Сetacean occurrence, group size and abundance estimation in the waters of Zonguldak Province, Türkiye, southwestern Black Sea

In spring 2019 and 2022 and summer 2020 and 2022, vessel surveys tracked the seasonal dynamics of cetaceans along Zonguldak’s coast (Türkiye). This study analyses the occurrence, group size, and density/abundance estimates for the common dolphin (Delphinus delphis), bottlenose dolphin (Tursiops truncatus), and harbour porpoise (Phocoena phocoena) over a survey area of 590 km2. Harbour porpoises were the most frequently observed in spring; bottlenose dolphins dominated in summer. Mean group sizes were larger in spring of 1.9 (median = 2) for harbour porpoises, 3.3 (median = 2) for bottlenose dolphins, and 4.4 (median = 2) for common dolphins. The lowest abundance and density estimates were observed for harbour porpoises in summer 2020 (94 individuals, 95% confidence interval (CI) = 17-525; 0.2 ind/km2, coefficient of variation (CV) =81.2%). The highest were for common dolphins in spring 2022 (5929 individuals, 95% CI = 1682–20,908; 9.3 ind/km2, CV = 67.6%). Spring peaks in harbour porpoise abundance may reflect annual migrations, while Delphinidae dynamics could be related to prey availability. These findings provide a rare glimpse into how seasonal patterns shape cetacean communities in the Black Sea. Understanding these dynamics is critical for informing conservation strategies, especially in regions where prey distribution and human activities are rapidly changing.