Relative influence of environmental factors and fishing on coral reef fish assemblages

Understanding whether assemblages of species respond more strongly to bottom-up (availability of trophic resources and/or habitats) or top-down (predation pressure) processes is important for effective management of resources and ecosystems. Here, we determine the relative influence of environmental factors and predation by humans in shaping the density, biomass and species richness of four medium-bodied (10-40cm total length; TL) coral reef fish groups targeted by fishing (mesopredators, planktivores, grazer/detritivores, scrapers), and the density of two groups not targeted by fishing (invertivores, small fish ≤10cm TL) in the central Philippines. Boosted regression trees were used to model the response of each fish group to 21 predictor variables: 13 habitat-level, 5 island-level, and 3 fishing variables; no-take marine reserve (NTMR) presence/absence, size, and age. Targeted and non-targeted fish groups most strongly responded to habitat-level, then island-level variables, with fishing (NTMR) variables having lesser effects. Of the habitat-level variables, live hard coral cover and structural complexity (or a habitat complexity index), and depth had the greatest effect on the density, biomass and richness of targeted fish groups, and density of non-targeted fish groups, with proximity to the nearest river, and island elevation the most influential island-level variables of fish groups. NTMRs were influential only on fishes targeted by fishing, with NTMR size positively correlated with density, biomass and species richness of targeted fishes, particularly mesopredatory and grazing/detritivorous fishes. Importantly, NTMRs as small as 15 hectares showed positive effects on the medium-bodied fish groups. This finding provides reassurance for regions that have invested in small-scale community-managed NTMRs, often following a history of high fishing pressure. However, management strategies that integrate sound coastal land-use practices to conserve adjacent reef fish habitat, with strategic NTMR placement, and the implementation of larger NTMRs, will be crucial for maintaining biodiversity and fisheries. Article Impact Statement: Prioritizing strategic land-sea management that protects habitat for coral reef fishes will support biodiversity and fisheries. This article is protected by copyright. All rights reserved.