We assessed the ability of socioeconomic variables (population size, perceived infringement, and community infrastructure) and design features (closure age and area) to predict ecological indicators of “success” in seventeen coral-reef fishery closures in the Western Indian Ocean. Success was measured as absolute fish biomass and coral cover in closures, and the response ratio of these variables indicating the level of difference relative to control sites outside closures. Fish biomass had a greater and more consistent response to protection than coral cover. Human population density had a strong positive association with the response of fish biomass, which was driven by lower biomass outside marine protected areas (MPAs) in high human population density sites, rather than higher biomass within MPAs. Perceived infringement was negatively associated with the absolute and response ratio of fish biomass. Coral cover was variable, weakly related to closure, and positively related to closure size and human population density. This, and previous regional studies, indicate that physical design features have a modest effect on the response of fish to MPAs, that design features interact with compliance, and that human population density around closures can indirectly influence the effect of MPAs on fish populations.