Fish as indicators of coral reef ecosystem status

• Coral reef fish indicator relationships with reef benthic condition were tested. • Butterflyfish and grouper indicators are positively influenced by high coral cover. • Surgeonfish abundance and length are positively influenced by high turf algae cover. • Parrotfish biomass responds primarily to human gravity not benthic condition. • Most coral reef fish indicators are strongly related to human gravity. Monitoring coral reef health is essential for managing biodiversity and ecosystem services in the face of accelerating environmental change. Here, we assess whether aggregated reef fish indicators at the assemblage and family level can reflect variation in coral reef benthic condition, structural complexity, and human pressure, across 77 sites in the western and central Indian Ocean. Our findings reveal that fish indicators exhibit variable sensitivity and specificity to these ecological gradients. Human gravity was the strongest and most consistent predictor across both assemblage-level and family-level metrics, particularly for biomass and length-based indicators. Mean assemblage trophic level, grouper (Epinephelidae) and butterflyfish (Chaetodontidae) biomass, and surgeonfish (Acanthuridae) abundance were responsive to benthic condition, while grouper length was the most specific indicator of reef structural complexity. In contrast, parrotfish (Scarinae) indicators were not significantly associated with benthic condition, challenging the assumption that they are tightly linked to reef condition or degradation in this region. These results underscore the need for careful selection of fish indicators of coral reef status and demonstrate how family-level indicators can provide specificity in response to pressures that is not captured within aggregate assemblage indicators. We recommend that additional family-level fish indicators of coral reef ecosystem integrity and function be considered within Red List of Ecosystems assessments, and that targeted family-level indicators, such as butterflyfish and grouper biomass for benthic status, grouper length for rugosity and surgeonfish abundance for benthic status, complement existing reef benthic monitoring within global coral reef reporting frameworks such as the Global Coral Reef Monitoring Network.