Observations on the interaction of dolphins with trawl fishing vessels in the northwestern Black Sea in 2018–2021

Abstract

Implementation of large-scale fishing for sprat Sprattus sprattus (L., 1758) in the northwestern Black Sea (NWBS) in the late 1970s caused the emergence of a special form of protocooperative interaction of dolphins with the trawl vessels, in which dolphins consume the fish lost or discarded during trawling operations. The time of pulling the trawl out of the water is the most important for feeding dolphins, as the largest number of animals gather near the vessel to collect injured fish. Visual observations of dolphins aboard fishing vessels were conducted in 2018–2019 and in 2021 during 184 trawl operations. During observations the presence of two species - common dolphin Delphinus delphis ponticus (Barabasch-Nikiforov, 1935) and bottlenose dolphin Tursiops truncatus ponticus (Barabasch-Nikiforov, 1940) was noted. The proportion of common dolphins of all registered animals was 82.8%, bottlenose dolphins – 17.2 %. The presence of dolphins during trawl operations was observed in 65.2 % of the cases. Of the 120 recorded cases of interaction common dolphins were noted in 83 (69.2 %), bottlenose dolphins in 20 (16.7 %) and common and bottlenose dolphins in 17 (14.1 %). A total of 1035 common dolphins and 170 bottlenose dolphins were recorded in single-species groups, 189 common dolphins and 85 bottlenose dolphins in mixed groups. The mean size of common dolphins group was 12.2, bottlenose dolphins – 6.9 individuals. One case of death of common dolphin in a trawl was recorded in May 2019. Feeding of dolphins near trawl vessels in the NWBS is presently the norm. Despite the potential danger of death in fishing gear, it is obvious that the benefits for dolphins from interacting with trawl vessels outweigh the risk. Compared to the beginning of the 2000s and in conjunction with significant decrease in the intensity of sprat fishing in the NWBS, the frequency of interaction of dolphins with trawls doubled. The mean size of common dolphins group increased 2.6 times, bottlenose dolphins almost tripled.