Studysites

Testing was carried out on wild harbor porpoises off the coast of Sprogø in the Great belt, Denmark from July-October 2016 and off the Kullaberg peninsula, Skåne, Sweden, from October 2016 – January 2017.

Population

Two harbor porpoise populations are found in the study areas– one in the western Baltic, Belt sea and southern Kattegat, and the other one in northern Kattegat, Skagerrak and the North sea.

The Aquatec 2446 pingers transmitted eight different sounds, all of which were high-pass filtered to follow the hearing curve of a grey seal. The source level was approximately 145dB 1 µPa at 1m, and the inter-ping interval varied semi-randomly between 5 and 30 s. The pinger sound transmissions were programmed to 60-hour ON/OFF cycles to mimic the presence and absence of a net.

The Banana pinger (www.fishtekmarine.com) transmitted multi-harmonic sounds with a frequency spectrum ranging from 60 - >100kHz and at a source level of 145dB re. 1 µPa at 1m and with 12-17-hour ON/OFF cycles to mimic the presence and absence of a net with a shorter soak time.

The responses of the porpoises were monitored by click train detectors, so called C-PODs (Continuous Porpoise Detectors; www.chelonia.co.uk),  that can detect clicks of a maximum range of 500m. They were placed 0m, 100m and 400m from the pingers. These self-contained devices log the sonar click trains generated by porpoises in the area which are then used as a proxy for porpoise presence.