Mass settlement and post-settlement mortality of Epinephelus merra (Pisces: Serranidae) on Reunion coral reefs

An unusual mass settlement event of the grouper Epinephelus merra occurred on the fringing coral reefs of Reunion Island (south-western Indian Ocean) in April 1994. This major event coincided with stormy conditions associated with the new moon period. The abundance of new settlers was surveyed for 45 days following settlement on both the reef flat and the back reef area of the coral reefs of St Gilles, La Saline and St Leu. Mean densities of settlers were higher on reef flats (maximum 474 ind/20 m(2) at La Saline) than on back reef areas (maximum 156 ind/20 m(2) at La Saline). Seven weeks later, densities of settled E. merra had drastically decreased, and did not differ with site and zone (18.5 ind/20 m(2)). Percentages of post-settlement mortality at that period ranged from 85 to 88% in back reef areas and from 93 to 95% on reef flats. A strong correlation between density of recruits and per capita mortality suggests density-dependent early postsettlement mortality for E. merra. The limitation of food resources and shelters, perhaps combined with an increase of diseases due to high population densities of settlers, was probably the main cause of mortality just after the mass settlement. Predation by reef fish, including cannibalism between settlers, is probably responsible for most subsequent mortality.