Bottlenose dolphins (Tursiops truncatus) in the north-western Adriatic Sea: Spatial distribution and effects of trawling

Many species and populations of odontocetes have modified their behaviour to take advantage of feeding opportunities provided by fishing activities, with depredation of fishing gear being the most common type of adaptation. The northern Adriatic Sea has been identified as an important marine mammal area because of a regular occurrence of common bottlenose dolphins. Boat surveys were conducted within a 3,000 km(2) sector of the Adriatic Sea off the coast of Veneto, Italy, between April and October 2018-2019. Based on 76 days at sea, 10,711 km of navigation, and 81 h 26 min of dolphin tracking, this study contributes novel quantitative information on dolphin spatial distribution, and on their occurrence in the wake of beam trawlers, otter trawlers, and midwater pair trawlers. A combined generalized additive model and generalized estimation equation framework indicated that trawling-along with other physiographic, biological and anthropogenic variables-influenced dolphin distribution. In days of trawling, the chance of encountering dolphins increased by similar to 4.5 times (95% confidence interval 1.8-11.0) near active beam trawlers, by similar to 16.0 times (7.1-36.0) near otter trawlers, and by similar to 28.9 times (12.0-69.6) near midwater pair trawlers. Spatial modelling was used to create maps of predicted distribution, suggesting differences in habitat use between trawling and no-trawling days. Spatial modelling for all days identified a dolphin distribution hotspot of 832 km(2), situated off the Po river delta. Evidence contributed by this study can be used to inform management action within one of the world's areas most heavily impacted by fishing and other human encroachment. Such management action would help enforce the European Union's Habitats Directive and Marine Strategy Framework Directive, while also informing EU's Maritime Spatial Planning.