The harbour porpoise (Phocoena phocoena) is the UK's most common cetacean, yet it faces significant bycatch mortality in bottom-set gillnets across the North Sea and Celtic Sea. ICES estimates that annual bycatch exceeds sustainable levels in some regions.
Porpoises entangled in gillnets experience acute hypoxia: as air-breathing mammals, they drown within 4-8 minutes of entanglement. Their echolocation systems can detect nets but may not avoid them at high swimming speeds or in areas of high net density. Post-mortem examination of bycaught porpoises shows traumatic compression injuries from net entanglement as well as drowning. Acoustic pingers disrupt porpoise foraging behaviour, creating a welfare trade-off between bycatch death and foraging disruption in protected areas. Welfare improvements include mandatory pinger deployment, minimum mesh size requirements, and real-time bycatch monitoring using onboard observers or electronic monitoring systems.