Let the fish do the cropping: identifying fish grazers to improve coral aquaculture

Controlling the growth of fouling organisms in coral aquaculture is a recognised approach to enhance survival during grow-out of recruits. Herbivorous fish can reduce algae growth, though indiscriminate grazing by the fish pose a risk to the early life stages of corals. To identify a suitable age or size to introduce fish to coral recruit culture, settlement tiles with 1-week-old, single-polyp and 1-month-old, multi-polyp Acropora millepora, Acropora kenti and Goniastrea retiformis were exposed to “brusher”, “cropper” and “concealed cropper” fish grazers for 24 h, in addition to a manual aquarist cleaned treatment and a control uncleaned treatment for comparison. In general, acroporid recruits displayed lower mortality than Goniastrea recruits across all types of grazing, and younger, smaller recruits were more vulnerable to grazing, with the “brusher” fish functional group more likely to cause mortality. Morality was low in the uncleaned controls for all corals, whilst single-polyp recruits generally saw slightly elevated mortality under manual aquarist cleaning. Grazing by the “brusher” Ctenochaetus binotatus resulted in the highest mortality across all treatments with week old, single-polyp recruits experiencing 2.5% and 8.6% mortality for A. millepora and A. kenti, respectively, and as high as 88.9% mortality for G. retiformis. In contrast, month old acroporids that were 2–7 polyps in size displayed < 1% probability of mortality when exposed to the same C. binotatus grazing. Grazing intensity of the fish also played a role, as fish belonging to the same functional group with higher bite rates caused higher recruit mortality. Overall, “cropper” Acanthurus nigrofuscus represented the best trade-off between minimising recruit mortality whilst reducing algae coverage on the settlement tiles. Based on our results and knowledge of coral recruit growth, coral grow-out operations would gain the most benefit by introducing fish grazers once corals reach the multi-polyp stage at > 1 month old for acroporids and other fast growing species, and later for smaller, slower growing species such as G. retiformis.