Threats Facing the Marine Environment: Bycatch


Commercial fishing with large sophisticated gear, nets and long-lines is operated on a very large scale in most seas of the world. Accidental entanglement in nets and swallowing of baited hooks is a significant threat to marine life.

Bycatch of cetaceans

Dolphins and porpoises in particular are vulnerable to being caught and drowned in fishing nets, but larger whales are also prone. In excess of 300,000 cetaceans are estimated caught in fishing nets every year worldwide (Read et al 2003[1]). Although the majority of these are dolphins and porpoises, some large whales are vulnerable. The small and vulnerable North Atlantic Right Whale population in the north western Atlantic suffers several entanglements in nets every year. It is estimated that 72% of the current population have suffered entanglement at some time[2].

Bycatch of other marinelife

  • The scale of seabird bycatch is impossible to quantify, but rates calculated in specific studies frequently exceed the breeding success of the species affected hence leading to population declines and in some cases threatens extinction (for instance Albatross species in the souther oceans and Balearic Shearwater in the western Mediterranean)
  • 250,000 Loggerhead and 60,000 Leatherback turtles are caught on long-lines every year world wide
  • Sharks and other predatory fish are frequently hooked on long-lines and caught in nets not set for them
  • Several species of seal and sea lion are vulnerable including in British and Irish waters.

Long line fishing

The threat of long-line tuna fisheries to albatrosses in the southern hemisphere oceans has receiving much publicity as several species there are threatened with extinction as a result, but there is also a large scale impact in European Atlantic and Mediterranean waters, including off the west coast of Scotland and Ireland.

  • Over 20,000 Northern Fulmars are estimated to be caught by Norwegian longliners alone (Dunn and Slater 2001[3])
  • In the western Mediterranean mortality rates of 0.25 birds/1,000 hooks set is extremely high, and was each year taking 4% of the Cory's shearwaters and 0.9-1.4% of the Balearic shearwaters (Critically endangered species) breeding in the area (Dunn 2007[4])
  • An estimated 55,000 Great Shearwaters are killed each year on the Gran Sol fishing grounds of western Ireland (Dunn 2007)
  • Not only seabirds are killed in longline bycatch. A study in the western Mediterranean found that Loggerhead Turtles were caught at a rate of 0.69/1,000 hooks

A range of mitigation measures are being trialled and evaluated, including setting lines at night, use of tube devices to set the lines below the surface, and decoy lines and other scarers. Some of these have good levels of success, but depend on uptake by the fishing fleets to succeed. One incentive is the number of baited hooks lost to bycatch is a cost to the operation in lost fish and and replacement hooks. Where these have been implemented such as in Australian waters the numbers of Albatrosses caught has dropped very significantly.

Bycatch in fishing nets

A range of net types are used to catch different fish. Bycatch can occur in almost any, however some types catch significantly more than others.

Gill nets are implicated in the biggest bycatch issues in many parts of the world, including European waters. These are usually fine filament and difficult to see when in the water, and are usually set in two different ways; bottom set gill nets are set in shallow inshore waters often to catch coastal migratory fish, including salmon, and are believed to be responsible for a high number of small cetacean and seal bycatch. Drift nets are more infamous for bycatch of a wide range of marine life from large whales to seabirds; they are set to drift in the ocean, often over deep water, and are indiscriminate in what they catch. They were often of inordinate length, up to 50 km long, and because they drift there is the risk that they cannot be refound and are known as ghost nets, which continue to pose a significant threat to marine life.

  • In the Celtic Sea up to 2,000 Harbour Porpoise may be caught every year, and over 5,000 a year in the Danish bottom net fishery in the North Sea
  • Various fisheries in the western English Channel are believed to bycatch 4,000 Short-beaked Common Dolphins a year; 3-5% of the Biscay and Celtic Sea population

Depending on the fishing techniques, there are some measures which can reduce bycatch of small cetaceans. Pingers that emit sounds to warn the cetaceans off the nets can reduce bycatch by as much as 90%[5] although there are still technical problems to be resolved and net designs are being trialed that allow cetaceans to escape.

Seabirds, seals and turtles are also highly vulnerable to drift nets in particular. The combined impact on marine species has been such that drift net fishing was controlled in 1989 by a UN moratorium on all large-scale pelagic drift net fishing, which was adopted by the major fishing nations. The EU and USA and several others restricted the permissible length to 2.5km. The EU has also banned outright drift net use for catching tuna, swordfish etc. However, it is clear that this has not solved the problem; illegal drift nets are still widely used around the world; 600 vessels are thought to be illegally using drift nets in the Mediterranean alone.

Trawling is generally less likely to cause bycatch of cetaceans, but paired trawling, where a very large wide-mouthed net is towed between two fishing vessels, is implicated in significant bycatch of Short-beaked Common Dolphin in the outer English Channel and Celtic Sea.

Pair trawling is now banned in UK territorial waters (12NM), but this only applies to UK-registered vessels so is of limited value. The UK government asked the EU to extend the ban to other vessels, but this was turned down.

EU regulations do however, require that pingers are used on drift and bottom set gill nets, however this regulation doesn't apply in Biscay or west of Ireland, and doesn't apply to small vessels.