Depth structure and habitat associations of flatfish assemblages in a productive semi-enclosed sea

Flatfish are key components of demersal ecosystems and valuable targets of fisheries, yet their community structure and habitat associations remain poorly resolved in the Mediterranean. Using 20 years of data from a standardized bottom-trawl survey in the western part of the North–Central Adriatic Sea, we aimed to describe flatfish assemblage composition along habitat gradients and identify spatially specific indicator taxa. Using haul-level data from 2021, 2023, and 2024, we further related flatfish assemblages to benthic invertebrate biomass, bottom water temperature, and seafloor substrate type to examine habitat associations. Assemblages differed significantly among the bathymetric strata investigated (North: 10–50 and 51–100 m; Central: 10–50, 51–100, 101–200 and 201–500 m; PERMANOVA, R² = 0.73). The shallow-shelf communities (10–50 m) were characterised by soleids, pleuronectids, bothids and scophthalmids, while in central deep-shelf (101–200 m) and upper-slope assemblages (201–500 m), scophthalmids and cynoglossids became progressively more prominent. Bathymetric strata were characterised by different indicator taxa, with the highest number found in the northern shallow shelf. Despite sharing the same depth range, the northern and central shallow-shelf assemblages remained markedly different, indicating that depth alone does not explain community organisation. Variation partitioning confirmed significant independent contributions of bottom temperature and substrate type. Together, these results provide the first spatially resolved baseline for flatfish assemblage organisation and habitat associations for the North–Central Adriatic Sea and highlight the value of long-term standardized surveys for monitoring future reorganisation of demersal communities under climate change