Harbour porpoise displacement by a bottlenose dolphin in Southern fyn

The most common cetacean species in Danish waters is the harbour porpoise. A few years ago, a group of bottlenose dolphins arrived from Scotland and settled around Thyborøn and Svendborg. Bottlenose dolphins are known to harass and kill harbour porpoises. The male dolphin that settled in Svendborg was reported killing harbour porpoises at least twice, in August 2020 and 2022. It is therefore important to monitor the effects of dolphin presence on the local population of harbor porpoises. Here we report the results of passive acoustic monitoring of the dolphin and porpoise presence in Svendborg and other areas of southern Fyn. We used F-PODs (underwater dataloggers) to detect click trains of porpoises and the dolphin. We found that there were significantly fewer detections of porpoises in Svendborg harbour and in Svendborgsund where the dolphin regularly occurred, compared to the nearby dolphin-free areas in the same period. After the dolphin left the area in April 2023, the occurrence of porpoises within his former range increased significantly. The results of our study demonstrate that harbour porpoises actively avoided the area inhabited by the bottlenose dolphin.