O‘ahu’s marine protected areas have limited success in protecting coral reef herbivores

Abstract

Herbivorous fishes and urchins can contribute to reef resilience by removing algae that are competitive with corals, yet herbivorous fishes are frequently targeted by fishers. The deleterious effects of fishing on coral reef herbivore populations are thought to be decreased through the establishment of no-take marine protected areas (MPAs). We conducted surveys to compare the biomass, diversity, and size structure of herbivorous reef fish within three functional guilds (i.e., grazers, scrapers, and browsers) and herbivorous urchin populations inside and outside of four of O‘ahu’s current MPAs to determine if they were effectively protecting herbivorous fishes and assess any resulting effects on sea urchin competitors. We also characterized the benthos, including the identification of common taxa in algal turfs, to evaluate interactions between herbivory and benthic communities. Urchins and grazing fishes accounted for the majority of herbivore biomass across regions of O‘ahu, and the dominance of urchins suggests that they significantly contribute to herbivory. Coral cover was a significant predictor of urchin biomass, but urchins did not have a significant relationship with herbivorous fish biomass or MPA protection. MPA protection and depth had positive effects on herbivorous fish biomass when examined in aggregate. However, these results were largely influenced by a large MPA protection effect in the Hanauma Bay MPA, compared to weaker MPA effects in Kāne‘ohe Bay, Waikīkī, and Pūpūkea. Hanauma Bay had stark differences in the condition of reef habitats between the MPA and fished areas which may also confound the interpretation of a strongly significant protection effect. Depth and habitat complexity were significant predictors of both grazer biomass and aggregate herbivore community composition, suggesting that these bottom-up factors may have a greater impact on herbivorous fish biomass than protection. Turf communities were speciose and displayed similar trends to herbivore communities across regions, but within-region trends in the Hanauma Bay region suggest that turf species composition is not only dictated by top-down factors. Within-guild fish diversity was low across site and region, suggesting herbivory was dominated by a single or few species. The small size structure and densities of herbivorous fishes in MPAs across all regions limits their population-level functional and reproductive contributions. While MPAs serve as a popular fisheries management tool for coral reef environments, the limited benefits of MPAs for herbivorous fish and urchin populations around O‘ahu suggest a general need for improved fisheries management and enforcement, especially prior to the consideration of an expansion of the existing MPA network.