Acoustic pingers to mitigate dolphin bycatch and depredation, barely a one third octave between them.

Abstract

Bycatch mitigation alarms/pingers for dolphins/porpoises and whales are developed in the mid 1980s in Japan and Canada, respectively. Alarms are attached to nets so the acoustic warning is associated with the obstruction, avoided, and the behavior reinforced via associative learning. A continuous association between alarms (the warning) and a net (the obstruction) is essential. What constitutes an appropriate alarm is not fully understood but should result in reduced entanglement in fishery conditions, irrespective of the mammals behavior observed by human observers. Dolphins are rarely deterred with bycatch pingers; they are alerted if they were inattentive but often maintain a close association with nets. Both bycatch and depredation may be reduced. These puzzling results should be investigated. At the other end of the pinger cline are the pingers that move dolphins from the vicinity of nets or lines to mitigate depredation, and by default, bycatch. The mechanisms are not known how these pingers can be successful at sound pressure levels comparable to dolphin whistles yet the results are becoming clear. The avoidance behavior from these pinger types should be investigated with captive animals. Interactive pingers and net material/reflectors with higher target strength are also seen as other important developments.