Species that are not targeted by the fishery, such as marine mammals, seabirds, turtles and other fish species, as well as juvenile fish may be caught unintentionally in fishing gear.
Practically all fisheries have some level of bycatch. On average around 40 percent of annual global marine catch is bycatch. Every year, around 38 million tonnes are thrown overboard again, dead or dying. Fish that are of the target species but are too small to fetch a reasonable price or are below the legal minimum landing size are also discarded at sea. In some fisheries, such as the whithing fishery west of Scotland, the number of discarded fish has often exceeded that landed.
In the North Sea more than 4,400 harbour porpoises are incidentally killed in fisheries operations annually. The main threat being gillnets targeting cod, turbot and other species. Sharks, rays and skates are also caught in large numbers, often in trawls, and many species have shown a dramatic decline over the last decades. The common skate, for example, was once abundant in the North-East Atlantic. It can live for 50 years and matures late, making it vulnerable to overfishing. The skates are now too few to be the target of a fishery, but many are still caught in other fisheries. This has led to the common skate being listed as an endangered species by the International Union for the Conservation of Nature (IUCN).
Bycatch is mainly a result of the use of non-selective gear, such as trawls and bottom-set nets. It can to a certain extent be avoided by using different bycath reduction devices, such as grids or panels inserted into parts of the trawl nets. These so-called exclusion devices help, for example, turtles or dolphins or unwanted fish to escape from the trawl before it is taken out of the water. Larger mesh sizes or use of square meshes rather than conventional diamond-shaped meshes also greatly reduce bycatch in certain fisheries. For other fishing methods, such as longlines, it may help to set the lines at night. Means to reduce bycatch are increasingly required by law, but much still remains to be done.