MASS MORTALITY OF DIADEMA-ANTILLARUM .2. EFFECTS ON POPULATION-DENSITIES AND GRAZING INTENSITY OF PARROTFISHES AND SURGEONFISHES

It has been hypothesized that herbivorous fishes and the regular echinoid Diadema antillarum Philippi compete for benthic algae as their major food resource. Mass mortality of D. antillarum in February 1984 provided the opportunity to test the hypothesis that herbivorous fishes and sea urchins were competing previously. Visual censuses of herbivorous fishes conducted over 4 yr in four reef zones on Tague Bay Reef, St. Croix, U.S. Virgin Islands, before and after the mass mortality indicated that population densities increased approximately three-fold in backreef and shallow (2m) forereef zones and two-fold, and four-fold in mid (5m) and deep (10m) forereef zones, respectively. Juvenile parrotfishes constituted the major component of these increases, except in the shallow forereef where acanthurids became most abundant. Grazing intensity by herbivorous fishes increased in three of the four reef zones immediately following the mass mortality. These data support the hypothesis that exploitative competition for algal resources was occurring prior to the sea urchin mass-mortality, although alternative hypotheses cannot be discounted completely. Despite the increases in the abundances of, and grazing by, herbivorous fishes, the algal community continued to increase in percent cover and biomass, indicating that increased grazing by fishes does not compensate for the loss of grazing by D. antillarum in controlling algal abundance and community structure. © 1990 Springer-Verlag.