Passive acoustic monitoring to assess depredation of killer whales on demersal longlines during soaking

Abstract

Toothed whales feeding on fish caught on longlines is a growing issue worldwide. This issue named depredation has a serious socio-economic impact and raise conservation questions. In the French Patagonian toothfish fishery (Southern Ocean), a good understanding of the depredation behavior by killer whales is urgently needed to find adequate mitigation solutions. However, depredation on demersal longlines has been historically assessed from surface observation during hauling phases, thus the underwater dimension of depredation is not understood. Here, we used passive acoustic monitoring to record echolocation activity of killer whales and fishing vessels’ acoustic signatures during the fishing activity. Fixed hydrophone deployed on soaking longlines without any fishing activity nearby has recorded echolocation sounds. These recordings suggest that killer whales were feeding within the vicinity of longlines and thus very likely depredating on soaking longlines. Additionally, we also investigated fishing vessels’ acoustic signatures. We observed differences of acoustic cues emitted by the vessels between the setting and hauling phases. This suggests that killer whales may be able to recognize the boat activity and therefore localize longline positions to depredate.