Fish and sea urchin herbivory and competition in Kenyan coral reef lagoons: the role of reef management

Abstract

The impact of “overfishing” on coral-reef herbivores was studied using Thalassia and Sargassum bioassays at two reefs protected from fishing for over 15 yr, one “transition reef” protected for ~ 2 yr, and three unprotected reefs. The primary goals of this research were to (1) assess the ability of an herbivory assay to distinguish between (a) herbivore types such as sea urchins and herbivorous fishes (i.e. parrotfish and surgeonfish), (b) rates of herbivory by sea urchins and herbivorous fishes, (2) potential impacts of coral-reef herbivores on seagrass species composition and abundance, and (3) the role that fishing plays in mediating competitive interactions between sea urchins and herbivorous fishes. Studied reefs differed in their management regulations and enforcement such that impacts of fishing regulations could be partly distinguished from species interactions. Parrotfishes appear to be the dominant fishes feeding on the assay. The sea urchin Echinothrix diadema (Linnaeus) exhibited a much higher preference for the assay than other sea urchin species. Consequently, the variable species composition of herbivorous fishes and sea urchins and their feeding preferences make between-site comparisons of herbivory problematic. However, if sites with high numbers of Echinothrix are excluded and the majority offish bites on the Thalassia assay are attributabed to a few species of parrotfish, then between-site and between-treatment comparisons of relative herbivory can be made. Experimental reduction of sea urchin abundance led to increased bite rates on herbivory assays by parrotfishes at protected site but not the unprotected sites. These results imply that sea urchins can reduce grazing rates of some species of parrotfishes. The species composition of seagrass communities in protected and unprotected reefs appears to be partially affected by prey choices of the dominant grazers such that parrotfish and Echinothrix sea urchins favor Thallasodendron dominance while other species of sea urchin such as Diadema setosum, D. savignyi (Audoin) and Echinometra mathaei de Blainville favor Thalassia dominance.