Impacts of parrotfish predation on a major reef-building coral: quantifying healing rates and thresholds of coral recovery

Parrotfishes are important Caribbean herbivores that are believed to indirectly benefit corals by grazing algae; yet, some species also feed on live coral, which may have direct negative impacts on coral growth and survivorship. Caribbean parrotfishes prey upon multiple coral species but have particularly high rates of predation on Orbicella annularis, a major framework building coral and an endangered species. While some researchers have suggested that parrotfishes may have significant long-term impacts on heavily targeted species such as O. annularis, the patterns of coral recovery from parrotfish predation scars remain poorly understood. To address this knowledge gap, we tracked the fate of parrotfish bite scars on O. annularis colonies across two Caribbean islands for up to 2 months. We evaluated differences in coral healing between islands in response to a number of variables including the initial scar surface area, scar abundance per coral colony, colony surface area, and water depth. We used these data to develop a predictive model of O. annularis tissue loss from recent parrotfish bite scars. We then applied this model to surveys of the distribution of bite scars at a point in time to estimate long-term tissue loss of O. annularis colonies from a standing stock of bite scars. Our findings suggest that the initial scar surface area is one of the most important predictors of coral tissue loss. The data also indicate that there are thresholds in patterns of coral tissue regeneration: we observed that small scars (≤ 1.25 cm2) often fully heal, while larger scars (≥ 8.2 cm2) had minimal tissue regeneration. The vast majority of observed scars (~ 87%) were 1.25 cm2 or less, and our model predicted that O. annularis colonies would regenerate nearly all the corresponding scar area. In contrast, while scars greater than or equal to 8.2 cm2 were infrequent (~ 6% of all observed scars), our model predicted that these larger scars would account for over 96% of the total tissue loss for grazed colonies. Overall, our results suggest that the immediate negative impacts of parrotfish predation on coral tissue loss appear to be driven primarily by a few exceptionally large bite scars. While further work is needed to understand the long-term impacts of corallivory and quantify the net impacts of parrotfish herbivory and corallivory on Caribbean coral reefs, this study is an important step in addressing factors that impact the recovery of a heavily targeted and ecologically important Caribbean coral from parrotfish predation.