POPULATION-DENSITY, RECRUITMENT AND 1991 MORTALITY EVENT OF DIADEMA-ANTILLARUM IN THE FLORIDA-KEYS

Population densities and recruitment of Diadema antillarum were monito red on reefs of the Florida Keys beginning in 1990/1991. Signs of a mo rtality 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 of f Key West in April 1991, reducing densities, which ranged from 0.02-0 .60.m-2, by 97%. Size frequency distribution shifted from one dominate d by large individuals to one dominated by small recruits. Between fal l 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 19 91 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 highe r 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 t his settlement, densities of sea urchins did not increase at any of th e study reefs. Low adult densities, low recruitment and the chronic oc currence of mortality events in the Florida Keys indicate it is possib le that densities of D. antillarum will never regain pre-mass mortalit y levels.