Some fishing techniques, like the drift nets yield not only tons of fish but kill millions of birds, whales and seals and catch millions of fish not intended. Small net holes often capture juvenile fish who never have a chance to reproduce. Millions of fish may be killed because of being caught by lost or abandoned fishing equipment. Some forms of equipment destroy natural habitats, for example bottom trawling may destroy natural reefs.
Trawling is a method of fishing that involves pulling a fishing net through the water behind one or more boats. The net that is used for trawling is called a trawl.
Trawling can be divided into bottom trawling and midwater trawling, depending on how high the trawl (net) is in the water column. Bottom trawling is towing the trawl along (benthic trawling) or close to (demersal trawling) the sea floor. Midwater trawling is towing the trawl through free water above the bottom of the ocean or benthic zone.
One of the most destructive methods of catching fish is bottom trawling.
The biggest bottom trawl nets have mouths as wide as the length of a rugby field. Weighted with heavy metal rollers, the nets smash and crush everything in their path as they indiscriminately swallow vast quantities of marine life.
Bottom trawlers usually fish for particular species such as plaice, hake or sole, but they are normally only a small part of the catch. In 2004 it was reported that bottom trawling killed up to 16lb of other animals in order to produce just 1lb of marketable sole. Most of the dead or dying marine animals are considered worthless and dumped over the side.
Beam trawling is one of the most destructive form of bottom trawling, in which a large net attached to a heavy metal beam is dragged across the sea bed behind a boat, digging into and ploughing up the ground. The beam, which can be up to 12m long, keeps the net open horizontally while metal frames at each end keep it open vertically. Beam trawlers tow two nets, one each side of the vessel. On larger boats, several tons of 'tickler' chains can be used ahead of the ground rope to raise fish which may otherwise be crushed by the beam.
The target species are usually shrimp or bottom-dwelling flat-fish such as plaice and sole, but its indiscriminate nature means that hundreds of other species with no commercial value are killed in the process. In the fishing industry these unwanted species are known by the deliberately inoffensive term 'bycatch'.
Gill nets are long panels of netting that can be set at any depth in the sea, some are set just below the surface of the water, others on the sea bed. Their length varies enormously, the smallest around 70m long, the largest several kilometres. The size of the holes in the net determines the size of the fish that can be caught. Gill nets are a danger to ocean life because they cannot be used to catch targeted species. This makes them a threat to a diverse range of sea life that can become entangled in the nets.
In the North Sea, gill nets are a particular problem for the harbour porpoise population. They are frequently caught in bottom-set gill nets, probably due to their feeding behaviour on or near the sea bed. Porpoises, which depend on echolocation to make sense of their surroundings, often fail to detect the thin but strong nylon fibres of gill nets and become entangled and drown. One study estimated that the annual bycatch of harbour porpoises in the Danish gill net fishery alone was around 7000 animals. Gill nets are primarily used to target cod, hake, pollack, ling, flatfish and dogfish.
Industrial fishing targets small fish species for conversion to fishmeal and fish oil as opposed to direct human consumption, pose another major threat to the North Sea. The main target species are sandeels, Norway pout and sprat.
Sandeels are a vital link in the North Sea ecosystem. Feeding on zooplankton, they are fed on in turn by many predatory fish, seabirds, seals and cetaceans. Removing large quantities of these small fish may lead to a shortage of food for their predators, which include several commercially important fish species, such as cod and haddock.
It is difficult to determine the impacts that industrial fishing has on the ecosystem, but the breeding failure of kittiwake in Shetland in the 1980s and eastern Scotland in the 1990 was traced back in both cases to a local failure in the sandeel stock.
Both gill netting and industrial fishing would be prohibited from the marine reserves that Greenpeace is proposing.