Abundance and behaviour of detritivorous fish as indicators of reef degradation in the Pemba Channel, Tanzania

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Coral reef degradation is changing benthic habitats globally, with increasing transitions from structurally complex coral frameworks to rubble- and sediment dominated substrates. This study examined how habitat structure influences the abundance and behaviour of two surgeonfish, Naso brevirostris and Naso hexacanthus , across a coral–rubble gradient around Pemba Island, Tanzania, using Baited Remote Underwater Video systems (BRUVs). Forty deployments were analysed to quantify relative abundance (MaxN), behavioural prevalence and intensity, and temporal behavioural variation during each BRUV deployment. Depth and benthic composition were also analysed as environmental predictors. Abundance showed significant species-specific responses to habitat condition. N. brevirostris was associated with structurally complex coral habitats, whereas N. hexacanthus was more abundant in degraded rubble-dominated areas. In contrast, behaviour remained consistent, with no significant differences in behavioural prevalence or intensity between species or habitat types. Temporal analyses revealed short-term variation in feeding and social behaviours linked to changes in resource availability and aggregation dynamics, rather than habitat condition. Depth influenced the benthic composition but was not a significant predictor of fish abundance or behavioural responses. The findings suggest that habitat degradation changes the distribution of detritivorous fishes, while their behaviour remains stable across habitat conditions. The contrasting abundance patterns challenge broader generalisations of detritivore responses to reef degradation. Stable behaviour despite changes in species distribution suggests that key detrital processing functions may persist despite ongoing habitat simplification

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