Fish-based herbivory and the natural maintenance of algal fouling on coral nurseries used across the Great Barrier Reef

Abstract

In-situ coral nurseries have become widespread across the Great Barrier Reef (GBR) as part of tourism-led stewardship efforts to grow and replant coral at sites of high socio-economic value. Practices are aimed at reducing costs and increasing propagation efficiency. Whilst the role of herbivorous fish in regulating coral-algal competition has been extensively studied on natural reefs, limited research has examined how similar benefits on nurseries could be harnessed to improve coral propagation success. Here, we assess the extent to which fish assemblages on the GBR reduce the growth of biofouling algae on coral nursery frames and improve the survival of Acropora species growing on frames. Cage-exclusion treatments, coupled with grazing plate assays and video fish surveys, were employed to quantify the maintenance effect of fish (>2.5 cm in length) at two reef stewardship sites on Opal Reef in the northern Great Barrier Reef. Exclusion of grazers from frames for 112 days led to a fourfold increase in algal biomass compared to treatments where fish were allowed access. Removal of algae by fish, in turn, led to a mean increase in coral survivorship by >25%, although coral responses to algal composition were species-specific, and Acropora verweyi exhibited greater survivorship than either A. muricata or A. cerealis. Analysis of fish biomass and bite rates revealed nursery fouling maintenance was almost entirely driven by individuals from the parrotfish and surgeonfish families, notably, Scarus globiceps (Globehead parrotfish) and Acanthurus nigricauda (Eye-line surgeonfish). Our results highlight the importance of accounting for the broader ecology of coral reef stewardship practices and detail important early considerations for more effective nursery design, positioning, and coral stocking arrangements.