Data from: Effects of recreational boats on harbour porpoise swimming speed and surfacing interval investigated by two synchronised UAVs

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 endangered
Belt Sea population (HELCOM, 2024). 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 the 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.