Bottlenose dolphin reliance on trawlers in prey-depleted Adriatic Sea regions

The Adriatic Sea is one of the areas most exposed to trawling worldwide. The common bottlenose dolphin Tursiops truncatus—the only cetacean species regularly present in the northern and central portions of the basin—is known to forage and scavenge behind trawlers. We investigated dolphin occurrence behind three types of trawlers off the Italian regions of Veneto (3000 km²) and Marche (2000 km²), using 8 years (2018–2025) of survey data and 17,755 km of navigation effort from independent research boats. A total of 859 trawlers were checked for dolphin presence/absence across 148 days of trawling. Overall, dolphins were observed in the wake of 24.7% of the trawlers: 41.3% of the bottom otter trawlers (n = 375), 35.1% of the midwater pair trawlers (n = 148), and 1.5% of the beam trawlers (n = 336). Dolphin occurrence behind otter trawlers varied significantly between regions: dolphins followed one quarter of the vessels off Veneto (25.9%, n = 259), and three quarters of those off Marche (75.9%, n = 116). In the sampled regions, the large majority of bottlenose dolphins appear to be strikingly reliant on trawl gear, not only specialising in foraging and scavenging behind operating trawlers, but also targeting considerable proportions of the trawl fleets, possibly in response to severe prey depletion and habitat disruption by fisheries. The net effects on dolphin fitness and demography remain unknown. As trawlers have been causing profound damage to Adriatic ecosystems, the promotion of their continued use is inadvisable, not only to protect dolphins but also to conserve marine biodiversity writ large.