Human exploitation shapes productivity–biomass relationships on coral reefs

Coral reef fisheries support the livelihoods of millions of people in tropical countries, despite large‐scale depletion of fish biomass. While human adaptability can help to explain the resistance of fisheries to biomass depletion, compensatory ecological mechanisms may also be involved. If this is the case, high productivity should coexist with low biomass under relatively high exploitation. Here we integrate large spatial scale empirical data analysis and a theory‐driven modelling approach to unveil the effects of human exploitation on reef fish productivity–biomass relationships. We show that differences in how productivity and biomass respond to overexploitation can decouple their relationship. As size‐selective exploitation depletes fish biomass, it triggers increased production per unit biomass, averting immediate productivity collapse in both the modelling and the empirical systems. This ‘buffering productivity’ exposes the danger of assuming resource production–biomass equivalence, but may help to explain why some biomass‐depleted fish assemblages still provide ecosystem goods under continued global fishing exploitation. Using a large spatial scale dataset we find that, unlike often assumed, standing biomass is not an adequate proxy for biomass production rates on coral reefs. Distinct local productivity–biomass relationships emerged in regions with high and low average biomass. Low‐biomass regions had larger human populations, smaller fish sizes and higher production per unit biomass. Simulations showed that size‐selective fishing can trigger compensatory productivity at low biomass, generating the patterns observed. This novel ecological mechanism could help to explain why sustained fisheries yields occur even with widespread biomass depletion due to overexploitation.