The Distribution of Herbivorous Coral Reef Fishes within Fore-reef Habitats: the Role of Depth, Light and Rugosity

Abstract

Examining the relationship between habitat characteristics and utilization patterns by herbivorous fishes on coral reefs will add to our understanding of the factors that influence the abundance and distribution of this important group. The abundances of parrotfishes (Scaridae) and surgeonfishes (Acanthuridae) on fore-reef habitats were sampled along an inshore-offshore gradient to provide for within reef and crossshelf comparisons in relation to the environmental parameters of depth and topographic relief. Temporally replicated visual surveys were conducted along permanent belt transects (100m 2 ) at three depth intervals (3, 10, 15 m) to obtain data on fish species density and lengths, which were used to calculate biomass. The roving herbivorous fish assemblage was dominated by three species of parrotfishes ( Scarus iseri, Sparisoma aurofrenatum and S. viride ) and three surgeonfishes ( Acanthurus bahianus , A. chirurgus and A. coerulus ). Overall the biomass of both families was highest at 3m compared to 10 or 15m (p<0.05). However, the relative decrease in biomass across depths for both families was greatest at inshore reefs where water transparency is lowest. The mean biomass for both families differed between inner and mid-shelf reefs at 10 and 15 m (p<0.05) but not at 3m. Fish biomass was correlated to reef topographic relief at 3m for parrotfishes (p<0.05) and at all three depth intervals for surgeonfishes (p<0.05). Overall patterns of herbivore biomass across the shelf reflect differences in light penetration, suggesting that fish may be responding to algal productivity. Thus, within fore-reef habitats along a cross-shelf gradient water transparency and topographic relief may interact to structure biomass patterns.