Natural maintenance of biofouling on coral nurseries used across the Great Barrier Reef

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 ecological and socio-economic value. Such coral nursery practices are aimed at reducing costs and increasing propagation efficiency to remain effective at scale. Coral nursery structures are therefore routinely placed in close proximity (1-3 m) to natural reefs with diverse fish assemblages, where fish presence is assumed to reduce biofouling on nursery structures that compete with coral for space. However, the extent to which biofouling occurs has not yet been examined. We therefore assessed the effect of excluding fishes >2.5 cm on sessile community composition of coral nurseries at two GBR sites. Settlement substrates and coral fragments (Acropora species typically used for propagation on the GBR), were placed inside fish cage-exclusion treatments to assess this biofouling on nursery structures at the two reef sites for 112 days. Short-term video surveys were used to identify fish species present on nurseries that would be excluded by the cages. Across sites, mean filamentous turf biomass observed on settlement substrates after 30 days was ∼275-400% greater within cages than within uncaged treatments. At site Rayban, survival of Acropora kenti declined from 83% in controls to 17-33% in fish-exclusion treatments, while A. donei experienced complete mortality in full cages and higher, yet variable survivorship in control and partial-cage treatments (16-33%). In contrast, at site Mojo, A. aff. Abrolhosensis survived irrespective of treatment. Collectively, coral survivorship under enhanced biofouling was therefore species-specific and varied between sites. Fish assemblages on nurseries at both sites were numerically dominated by damselfish (pomacentridae); however, biomass largely comprised of species belonging to Labridae:Scarinae (parrotfishes) and Acanthuridae (surgeonfishes). Bite rate analysis indicated that Scarinae and Acanthuridae interacted most frequently with the nursery frames. We discuss the importance of understanding natural fish assemblage support of localised nursery operations for ensuring effective coral propagation, including use of low-cost experimental approaches we present here for the GBR.