Avoidance failure, not detection failure drives gillnet bycatch in a small toothed whale: The harbour porpoise

Understanding small‐odontocete behaviour near static nets is central to reducing bycatch, the main source of anthropogenic mortality. The causes of bycatch however remain unknown due to a lack of direct observations of net–animal interactions. We conducted the first in situ drone‐ and acoustic‐based experiment examining harbour porpoise behaviour around a dummy bottom set net, deployed alone and in combination with an acoustic alerting device, describing movement patterns, quantifying swimming speeds and respiration rates, and echolocation responses. Porpoises frequently crossed the net swimming over the float‐line. The net and the alert device did not elicit strong changes in swimming speeds, respiration rates, or acoustic behaviour. Porpoises consistently avoided direct contact with the net at short observed reaction distances (0.8–16.4 m). Policy implications . Our findings suggest that bycatch is not simply due to a failure to detect nets but rather occurs when avoidance fails, highlighting the need for mitigation strategies that enhance detectability and support avoidance behaviour to overcome the global gillnet bycatch issue. Read the free Plain Language Summary for this article on the journal's blog .