Monitoring coral reef health is essential for managing biodiversity and ecosystemservices in the face of accelerating environmental change. Here, we assess whetheraggregated reef fish indicators at the assemblage and family level can reflect variationin coral reef benthic condition, structural complexity, and human pressure, across 77sites in the western and central Indian Ocean. Our findings reveal that fish indicatorsexhibit variable sensitivity and specificity to these ecological gradients. Human gravitywas the strongest and most consistent predictor across both assemblage-level andfamily-level metrics, particularly for biomass and length-based indicators. Meanassemblage trophic level, grouper (Epinephelidae) and butterflyfish (Chaetodontidae)biomass, and surgeonfish (Acanthuridae) abundance were responsive to benthiccondition, while grouper length was the most specific indicator of reef structuralcomplexity. In contrast, parrotfish (Scarinae) indicators were not significantlyassociated with benthic condition, challenging the assumption that they are tightlylinked to reef condition or degradation in this region. These results underscore theneed for careful selection of fish indicators of coral reef status and demonstrate howfamily-level indicators can provide specificity in response to pressures that is notcaptured within aggregate assemblage indicators. We recommend that additionalfamily-level fish indicators of coral reef ecosystem integrity and function be consideredwithin Red List of Ecosystems assessments, and that targeted family-level indicators,such as butterflyfish and grouper biomass for benthic status, grouper length forrugosity and surgeonfish abundance for benthic status, complement existing reefbenthic monitoring within global coral reef reporting frameworks such as the GlobalCoral Reef Monitoring Network.

Monitoring coral reef health is essential for managing biodiversity and ecosystemservices in the face of accelerating environmental change. Here, we assess whetheraggregated reef fish indicators at the assemblage and family level can reflect variationin coral reef benthic condition, structural complexity, and human pressure, across 77sites in the western and central Indian Ocean. Our findings reveal that fish indicatorsexhibit variable sensitivity and specificity to these ecological gradients. Human gravitywas the strongest and most consistent predictor across both assemblage-level andfamily-level metrics, particularly for biomass and length-based indicators. Meanassemblage trophic level, grouper (Epinephelidae) and butterflyfish (Chaetodontidae)biomass, and surgeonfish (Acanthuridae) abundance were responsive to benthiccondition, while grouper length was the most specific indicator of reef structuralcomplexity. In contrast, parrotfish (Scarinae) indicators were not significantlyassociated with benthic condition, challenging the assumption that they are tightlylinked to reef condition or degradation in this region. These results underscore theneed for careful selection of fish indicators of coral reef status and demonstrate howfamily-level indicators can provide specificity in response to pressures that is notcaptured within aggregate assemblage indicators. We recommend that additionalfamily-level fish indicators of coral reef ecosystem integrity and function be consideredwithin Red List of Ecosystems assessments, and that targeted family-level indicators,such as butterflyfish and grouper biomass for benthic status, grouper length forrugosity and surgeonfish abundance for benthic status, complement existing reefbenthic monitoring within global coral reef reporting frameworks such as the GlobalCoral Reef Monitoring Network.

Within low nutrient tropical oceans, islands and atolls with higher primary production support higher reef fish biomass and reef organism abundance. External energy subsidies can be delivered onto reefs via a range of physical mechanisms. However, the influence of spatial variation in primary production on reef fish growth and condition is largely unknown. It is not yet clear how variability in food delivery onto a reef interacts with reef depth and slope, and affects reef fish productivity. Here we test the hypothesis that with increased proximity to deep-water oceanic allochthonous nutrient sources, or at sites where transportation of these water bodies onto reefs is facilitated by shallower reef slopes, parameters of fish growth and condition will be higher, and this pattern will be further emphasised in areas naturally higher in primary production. Contrary to expectations, we found no association between fish growth rate and sites with higher mean chlorophyll values. There were no differences in δ15N or δ13C values in fish collected at greater depths across reefs, suggesting a homogeneous primary production resource. However, the relationship between fish condition and primary production was influenced by depth of collection, driven by higher fish condition at shallow depths within a study site which is a ‘hotspot’ of primary production. Carbon δ13C values were depleted at sites with increasing primary production, and this trend was reversed by an interactive effect with shallower reef slopes. Our results indicate that deep-water ocean nutrient influences did not translate into observable increases in overall population growth in planktivorous Chromis fieldi within the 10–17.5 m depth range, but show the importance of site specific variation in hydrodynamics and reef physical characteristics influencing fish carbon isotopic composition and condition.