First detailed insights into harbour porpoise behaviour during a bycatch event

Bycatches in static nets are the greatest cause of anthropogenic mortality in small cetaceans globally; however, the mechanism underlying how animals with highly sophisticated biosonars become entangled in nets is still poorly understood and represents a key information gap. During the deployment of a multi-hydrophone passive acoustic monitoring system, we tracked the three-dimensional movements and acoustic behaviour of harbour porpoises (Phocoena phocoena) during a rarely observed bycatch event. These data provide, to our knowledge, the first comprehensive insights into the behaviour of a harbour porpoise (or any small-toothed whale) prior to and during a bycatch event in static nets. It also included behavioural information related to a second animal that was in close proximity but did not interact with the net. The results indicated that the porpoises were foraging around the net prior to the bycatch event, that the bycaught animal lifted the net by approximately 7 m while entangled and that the two animals were potentially a social unit, with the surviving porpoise exhibiting unusual communication behaviour for several hours after the bycatch occured. Understanding behaviours prior to and during a bycatch event could significantly improve bycatch mitigation strategies in static net fisheries, and this paper demonstrates a new method for how this can be achieved.