Population status of the urchin Diadema antillarum in the Florida Keys 25 years after the Caribbean mass mortality

The 1983-84 Caribbean-wide mortality of the urchin Diadema antillarum Philippi was followed by a second mortality event in the Florida Keys in 1991. The demise of this once ubiquitous herbivore is one factor contributing to wider Caribbean reef change during the past 25 years. Over an 8-year period from 1999-2007, we examined densities and test sizes of D. antillarum at 786 sites from the northern extent of the Florida Reef Tract to the Dry Tortugas, including two National Parks and the Florida Keys National Marine Sanctuary. Visual surveys along belt transects were used to enumerate individuals and test sizes in a two-stage stratified random sampling design that incorporated cross-shelf benthic habitats, geographic regions, and no-fishing management zones. While pre-1983 densities were as high as 5 individuals/m, surveys since 1999 from < 1 m to 27 m depth show that current densities are still well below 1 individual/m. During seven different annual sampling periods, the maximum site-level density was only 0.33 individuals/m, with the highest densities of larger (> 5 cm test diameter) individuals reported from only a few locations. The relative contributions of larval survivorship, predation, suitable recruitment sites, and reduced fertilization success to Diadema recovery are still largely unknown.