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

Abstract

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 km2 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 ~4.5 times (95% confidence interval 1.8–11.0) near active beam trawlers, by ~16.0 times (7.1–36.0) near otter trawlers, and by ~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 km2, 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.

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

“…The northern Adriatic is subject to strong oceanographic and trophic fluctuations (Degobbis et al, 2000; Russo, Rabitti & Bastianini, 2002; Solidoro et al, 2009; Fortibuoni et al, 2010; Mozetič et al, 2010; Lotze, Coll & Dunne, 2011) that may influence the movements of bottlenose dolphins and determine variations in abundance and distribution (Bearzi et al, 2008; Fortuna et al, 2018). Trawling effort also may influence dolphin distribution and movements, through dolphins foraging behind operating trawlers and possibly concentrating in trawled areas when this type of fishing is allowed (Sala et al, 2018; Genov et al, 2019a; Bonizzoni, Furey, & Bearzi, 2021). Finally, anthropogenic noise is another potentially important factor that may influence dolphin distribution (Rako‐Gospić et al, 2017).…”

“…The available information suggests that although bottlenose dolphins have managed to persist in the northern Adriatic – whereas common dolphins have not – their numbers have declined considerably from historical baselines (Bearzi, Holcer & Notarbartolo di Sciara, 2004). Clearly, the mere occurrence or persistence of a species as resilient and behaviourally flexible as the bottlenose dolphin should not be regarded as a proxy of ecosystem health (Borja et al, 2013; Bearzi, Piwetz & Reeves, 2019; Bonizzoni, Furey & Bearzi, 2021).…”

“…Conversely, a mere intensification of research effort clearly won’t help address the current threats, and therefore calls for ‘more research’ should not be used as an excuse for delaying conservation action (Bearzi, 2007). Notwithstanding such a caveat, a better understanding of dolphin abundance and distribution can be obtained through field monitoring – based on methods including those outlined in this study, spatially explicit capture–recapture modelling (Borchers & Efford, 2008; Pirotta et al, 2015), distribution modelling (Bonizzoni, Furey & Bearzi, 2021), and line‐transect distance‐sampling surveys (Fortuna et al, 2018). Such monitoring can help to clarify dolphin responses to human activities and support the implementation of management strategies that alleviate human impact, in compliance with formal commitments to protect marine mammals and marine biodiversity (EC, 1992; EC, 2008; EC, 2014).…”

Capitoline Dolphins: Residency Patterns and Abundance Estimate of Tursiops truncatus at the Tiber River Estuary (Mediterranean Sea)

“…The main targets of fishing are demersal species, and these may be the main prey for bottlenose dolphins in the Mediterranean [ 34 ]. Indeed, interactions among fishery and bottlenose dolphins are commonly observed in the area [ 26 ], as in other Mediterranean regions [ 35 , 36 ], but no data are available to quantify the effects in the study area, both on the commercial activity and on the dolphin population.…”

Addressing Cetacean–Fishery Interactions to Inform a Deep-Sea Ecosystem-Based Management in the Gulf of Taranto (Northern Ionian Sea, Central Mediterranean Sea)

“…Indeed, sharing the same marine resources can lead to conflicts between cetaceans and fisheries [24]. For instance, if several odontocetes recognize the depredation of fishing nets as a feeding opportunity, the excessive exploitation of fishery resources due to poor management of the sector can have a major impact on the marine food web and be a primary cause of odontocetes' decline [25,26]. Furthermore, overfishing consequences are driving fishing activities towards deep-sea environments [27][28][29] with possible amplification of their impact on cetaceans.…”