Fishing and environmental influences on estimates of unfished herbivorous fish biomass across the Hawaiian Archipelago

Abstract

A longstanding goal in coral reef ecology is to estimate the baseline states of ecological communities and the extent to which human activities have perturbed them. Often, baseline estimates rely upon observations from remote, uninhabited areas, highlighting dramatic differences in fish communities compared to populated areas. However, previous studies have focused on island- and archipelago- wide comparisons of reef communities that may mask finer-scale spatial variability. Here we describe spatial patterns of herbivorous fish biomass across fished and unfished regions of the Hawaiian Archipelago to evaluate whether accounting for environmental variability between these regions alters estimates of unfished biomass (Bunfished) and fisheries depletion. We found environmental factors were strongly associated with 4 measures of herbivorous fish community biomass (total, scraper, grazer, and browser), and failure to account for these influences on Bunfished resulted in different conclusions about herbivore depletion in Hawaii, USA. Overall, depletion estimates that controlled for environmental differences indicated that biomasses of the 4 herbivorous fish groups across much of Hawaii are near or above a commonly used sustainability reference point (0.5 × Bunfished). However, scraper and browser biomass on Oahu, the island with the highest human population density, were near or below 15% of Bunfished, highlighting the potential importance of spatial and functional group variability when assessing fishing effects on coral reefs. These findings are of immediate use to resource managers in Hawaii and applicable to other reef systems where embracing spatial variability can aid in identification of conservation targets on coral reefs.