First detailed insights into harbor porpoise behavior during a bycatch event

A monofilament gill net (267 mm stretched mesh, 10.5 meshes high, 10 mm braided float line with a foam core, twine diameter of 0.65 mm and a length of six nets at 120 m per net) was deployed in November 2019 at a depth of approximately 25 m off the coast of Cornwall (UK) during normal fishing operations. A single male bycaught porpoise (Phocoena phocoena) was recovered upon hauling the net. A multi-hydrophone passive acoustic monitoring system tracked the three-dimensional movements and acoustic behavior of the harbor porpoise, as well as 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 behavior for several hours after the bycatch occurred. Understanding behaviors prior to and during a bycatch event could significantly improve bycatch mitigation strategies in static net fisheries.