Response of fishes to algae reduction on Glovers Reef, Belize

Many Caribbean coral reefs have experienced an increase in erect brown algae (species of Sargassum, Turbinaria and Lobophora) over the past 18 yr. We explored the effects of fleshy algal overgrowth on coral reef fishes by reducing erect algae by similar to2.5 kg(wet) m(-2) on 8 patch reefs (average size similar to 1000 m(2)) whereby half were in a new no-fishing zone and half in an unrestricted fishing zone. Another 8 reefs were left as unmanipulated controls in the respective zones. Multivariate ordination indicated that the algal removal had marginal effect on whole-fish assemblages but that effect was highly significant on the biomass of common herbivores. The reduction of erect algae resulted in a rapid increase in the abundance of the blue-headed wrasse Thalassoma bifasciatum (Labridae), in the biomass of the blue tang Acanthurus coeruleus (Acanthuridae), and in both the abundance and biomass of the spotlight parrotfish Sparisoma viride (Scaridae). Bite rates and intra- and inter-specific aggressive encounters were used as measures of resource quality, and we found that these measures increased for surgeonfishes and damselfishes after the algal reduction, particularly in the center of the patch reefs, where most erect algae was originally located. Increased accessibility, net production and palatability of the early successional turf algae on the manipulated reefs are likely to account for the increased numbers, biomass and feeding rates of the dominant herbivorous fishes.