Bottlenose dolphin detection and approach to distant trawlers in the northwestern Adriatic Sea

Abstract

Common bottlenose dolphins Tursiops truncatus have adapted to foraging and scavenging behind fishing trawlers in many areas around the world. This opportunistic behaviour is widespread in Adriatic Sea waters off the region of Veneto, Italy. On October 1st, 2020, we tracked a group of bottlenose dolphins from a 6 m boat, north-east of the Po River Delta. The dolphins foraged behind midwater pair trawlers, then four individuals abandoned the vessels and engaged in fast, highly-directional movement toward another distant pair of midwater trawlers, at speeds of 8-14 km/h, as the trawlers moved away from the dolphins. After 29 min of fast directional travelling, the dolphins reached the midwater trawlers and immediately started foraging in their wake. GPS tracking of dolphin positions, combined with trawler positions obtained from AIS data matching vessel name/plate, revealed that the trawlers were 2.6 km away from the dolphins when the animals first appeared to have detected them – consistent with reports of detection-and-approach in other seas. While bottlenose dolphins in our 3,000 km2 study area frequently moved from one fishing trawler (or pair of trawlers) to the next, the trawlers’ detection-and-approach event reported here was the longest observed during our 7-year study, totalling 721 encounters with fishing trawlers and 17,432 km of survey effort during 129 days of monitoring trawling activities. Longdistance detection can theoretically enhance foraging opportunities, though the substantial negative effects of trawling on marine ecosystems make it difficult to disentangle causes and effects, and determine whether there may be net demographic benefits to the dolphins.