Benefits of herbivorous fish outweigh costs of corallivory in coral nurseries placed close to a Kenyan patch reef

Abstract

Coral mariculture involves time-consuming removal of fouling. On natural reefs, this service is provided by grazers. As natural reefs also harbour corallivores, it is debated whether reef-bound fish have a positive or negative effect on coral maricultured near natural reefs. This study quantified the net impact of herbivorous and corallivorous fish on coral mariculture. Nursery trees either uncaged, caged or as cage-control (15 per treatment) were placed near a patch reef at Wasini, Kenya, each hosting 10 Acropora verweyi fragments. From April to July 2016, survival and growth of the corals and bite marks on the corals were monitored. Using remote underwater video, bites by herbivorous and corallivorous fish were quantified. Upon termination of the experiment, dry weight of fouling from the nursery trees was determined. Caging of nurseries strongly reduced herbivory and corallivory. Results of cage-controls were not significantly different from uncaged trees. In caged nurseries, coral survival and growth were significantly lower than in uncaged nurseries, respectively 9% and 40% lower. Fouling was nearly 800% higher in caged nurseries. Herbivory was dominated by the surgeonfish Ctenochaetus striatus, which was responsible for 77% of the grazing. Monthly assessments showed bite marks on 10% of the uncaged coral fragments. Our study reveals that fouling control by herbivorous fish outweighs the costs of incidental corallivory on the survival and growth of A. verweyi. The vigour of unrestricted fouling, its negative impact on coral performance and the scarcity of corallivory justify the recommendation to place coral nurseries in Wasini near the reef.