Depth and habitat drive spatial patterns in fish functional diversity: a trait-based assessment across South Africa and southern Mozambique

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Understanding the full spectrum of biodiversity, including functional diversity, is essential for assessing ecosystem resilience and informing conservation strategies. This study assessed the functional structure of coastal fish assemblages across a broad latitudinal gradient spanning multiple Marine Protected Areas (MPAs) along South Africa's coastline and into southern Mozambique. We analysed over 1000 baited remote underwater video system (BRUVs) and stereo-BRUVs samples using a trait-based approach to explore spatial patterns in fish functional diversity and assess the influence of depth, habitat type, and protection level. Community structure varied significantly across ecoregions and habitat types, with depth emerging as the strongest environmental driver of functional composition. Deeper habitats were associated with lower functional richness, dispersion, and evenness, suggesting stronger environmental filtering. Notably, functional convergence was observed between the Agulhas and Delagoa Ecoregions, likely reflecting shared environmental constraints that favour similar functional traits, such as large, high-trophic-level predators, including sharks and rays. In contrast, sites in the Natal Ecoregion exhibited distinct but homogenous functional profiles, with assemblages characterised by smaller-bodied, mobile pelagic species and generalist foragers. MPAs demonstrated varying patterns of functional diversity, with no consistent patterns across protection levels. While some MPAs supported higher functional redundancy, others showed reduced functional richness and dispersion, suggesting site-specific influences, possibly different habitat types or specific management regimes. Our findings emphasise the importance of considering functional metrics to complement taxonomic approaches and to better understand biodiversity patterns and conservation outcomes. By highlighting spatial variability and ecological drivers of functional diversity, this study contributes to a more nuanced understanding of ecosystem functioning and supports the need for regionally tailored conservation planning and sustained monitoring across South Africa's marine ecosystems.

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