Acoustic deterrent devices
Underwater sound-emitting devices (maximum level of intensity equivalent to approximately 175 dB re 1 µPa @ 1m) attached to fishing gear, principally gillnets. ADD’s such as acoustic pingers are now mandated for use in some fisheries in the U.S. Northwest Atlantic, California driftnet, and in Europe. The sound of these devices is believed to alert an animal to the presence of the net and thus decrease the probability of entanglement. Although some studies have shown that pingers can have the unintended consequence of attracting pinnipeds to fishing operations (Bordino et al., 2002) , this may be controllable by raising the emitted frequency of the pingers above seal hearing (Kraus et al., 1997). Other ADD’s emit sounds of such high intensity that they cause pain or alarm in certain underwater species. The minimum sound level is approximately 200 dB re 1 _Pa @ 1m (Olesiuk et al., 2002). Other ADD’s include audio recordings of an animal in distress, or of its predator, played to deter individuals of that species from entering into a fishing area. Jefferson and Curry concluded that this technique was largely ineffective for reducing marine mammal interactions with fishing activity based on their review of multiple studies. Sounds produced to disrupt the normal echolocation abilities of cetaceans are also considered ADD’s. Preliminary research in Europe has shown some promise that these devices reduce depredation by bottlenose dolphins in gillnets and trammel nets, although habituation may be a challenge (Mooney et al. 2009) . The use of loud explosive devices, including gunshots, to scare non-target species such as sea lions away from a fishing operation can also be used. Deterrence may result from noise or tactile annoyance. Anecdotal evidence from some fishermen suggests this practice is widespread though its efficacy is not backed up by a number of studies, and it obviously threatens animal survival (Matkin 1994).
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