Stephen Szedlmayer

The floor of the Gulf of Mexico off the coast of Alabama is primarily flat and sandy with no natural reefs or structures to break the terrain. However, in the aquatic environment, almost any material that adds topographical relief will attract fish and increase catch rates. This concept has been applied extensively in coastal Alabama with the placement of more than 14,000 artificial reefs. AAES research is helping define the uses and implications of these reefs and how and why they affect fish populations.

One reason for the excitement over artificial reefs is that commercial and sport fishers prefer to catch certain species of fish compared to others. Many scientists believe that reef construction does not increase the number of fish but simply changes the species in an area. For example, extensive long spine porgy/sand perch habitat has been changed to red snapper reef habitat with the reef construction off coastal Alabama. Another example is the conversion of croaker habitat to tautog habitat with the construction of the 15-mile-long Chesapeake Bay Bridge tunnel in Virginia, or the 1,000-plus oil rigs off Louisiana that also increased red snapper habitat similar to Alabama's coastal waters.

Another reason for the excitement over artificial reefs is the impression that these reefs create a private spot that anglers believe draw little or no competition from other anglers. In Japan, this concept has been taken to the extreme in that all reefs are considered private property and other anglers or commercial fishers would no sooner fish a private reef than walk into a neighbor's corn field and pick a bushel of corn. However, this is not the case in the United States and fishers rely on the difficulty of finding reefs to keep fishing pressure down.

Artificial reefs have been a tremendous help in coastal Alabama to both the commercial and sport fishing industry. Catches of reef fish species, such as red snapper and grey triggerfish, probably exceed all other coastal areas in the United States. The Alabama Marine Resources Division along with a great many private fishers have been intensely active in constructing artificial reefs. Other states are following the example set by Alabama and are planning or already have in place their own special reef-building zones.

Concrete block study reef, showing some resident red snapper and gray triggerfish.

Despite the popularity and effectiveness of artificial reefs and more than 50 years of study, scientists still have not completely explained how artificial reefs work. The obvious answer is that artificial reefs provide hard substrate for sessile benthic invertebrates and various species of algae plants that normally do not settle on shifting sand substrate. Small fishes come to feed on these organisms and larger fishes are drawn to feed on the smaller ones. Thus, artificial reefs set up a food chain. The problem with this explanation is that science has continuously failed to prove the food chain hypothesis. Many diet studies of fishes on artificial reefs show that they are foraging on the animals found in the surrounding sand substrate.

AAES researchers investigated the question of whether or not artificial reefs actually produce food and cause the food chain effect. Concrete block reefs were built in the Hugh Swingle reef building zone off the coast of Alabama in the Gulf of Mexico. Each reef was made of 48 standard concrete blocks for a total size of about one cubic yard. Half these reefs were painted with a copper-based antifouling paint (the same that is used on boats) in an attempt to prevent any benthic invertebrate or algae growth. The idea was to cut off the food chain on half the reefs and compare the fish community to exactly the same type of reef that had not been painted with copper-based paint.

The copper paint prevented almost all growth on the benthic invertebrates and algae for more than 1.5 years. However, some of the results of this study were surprising. For the most part, painted reefs showed the same number of fish as the unpainted reefs. The one exception was the white spotted soapfish, which was more abundant on the unpainted reefs than on the reefs with food. It is interesting that the white spotted soapfish is probably the fish most closely tied to reef structure. This fish is almost never observed in the open and can be found tightly wedged within cracks or holes of almost any artificial structure.

However, two other lines of evidence suggest that artificial reefs may be more important than at first realized. One AAES study evaluated the residence of red snapper, proposing that if fish spend most of their time on artificial reefs, then production would be supported. Using conventional tags, the scientists studied the movement, abundance, age, and growth of red snapper (Lutjanus campechanus) and the age and growth of lane snapper (L. synagris) from artificial reefs in the northeastern Gulf of Mexico. Growth rates of L. synagris appeared faster off Alabama, compared to growth rates reported in previous studies. Also, L. campechanus showed larger sizes and older ages (42 years) from this area off Alabama, a substantial increase over the oldest previously reported age (13 years). Most (74 %) of the marked and recaptured L. campechanus were recaptured within two miles of their release site. Also, distance moved was not related to amount of time at large, even after extended periods (up to 430 days).

Survival from tag implantation was assumed high because of the high recapture rate (52%), and from visual observations of tagged fish by scuba divers up to five months after release. No seasonal movements were detected, with several fish located at their release site over all seasons. No relations were detected for distance moved compared to days tracked or fish size, and days tracked compared to fish size. In conclusion, red snapper showed year-round high fidelity to artificial reef sites in coastal Alabama. This consistent association suggests at least some benefit from artificial reefs for this species.

What does all this mean, concerning the production or attraction question of artificial reefs? These studies show that both mechanisms are working. Clearly, artificial reefs attract fish and some fish may not get any direct benefits. However, the close association of red snapper to reef structure, information from the diets of gray trigger fish and red snapper showing direct feeding on reef invertebrates, and the large numbers of reef fishes in an area that naturally has very little natural reefs, all suggest that the artificial reef system off Alabama has an important production component. Other issues also have been addressed in relation to artificial reefs and fish populations in the Gulf of Mexico. In future Highlights articles, results of research on fish diets and by-catch issues will be discussed.