Harbour porpoises respond to small boats by speeding up and moving away

Recreational boats are common in many coastal waters, yet their effects on cetaceans and other sensitive marine species remain poorly understood. To address this knowledge gap, we used drone videos to quantify how harbour porpoises (Phocoena phocoena) responded to a small motorboat approaching at different speeds (10 or 20 knots). The experiment was carried out in shallow waters near Funen, Denmark (55.51° N, 10.79° E) between July and September 2022. Porpoises moved further away from the boat path during approaches at both boat speeds. In addition, porpoises swam faster when approached at 20 knots but not when approached at 10 knots, and they had a higher likelihood of moving away from the boat path when approached at 10 knots but not at 20 knots. Importantly, the received sound level did not depend on how fast the boat approached, suggesting that differences in porpoise responses were related to the speed of the boat's approach rather than to sound itself. The porpoises' behaviour during the minute where the boat was closest did not differ from their behaviour before boat exposure, indicating that the direct impact of small vessels on porpoise behaviour was most likely small. Nevertheless, repeated exposure to noise from small vessels could influence porpoises' foraging efforts and cause them to relocate from disturbed areas. The approach used in this study increases our understanding of recreational boats’ impact on harbour porpoises and can be used to inform efficient mitigation measures to help conservation efforts.