Degradation of coral reef:
Are indirect effects of overfishing a major contributor to coral decline?
Alina M. Szmant1), Margaret W. Miller2), and Iliana Baums3)
1) Dept. of Biological Sciences, University of North Carolina at Wilmington, U.S.A.
2) National Marine Fisheries Service/Southeast Fisheries Center, U.S.A.
3) Rosenstiel School of Marine and Atmospheric Science, University of Miami, U.S.A.
Many coral reefs world-wide show evidence of recent decline. The causes for this decline are multi-faceted and not always easy to discern. The most common causes of coral reef degradation are thought to be over-fishing, poor water quality (nutrients, sediments, turbidity), and most recently, wide-spread coral bleaching followed by death of the corals. Sometimes the causes of decline are easily identifiable (e.g. Point-source pollution; recent bleaching events), but often the changes are gradual, and not easily attributable. Over-fishing is recognized as the most extensive and serious impact to coral reefs world-wide, yet some mechanisms by which over-fishing can impact coral reef health are not obvious (except for the absence of the fishes) and thus not easy to study. However, over-fishing may indirectly cause severe coral decline due to a release of predation pressure on the prey organisms of the heavily fished predatory fishes.
One possible example of such an indirect effect of is that of the corallivorous snail, Corallophila abbreviata, Lamarck. This coral-eating snail is very abundant on coral reefs of the Florida Keys and Puerto Rico, and possibly elsewhere in the Caribbean as well. We hypothesize that high snail infestation is due to the loss from Caribbean coral reefs of invertivorous fishes (puffers, hogfish, filefish) and invertebrates (lobsters, octopii) that likely prey on these snails, in combination with the declining abundance of acroporid prey. We have studied the abundance and distribution of this snail on six reefs in the northern Florida Keys, and made measurements of their growth rates on two major reef-building species of coral, Acropora palmata and Montastraea spp., and feeding rates on the former only. The percent of colonies infested with snails ranged from 7 to over 69 % for A. palmata (mean 1998 10%, mean 1999 20%), and from 22 to over 80 % for Montastraea (mean 1998 56%, mean 1999 53%). Snail density averaged ca. 8 per colony for Montastraea and could be as high as 50 for larger colonies; feeding aggregations were observed causing large lesions on colonies of this species during the summer of 1999. Snail density was ca. 3 snails per colony for A. palmata; lesion size of recent feeding scars could be quite large (＞ 100 cm2). The snails on A.palmata were about 1.5 times the size of those on Montastraea. Feeding experiments showed that snails can consume ca. 1-9 cm2 per day of A. palmata (mean 4 cm2), and calculations based on snail respirometry and the C and N content of the coral tissues suggest that snails can consume upwards of 130 cm2 of live coral tissue per snail per year on A. palmata, and about 300 cm2 per snail per year of Montastraea. Given the large number of snails observed per reef, and the low percent cover by A. palmata on most Florida reefs, this snail could be a major contributor to the loss of a this important species of coral from these reefs.
These snails are small and often cryptic, and not as easy to notice as another major coral predator, Acanthaster, that devastated many Indo-Pacific coral reefs in past decades. Thus the extent of its impact to the Acroporas and Montastraeas of the Caribbean is not known and snail predation is likely often mistakenly attributed to other causes such as "disease". However, based on these results, the extent of snail impacts merits immediate investigation throughout the Caribbean region. If in fact this snail is over-abundant because of lack of reduced predation pressure, it will be another justification for why harvesting of coral reef fishes and macro-invertebrates must be greatly reduced if we want to save and protect coral reefs.
