Effects of Recreational Boats on Harbour Porpoise Swimming Speed and Surfacing Interval Investigated by Two...

Cetaceans are negatively affected by anthropogenic activities, including acoustic and physical disturbance from boat traffic. Behavioural responses to such disturbances are context-dependent, and site-specific insights are needed for effective local management plans. In this study, the impact of speed and proximity of recreational boats on the swimming speed and surfacing interval of one of the most common coastal cetacean species, the harbour porpoise (Phocoena phocoena), is investigated using data collected by unmanned aerial vehicles (UAVs) within a key habitat for the vulnerable Belt Sea population. In August 2024, two UAVs were flown simultaneously on predefined routes within the area. One UAV searched for and followed detected porpoises, while the other monitored recreational boats. All data was captured as UAV video and used to determine surfacing intervals of individual porpoises, measure speed of porpoises and boats and calculate the closest distance between porpoises and boats for each simultaneous sighting. A total of 91 synchronous flights were conducted, resulting in 28 porpoise observational events. GLM analyses showed that an interaction between mean boat speed and distance to the boat influenced the mean speed of the porpoises. In the presence of boats with high mean speeds, porpoises at close range reduced their swimming speed, while porpoises at greater distances instead increased their swimming speed. Also, porpoise surfacing intervals decreased (i.e., porpoises surfaced more frequently) with decreasing distance to boats. This study demonstrates that recreational boats influence the behaviour of porpoises, which could lead to potential negative individual- and population-level effects.