Population status of the long-spined urchin Diadema antillarum in Puerto Rico 20 years after a mass mortality event

A mass mortality event of the long-spined sea urchin Diadema antillarum (Philippi, 1845) occurred during 1983–1984 across the western Atlantic. Recovery to pre-mortality densities has been slow throughout most of the Caribbean as current stocks remain low, existing at a small fraction of previously recorded levels. To measure population recovery at Puerto Rico, we surveyed 26 localities around the island during 2003–2004. At each reef, we deployed 15 × 2 m belt transects (n = 12) to detect and quantify the presence of D. antillarum and obtain data on benthic cover. Density of D. antillarum varied significantly among geographic regions and localities, though variability was high. The highest density was documented in the Culebra Island region (0.44 ind m⁻²), followed by the northern region of Puerto Rico (0.23 ind m⁻²). No individuals of D. antillarum were found in 11 of the 26 surveyed localities. These densities are still substantially lower than pre- and post-mortality estimates from Puerto Rico. Most of the reefs were characterized by high macroalgal cover and horizontal water transparency < 3 m. Urchin densities were significantly and positively correlated with percentage of coral cover, percentage of coralline algae, and water transparency.