Fish response to macroalgal removal

Abstract

Coral reefs are rapidly degrading into macroalgae dominated systems sustained through positive feedback loops. Macroalgae can harm corals directly via competition and indirectly via suppression of recruitment and increasing susceptibility to disease. This suppresses coral resilience, enhances macroalgal resilience and abundance, and decreases fish herbivory and recruitment. To break macroalgal feedback loops manual macroalgae removal may be an option. This study removed macroalgae in paired 4m2 removal and control plots (n=12-14) in a backreef lagoon of Moorea, French Polynesia, and (1) analyzed fish responses to macroalgal removal at days 0, 7, 30, and 55, and (2) measured macroalgal recovery at day 55. Immediately following removal (day 0), total fish visitations per minute and bites per minute increased by 232% and 575% respectively. Herbivore visitations increased by 172% at day 0 and remained elevated by 26-63% from day 7 through 55 post removal. In contrast, after an initial 588% increase in invertivore visits, there were no significant differences in invertivore visits at later dates. These results support the argument that dense macroalgae deter herbivory, create refuges from herbivores and invertivores, and create feedback loops enhancing macroalgal resilience. Total macroalgal abundance did not recover over the 55 day duration of monitoring; at day 55, total cover of macroalgae was 21% lower and barren substrate was 25% higher than at day 0. Thus, at these spatial and temporal scales of manipulation, manual removal of macroalgae allows fishes to maintain low macroalgal cover. These results suggest that manual removal of macroalgae prior to coral recruitment seasons may allow herbivores to maintain lower cover or algae, greater cover of bare substrate, and possibly facilitate coral recruitment – potentially enhancing coral resilience and reef recovery. The spatial scales over which manual macroalgae removal can be used for long-term phase shift reversal are uncertain and an area for further evaluation.