Low-quality sediments deter grazing by the parrotfish 'Scarus rivulatus' on inner-shelf reefs

Elevated sediment loads in the epilithic algal matrix (EAM) deter grazing by herbivorous fishes and may compromise their critical roles on coral reefs. However, the properties of sediments that drive herbivore deterrence are unknown. Binary choice trials in aquaria were used to examine the effects of three sediment attributes-sediment source, grain size and organic load-on grazing by the abundant inner-shelf parrotfish, Scarus rivulatus. Fish were presented with a choice between EAM-covered rocks treated with (a) terrigenous or reefal sediments, (b) fine or coarse sediments or (c) sediments with high or low organic loads. Scarus rivulatus did not show a preference for sediments from different sources (terrigenous vs. reefal); however, a clear preference was evident for fine-grained sediments over coarse (109 % more bites) and sediments with high organic loads over low (147 % more bites). The avoidance of coarse sediments is likely to be a key factor driving the inhibition of grazing on mid-shelf reefs, which are dominated by coarse sediments. In contrast, on innershelf reefs, grazing by parrotfishes may be deterred primarily by high sediment loads, which reduce the proportional organic content in EAM sediments. Our study highlights the potential impact of sediments on critical ecological processes and the threats posed by changing sediment loads on inner-shelf reefs.