Benthic composition drives functional composition of fish assemblages despite varying recreational diving intensity

Abstract

Human activities are driving widespread degradation of natural ecosystems, with coral reefs among the most affected. While the impacts of climate change and pollution on coral reefs are well documented, the ecological consequences of recreational scuba diving, a rapidly growing industry, remain less understood. Namely, few studies have explored how diving influences trait diversity of coral reef fish assemblages, a key dimension of ecosystem resilience. Here, we investigated spatial variation in benthic composition and fish trait diversity across reef sites in Utila, Honduras, an island heavily reliant on dive tourism. We surveyed five sites of varying levels of diving intensity to quantify benthic cover and fish assemblages. Contrary to expectations, sites with high diving intensity did not show reduced coral cover, with the most heavily dived site (Stingray Point) maintaining the highest overall coral cover. Algae, however, dominated the benthos across all sites. While functional richness was maintained across the diving intensity gradient, functional composition shifted significantly among sites. This compositional shift was driven primarily by the Von Bertalanffy growth coefficient (k). Sites with low hard coral cover but higher benthic heterogeneity harboured fish communities dominated by fast-growing, early maturing species, whereas high hard coral cover sites supported assemblages of slower-growing, later-maturing species. Benthic composition, rather than diving intensity, emerged as the primary filter of fish life-history strategies. These findings suggest that while current diving intensity may not erode the volume of functional trait space, it is associated with distinct benthic states that filter for specific life-history strategies. Our results highlight the importance of maintaining benthic heterogeneity in reef systems, even in the absence of obvious coral loss. As functional diversity underpins key ecosystem services, it is important that coral reef monitoring focuses on changes in trait composition rather than just simpler taxonomic diversity metrics. Achieving this in collaboration with dive operators to promote low-impact diving is essential to sustaining reef resilience in tourism-dependent regions like Utila.