers, while reef-associated prey, decapod crustaceans and encrusting organisms account for large percentages by weight. Independent of reef age or prey level analyzed, reef-associated prey are important diet components, suggesting that, as with red snapper, artificial reefs are productive environments for the gray triggerfish.

The third and final article in this Highlights series on artificial reefs will focus on by-catch issues.