Deeper Caribbean reef fish communities show greater taxonomic and functional change in dominance structure over a nine-year period.

Fish communities at greater depths on a reef are thought to be less affected by disturbances that more strongly impact shallower areas. As a result, these deeper communities might be expected to show less change in their diversity and composition over time compared to those in shallow water. To test this hypothesis, we analysed changes in reef fish composition at 5-15 m and 25-40 m on reefs around Utila, Honduras, across two time periods: 2014-2015 and 2022-2023. We estimated taxonomic and functional α- and β- diversity using coverage-based standardisation and Hill-Chao numbers at orders q = 0 (species richness) and q = 2 (inverse Simpson index). Results showed that the α-diversity of fish communities was more consistent at 25-40 m than at shallower depths between the two time periods. However, β-diversity of dominant species and traits (q = 2) increased at greater depths, indicating that deeper fish communities became more distinct from one another in both structure and function, as well as more different from shallower communities at the same sites. Changes in diversity also varied between sites, highlighting the role of sitespecific conditions in shaping and maintaining fish communities across depths. Overall, the findings are not consistent with the expectation that greater depth reduces temporal community variability, and they raise questions about whether depth alone can serve as a refuge for reef fish.

Supplementary information: The online version contains supplementary material available at 10.1007/s00338-025-02709-7.