Pingers

Acoustic devices have been tested on harbour porpoises (Kraus et al. 1997, Kraus and Brault 1997). These devices are sometimes referred to as "acoustic warning/deterrent devices;" however, I prefer to call them pingers, which is a neutral label based on the sound that they produce. The pingers used by Kraus et al. (1997) emitted a loud broadband signal which lasted 300 ms and was repeated every 4 seconds. Two harbour porpoises were captured in nets with activated pingers and 25 were captured in nets with similar but silent devices. Thus the pingers effectively reduced the number of mortalities. However, exactly how this device worked and its effect on the porpoises is unknown. It was suggested that the pinger’s signal affected herring Clupea harengus which was the harbour porpoise’s main food item at that time (Kraus et al. 1997). Because the herring possibly avoided the sound signal and thus the nets, the porpoises did not get close to the nets and the number of porpoises captured was reduced. To test this hypothesis, replicates of the experiment were carried out in spring when there were no herring in the area (Kraus and Brault 1997). The number of harbour porpoise mortalities was still significantly smaller in nets with pingers than in nets with similar but silent devices (Kraus and Brault 1997).

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