Echosounders for fish detection disturb harbour porpoises

The marine world is an acoustic world that has become noisier with the increasing diversity and intensity of human activities at sea. The harbour porpoise (Phocoena phocoena) is one of the best-studied cetaceans regarding hearing and responses to human-made underwater sound. The species is most sensitive to high frequencies, yet most impact studies have focused on relatively low-frequency sources. As such, the effects of high frequency sonar - including echosounders - remain largely unstudied, despite their widespread use on vessels for depth sounding, fish or object detection, and seabed mapping. We investigated the effects of scientific echosounder use on harbour porpoise occurrence using 13 multi-sensor mooring deployments in the southern part of the North Sea. Moorings operated for an average of 57 days, with split-beam scientific echosounders active for an average of 51 days, for 10 minutes every hour (5 min at 70 kHz, followed by 5 min at either 185-255 kHz or 70 kHz again). Porpoise acoustic presence was continuously monitored with C/F-PODs and additionally validated with hydrophone detections at four of the locations. Across all 13 sites, detections declined by 65- 79% during echosounder transmissions and returned to typical levels within [~]30 minutes after the echosounder stopped pinging. Despite this relatively quick recovery, there was no indication of habituation, as responses did not diminish across observation periods over six weeks of hourly exposure. Spatial effects appeared local, as no deterrent effect was observed at 2.5 km from the source. We believe that our findings have important implications for studies that investigate both harbour porpoise and fish presence to understand predator-prey interactions, but they should also raise concern about the potentially cumulative impact on sensitive cetaceans from the wide use of relatively high frequency sonar in offshore practices.