Porpoises, dolphins, seals and whales are killed in fishing nets as bycatch worldwide. UK waters have significant bycatch mortality particularly for harbour porpoises and common dolphins.
Marine mammals caught in fishing gear drown slowly over minutes — a prolonged and distressing welfare event. For social species like dolphins, bycatch events also cause stress to pod members observing or responding to the caught animal. Acoustic deterrents and modified gear designs substantially reduce bycatch but are not universally required. UK fisheries regulations have moved toward pinger requirements in some fisheries, but coverage remains incomplete.