The importance of biophysical context in understanding marine protected area outcomes for coral reef fish populations

Multiuse marine protected areas (MPAs) that utilize a mosaic of no-take and sustainable use zones are an increasingly common tool for promoting fish population recovery while minimizing socioeconomic conflict. However, significant gaps remain in our understanding of the effects of multiuse MPAs on reef-fish populations, and how protection from fishing may interact with biophysical gradients to drive ecological outcomes. Here, we examine changes in fish assemblages inside and outside of two multiuse MPAs in Raja Ampat, Indonesia, over the first four to five years after their implementation. We use linear models to assess the impact of protection status, anthropogenic pressure, and biophysical gradients on changes in four biological metrics: abundance, biomass, mean size, and size-spectra. In addition, we use multivariate analyses to assess whether fish assemblages in protected and unprotected reefs diverged after MPA establishment. We find that both fish abundance and biomass were driven equally or more by benthic characteristics than protection status and that these environmental impacts were decoupled from the effects of MPAs. Contrary to expectations, increases in abundance were more pronounced in unprotected reefs, where fish assemblages diverged from protected reefs through an increase of herbivorous parrotfishes (Subfamily Scarinae) likely driven by increased algal production on hard-bottom reef areas. However, mean size and size-spectra improved in sustainable use zones compared to unprotected reefs, demonstrating that non-exclusionary management can provide ecological benefits, especially when combined with nearby no-take areas. Increased mean size was also associated with lower human population sizes, suggesting a strong influence of fishing pressure, which is mediated by MPAs. Our findings suggest that multiuse MPAs can quickly achieve some fisheries management objectives and that these effects are likely to increase over longer timescales. However, biophysical context can have significant impacts on reef-fish assemblages that are distinct from the effects of overlying fisheries management.