Fishing trawlers drive the fine-scale daytime distribution of bottlenose dolphins at the Tiber River estuary (Western Mediterranean Sea)

Fisheries can strongly alter marine ecosystem dynamics, influencing the ecology and behaviour of top predators such as cetaceans. At the Tiber River estuary (Tyrrhenian Sea, Western Mediterranean), bottlenose dolphins (Tursiops truncatus) frequently interact with bottom trawlers operating off the coast of Rome. We combined five years (2019-2023) of boat-based survey data with fishing data derived from the Vessel Monitoring System (VMS) to assess the influence of trawling activity on dolphin distribution. Spatial and temporal correlations between dolphin presence and fishing effort were analysed using multivariate analysis and overlap metrics. Dolphin occurrence was modelled relative to environmental and vessel-level predictors with generalized linear mixed-effects models (GLMMs), incorporating chlorophyll-a concentration (concurrent and lagged) as a proxy for productivity. In the area where sufficient survey effort was invested, dolphin presence was strongly associated with fishing hotspots, and dolphins shifted farther offshore on trawling days, indicating an ecological switch from nearshore natural resources to trawler-associated prey. Spatial overlap varied markedly among vessels, indicating preferences for certain trawlers. GLMMs identified specific vessels with a significant positive effect on dolphin occurrence, even after controlling for productivity. Catch analysis revealed these vessels landed higher amounts of European hake and common octopus, key dolphin prey species. Findings suggest dolphins exhibit flexible, and possibly socially acquired, foraging strategies in response to fisheries, with trawling acting as a strong driver of their distribution. While adaptive, such associations may reduce natural foraging, alter ecological dynamics, and pose conservation issues. This study provides a fine-scale perspective of fishery-driven behavioural change in dolphins and offers actionable insights for local management under EU marine biodiversity directives.