The ecology of reef fish varies with depth, although patterns in diversity remain largely undescribed, in particular the complementarity of their taxonomic, functional and phylogenetic facets. In the present study we investigated patterns of taxonomic, functional and phylogenetic diversity of fish on 21 reefs, at depths ranging from 3 to 31m, at Príncipe Island (Gulf of Guinea). Taxonomic and functional diversity decreased monotonically with depth; the pattern was less accentuated for phylogenetic diversity. Functional diversity was saturated at high levels of taxonomic diversity, reflecting redundancy in species traits, particularly at the shallower reefs. Functional diversity increased linearly with phylogenetic diversity; thus, increasing niche availability seems to translate into a larger diversity of phylogenies. Dissimilarities in the structure and composition of fish assemblages among reefs were correlated with differences in depth, including a progressive turnover in species. Depth affected the functional traits of nearshore reef fish. Trophic breadth decreased with depth; carnivores and planktivores increased with depth, whereas herbivores decreased with depth. Small-sized fusiform fish dominated on the shallowest reefs. In summary, the present study demonstrated decays in biodiversity, from different perspectives, of reef fish with depth, which are connected with shifts in fish traits.