University of Ottawa
The Horseshoe Reef is one of the longest bank-barrier reef of the St.Vincent Grenadine region. It is located away from direct anthropogenic stress with the closest inhabited island (Mayreau with a population of 250) located at 3km. Very little information is available on the Reefs of the Tobago Cays Marine Park; these reefs were previously studied by Lewis in 1972. A team of 3 scientists surveyed 5 reef sites along the 4km bank-barrier reef during two weeks of field work in June of 1999. A total of 60 coral line transects, 268 algae quadrats, 10 fish rover dives and 50 fish belt transects were conducted at two shallow sites and three deeper sites on the reef.
Our survey revealed that the coral assemblage at the Horseshoe Reef was dominated by Montastraea annularis (31%), Porites asteroids (23%) and Porites porites (23%), Montastraea faveolata (5%), and Millepora complanata (4%) which account for 86% of all corals (n=531). Standing dead colonies of Acropora palmata (2%), recognizable by their characteristic colony shape, were present at one of the shallow sites. Most shallow areas were composed of a dead Acropora palmata pavement largely encrusted by crustose coralline algae and colonized by Millepora and Porites. The size frequency distributions for the major reef building corals are skewed toward smaller colonies, the average diameter of corals being 50cm. The live coral cover at all sites was relatively low, ranging from 29 to 44%. The partial coral mortality was low to moderate, with recent mortality ranging between 1-3% and old mortality ranging between 23-28%. Relatively low incidence of coral disease was observed; diseases encountered were the yellow band disease (4% of star corals) and the black band disease (2% of massive corals). No total bleaching of corals was observed during our survey, but corals appeared to be recovering from last years low/moderate bleaching event. Pale discolored coral (9% of colonies) and partial patchy bleaching (5% of colonies) were encountered. The presence of an encrusting grayish-white tunicate (Trididemum solidum?) was present at many of the survey sites. This organism was mainly encrusting Porites and Monstastraea but occasionally Millepora. Algal communities at the Horseshoe Reef are dominated by crustose coralline at the shallow sites (coralline:macro:turf= 54:17:29) and by macroalgae at the deeper sites (coralline:macro:turf= 38:42:20). Macro algae communities were largely dominated by the calcareous algae of the genus Halimeda. Fish assemblages are dominated by herbivorous scarids and acanthurids, which are present in moderate densities (17-59/100m2 and 2-14/100m2, respectively). Commercially valuable fishes (e.g. groupers, snappers) are present in low densities (0-1/100m2) and are relatively small in size (mostly between 16-26cm).
Using the indicators established by the AGRRA protocol, our results suggest that the Horseshoe Reef is a relatively healthy reef but is also showing signs of past disturbances/recovery and overfishing. The AGRRA survey established a baseline of current conditions and enabled us to characterize the reef. This information will serve to evaluate future changes in the coral reef condition, this is especially important since the Horseshore Reef has recently obtained a marine park status.
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