Bottlenose dolphins in the north-western Adriatic Sea: Abundance and management implications

1. The Adriatic Sea is one of the Mediterranean areas most heavily impacted by fishing and other human stressors. The northern part of the basin has been certified as an Important Marine Mammal Area because of the regular occurrence of common bottlenose dolphins, Tursiops truncatus. 2. Boat surveys, totalling 76 days at sea and 10,711 km of navigation, were conducted between April 2018 and October 2019 to assess cetacean abundance within a 3,000-km(2) area off Veneto, Italy. Bottlenose dolphins - the only marine mammal species observed - were encountered on 52 days and were tracked for 81 h and 26 min, resulting in 15,066 dorsal fin photographs of high quality and resolution. 3. Various capture-recapture models were applied on individual photo-identification datasets. Model-based estimates indicate that approximately 600 individuals occurred within the study area during the sampling period in both years. Abundance varied monthly: minimum estimates were obtained in May 2018 (291 individuals; 95% CI 134-630) and May 2019 (121; 95% CI 20-721), whereas maximum estimates were obtained in September 2018 (385; 95% CI 310-477) and October 2019 (494; 95% CI 378-645). 4. Evidence provided by this study can be used to complement and validate coarse 'snapshot' information from recent aerial surveys of the entire Adriatic Sea, and to enforce management action mandated by the European Community (EC) Habitats Directive and Marine Strategy Framework Directive, as well as guiding the EC's Maritime Spatial Planning.

Automated summary

The study investigated the abundance of bottlenose dolphins in the Adriatic Sea, revealing monthly fluctuations in their numbers. The findings can complement aerial survey data, guide management actions, and support the European Union's maritime spatial planning initiatives.