Diadema antillarum on St. Croix, USVI: Current Status and Interactions with Herbivorous Fishes

The 1983-84 mass mortality of the long-spined sea urchin, Diadema antillarum, intensified the loss of herbivory that was a major factor in the degradation of coral reefs in the wider Caribbean. As determined from recent surveys, densities of D. antillarum populations at back reef locations on St. Croix, US Virgin Islands, are higher than densities immediately following the die-off – but still about an order of magnitude below pre-die-off densities and patchy in both time and space. Comparisons to similar surveys during the past twelve years and to earlier historical records indicate that recovery on St. Croix continues at the very slow rate that typified the first decade after the mass mortality. Populations of herbivorous fishes on St. Croix surged following the D. antillarum die-off but have experienced heavy fishing pressure ever since then. Reciprocal densities of D. antillarum versus roaming grazers (parrotfishes and surgeonfishes) on survey transects and stationary point counts indicate that negative interaction between these two groups is present despite the reductions to their populations during the last 35 years.