Population Density, Recruitment and 1991 Mortality Event of Diadema Antillarum in the Florida Keys

Population densities and recruitment of Diadema antillarum were monitored on reefs of the Florida Keys beginning in 1990/1991. Signs of a mortality event, with symptoms similar to the 1983/1984 mass mortality, were first observed at off shore reefs near the Middle Keys in January/February 1991. The mortality event occurred on the offshore reefs off Key West in April 1991, reducing densities, which ranged from 0.02–0.60·m⁻², by 97%. Size frequency distribution shifted from one dominated by large individuals to one dominated by small recruits. Between fall of 1991 and fall of 1992, abundance declined by 83% at a patch reef offshore of Long Key. The cause of this decline was unknown. During 1991 and 1992, densities on spur and groove reefs at Sombrero Key Reef, near the Middle Keys, and Molasses Reef, near the Upper Keys were less than 0.01·m⁻². Settlement on the reef-flat at Molasses Reef was higher in 1991 than 1992. The settlement that occurred at Molasses Reef in 1991 was also detected on reefs off Long Key and Key West. Following this settlement, densities of sea urchins did not increase at any of the study reefs. Low adult densities, low recruitment and the chronic occurrence of mortality events in the Florida Keys indicate it is possible that densities of D. antillarum will never regain pre-mass mortality levels.