Two harbor porpoises were caught in nets with active alarms, and 25 in those equipped only with controls. One possibility is that the alarms simply alerted the porpoises to the presence of the nets, and so enabled the cetaceans to avoid the danger. But there is an alternative hypothesis. The use of acoustic alarms would seem to be a promising method of reducing the number of harbor porpoises killed in sink gill-nets in the Gulf of Maine and offers hope for alleviating the bycatch problem for small cetaceans worldwide. S.D. Kraus et al. 1997.
Acoustic alarms reduce porpoise mortality.
Nature 7 August, 525.