Decadal changes in the fish assemblage structure and function within an insular coral community

Fish are a key group in any coral community, and their assemblage varies with differences in habitat structure among reefs, as well as with seasonal and interannual environmental fluctuations that influence community structure and ecosystem functioning. Long-term monitoring, combined with taxonomic and functional indexes, can provide valuable information on the effects of local and regional environmental fluctuations on any biological group. This study evaluated the decadal (2013–2022) variability of fish assemblage and their function in a Natural Protected Area composed of a coral community distributed along two islands, using diurnal visual censuses along belt transects (5 transects/site/season/year) in a 3–13 m depth range. The results showed higher values of species richness, functional richness, evenness, and redundancy on Isla Redonda, which also exhibited the highest observed diversity of benthic components, suggesting a potential association between benthic diversity and fish assemblage structure. Throughout the decade, fish assemblages exhibited clear temporal trends influenced by seasonal cycles, large-scale climatic events, and habitat changes. Especially during the El Niño 2015–2016 event, an abnormal increase in temperatures led to marked shifts in species composition and a decline in the abundance of schooling planktivorous species, such as Apogon pacificus. This was followed by a reduction in species richness in 2018, primarily due to habitat homogenization resulting from a decline in benthic diversity. The loss of benthic diversity and more frequent extreme meteorological events will likely disrupt species interactions, affecting coexistence, resource use, and long-term community stability. Although connectivity between islands currently acts as a buffer, this could be compromised if environmental conditions deteriorate (e.g., natural and anthropogenic stressors), causing a significant decline in the reef’s health and provision of ecosystem services.