Ecological damage caused by drift nets
Drift nets are lengthy, free-floating, 26-49 ft (8-15 m) deep nets, each as long as 55 mi (90 km). Drift nets are used to snare fish by their gills in pelagic, open-water situations. Because drift nets are not very selective of species, their use results in a large by-catch of non-target fish, sharks, turtles, seabirds, and marine mammals, which are usually jettisoned, dead, back to the ocean. Drift nets are an extremely destructive fishing technology.
Drift-net fisheries have been mounted in all of the world's major fishing regions, and unwanted by-catch is always a serious problem. This has proven true for pelagic fisheries for swordfish, tuna, squid, salmon, and other species. One example is the drift-net fishery for swordfish in the Mediterranean, 90% of which is associated with Italian fishers. This industry kills excessive numbers of striped dolphin and sperm whale, and smaller numbers of fin whale, Cuvier's beaked whale, long-finned pilot whale, and Risso's, bottlenose, and common dolphins, along with other non-target marine wildlife. As a result of concerns about the excessive by-catch in this swordfish fishery during the early 1990s, the European Union banned the use of drift nets longer than 1.5 mi (2.5 km) (prior to this action, the average set was 26 mi [12 km] in length). However, some fishing nations have objected to this regulation and do not enforce it. It remains to be seen whether this length restriction will prove to be useful in preventing the non-target, drift-net mortality in this fishery.
There are few monitoring data that actually demonstrate the non-target by-catch by drift nets. One measurement was made during a one-day monitoring of a typical drift-net set of 11 mile/day (19 km/day) in the Caroline Islands of the south Pacific. That single net, in one day, entangled 97 dolphins, 11 larger cetaceans, and 10 sea turtles. World-wide during the late 1980s, pelagic drift nets were estimated to have annually killed as many as one million dolphins, porpoises, and other cetaceans, along with millions of seabirds, tens of thousands of seals, thousands of sea turtles, and untold numbers of sharks and other large, non-target fish.
Although there are no hard data to verify the phenomenon, there are anecdotal reports of substantial reductions in the abundance of some of these groups of animals in regions that have experienced a great deal of drift netting. Consequently, the drift net by-catch is perceived (by proponents of this type of fishing technology) to be less of a problem than formerly, because the unintended by-catches are apparently smaller. However, this really reflects the likelihood that this rapacious fishing practice has created marine deserts, that only support sparse populations of large animals.
In addition, great lengths of drift nets and other fishing nets are lost at sea every year, especially during severe storms. Because the nets are manufactured of synthetic materials that are highly resistant to degradation, they continue to snare fish, sharks, mammals, birds, turtles, and other creatures for many years, as so-called ghost nets. Little is known about the magnitude of this problem, but it is undoubtedly an important cause of mortality of marine animals and other creatures.
In response to mounting concerns about unsustainable by-catches of non-target species of marine animals, which in some cases are causing population declines, the United Nations in 1993 banned the use of drift nets longer than 1.5 mi (2.5 km). Although this regulation would not eliminate the by-catches associated with drift netting, it would greatly reduce the amount of this unintended mortality, possibly by as much as two-thirds. Unfortunately, there has been a great deal of resistance from the fishing industry and certain fishing nations to the implementation of even this minimal regulation and have continued to allow the use of much longer nets. In addition, some illegal, or "pirate" fishers continue to use the extremely destructive, older-style drift nets.
Clearly, non-selective by-catches associated with drift nets cause an unacceptable mortality of non-target animals, some of which are threatened by this practice. A rapid improvement of this unsatisfactory state of environmental affairs could be achieved by using shorter drift nets, or by banning their use altogether. Unfortunately, because of economic self-interest of the world's nations and the fishing industry, this seemingly obvious and simple betterment has not yet proved possible.
Berrill, M., and D. Suzuki. The Plundered Seas: Can the World's Fish be Saved? Sierra Club Books, 1997.
Freedman, B. Environmental Ecology. 2nd ed. San Diego: Academic Press, 1995.
LaBudde, S. Stripmining the Seas. A Global Perspective on Drift Net Fisheries. Honolulu: Earthtrust, 1989.