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

Abstract

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. 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‐km2 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. 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). 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.

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

“…A combination of these factors may explain the increasing number of dolphins we measured at the end of each survey season (September-October), when fishery with trawling vessels is prohibited for almost 30 days to preserve marine resources and allow the recovery of commercial target species. Disturbance of and pressure on demersal prey by fishing are reported to strongly affect the abundance of bottlenose dolphins [59]. This seems coherent with the resident individuals in our study area, which show higher presence in September, probably as a consequence of the interruption in fishing activities.…”

“…Under the parsimony principle in the POPAN model selection, our results indicate that bottlenose dolphins are abundant in the study area. The estimated abundance of the superpopulation is 529 individuals (CI 0.95 = (456; 614)), similar to that of the North Adriatic Sea [59]. However, the heterogeneous survey effort in the study area (see Figure 1), due to different constraints (e.g., distance from the berth place of the research vessel, weather forcing, distribution of the fishing gears), is likely to have influenced the abundance estimate.…”

“…As the population was open (Stanley and Burnham Closure Test chi-square statistic = 392.353, df = 29, p < 0.05; Otis Closure Test z-value = −13.287, p < 0.05), we used the POPAN data type [55], which uses a parameterization of the Jolly-Seber model [56], to estimate its abundance. Open-population models are the most widely used to estimate the abundance of bottlenose dolphin populations [57][58][59]. These models venture the existence of a superpopulation N, which represents the total number of individuals that are potentially available in the study area between the first and the last sampling occasion [60,61].…”

An ecosystem-based system of variables to enhance marine species and habitat monitoring and conservation: The Adriatic Natura 2000 case study

“…For what concerns the ranking, among the ecological variables, "density", "abundance", and "spatial distribution" were those with the highest scores. Their feasibility score was based on the monitoring approaches adopted in the area, i.e., aerial and boat-based surveys (Fortuna et al, 2018;Bearzi et al, 2021). Furthermore, these variables might be monitored also by citizens (Embling et al, 2015;Giovos et al, 2016), as already happens in certain Mediterranean areas (Ricci et al, 2018;Alessi et al, 2019).…”

Are we overlooking Natura 2000 sites? Lessons learned from a transnational project in the Adriatic Sea

“…Even less information is available on the spatial and temporal density and distribution of T. truncatus, C. caretta, A. fallax and seabirds in the N2K sites, and further monitoring programs should be carried out on these species (see La Mesa et al, 2015;Fortuna et al, 2018;Bearzi et al, 2021).…”