Directional hydrophone clusters reveal evasive responses of small cetaceans to disturbance at offshore windfarms

Mitigation measures to disperse marine mammals prior to pile-driving
include acoustic deterrent devices and piling soft starts, but their
efficacy remains uncertain. We developed a self-contained portable
hydrophone cluster to detect small cetacean movements. Using an array of
clusters within 10 km of foundation pile installations, we tested the
hypothesis that harbour porpoises (Phocoena phocoena) respond to
mitigation measures at offshore windfarm sites by moving away. During
baseline periods, porpoise movements were evenly distributed in all
directions. In contrast, animals showed significant directional movement
away from sound sources during acoustic deterrent device use and piling
soft starts. We demonstrate that porpoises respond to measures aimed to
mitigate the most severe impacts of construction at offshore windfarms by
swimming directly away from these sound sources. Portable directional
hydrophone clusters now provide opportunities to characterise responses to
disturbance sources across a broad suite of habitats and contexts.