Warming-driven biotic differentiation of demersal fish across depths in the central mediterranean sea

Seawater warming, one of the facets of climate change, plays an important role in the redistribution of biodiversity by displacing cool-water species and promoting the expansion of warm-water species in certain regions. Many studies have assessed the effects of warming on species richness and composition at regional and global scales, but few have focused on vertical shifts in the composition of marine assemblages as a result of changing thermal conditions at different depths, particularly in the Mediterranean basin. Here, we analyze a time series from 2007 to 2023 of teleost fish assemblages monitored in 1678 trawl hauls in the Strait of Sicily (central Mediterranean), ranging from 10 to 800 m depth. The study area is considered an important biodiversity hotspot and a corridor for the westward expansion of warm-water species from the warmer Levantine basin. By estimating the rate of change in beta-diversity of demersal fish assemblages across depths, we observed a rearrangement of species composition driven by changes in bottom temperature. We found an increase in assemblage differentiation strictly associated with a change in depth range occupancy of species, while local and regional diversity did not change significantly. Our results suggest a key role for seawater warming in differentiating demersal fish community composition along depth gradients and anticipate potential similar responses in other areas of the Mediterranean. Understanding how bathymetric variation affects taxonomic composition, which may result from depth expansion or reduction of species with non-equivalent functional roles within ecosystems, could help predict changes in ecosystem functioning and services, and prevent overexploitation of fishery resources as their depth distribution changes.