Evidence for managing herbivores for reef resilience

Herbivore management is an important tool for resilience-based approaches to coral reef conservation. Yet, evidence-based science is needed to enact successful management. We synthesized data from multiple monitoring programs in Hawaii to measure herbivore biomass and benthic condition over a 10-year period preceding any major coral bleaching. We analyzed data from 20,242 transects alongside data on 27 biophysical and human drivers and found herbivore biomass was highly variable throughout Hawaii, with high values in remote locations and the lowest values near population centers. Both human and biophysical drivers explained variation in herbivore biomass, and among the human drivers both fishing and land-based pollution had negative effects on biomass. We also found evidence that herbivore functional group biomass is strongly linked to benthic condition, and that benthic condition is sensitive to changes in herbivore biomass associated with fishing. We show that when herbivore biomass is below 80% of potential biomass benthic condition is predicted to decline. We also show that a range of management actions, including area-specific fisheries regulations and gear restrictions, can increase parrotfish biomass. Together, these results provide lines of evidence to support managing herbivores as an effective strategy for maintaining or bolstering reef resilience in a changing climate.