The use of certain kinds of commercial fishing technologies can result in large bycatches—incidental catches of unwanted fish, sea turtles , seabirds, and marine mammals. Because the bycatch animals have little or no economic value, they are usually jettisoned, generally dead, back into the ocean. This non-selectivity of commercial fishing is an especially important problem when trawls, seines, and drift nets are used. The bycatch consists of unwanted species of fish and other animals, but it can also include large amounts of undersized, immature individuals of commercially important species of fish.
The global amount of bycatch has been estimated in recent years at about 30 billion tons (27 million tonnes), or more than 1/4 of the overall catch of the world's fisheries. In waters of the United States, the amount of unintentional bycatch of marine life is about 2.2 billion lb per year (1 billion kg per year). However, the bycatch rates vary greatly among fisheries. In the fishery for cod and other groundfish species in the North Sea, the discarded non-market biomass averages about 42% of the total catch, and it is 44–72% in the Mediterranean fishery. Discard rates are up to 80% of the catch weight for trawl fisheries for shrimp.
Some fishing practices result in large bycatches of sea turtles, marine mammals, and seabirds. During the fishing year of 1988–1989, for example, the use of pelagic drift nets, each as long as 90 km, may have killed as many as 0.3-1.0 million dolphins , porpoises, and other cetaceans. During one 24-day monitoring period, a typical set of a drift-net of 19 km/day in the Caroline Islands of the south Pacific entangled 97 dolphins, 11 larger cetaceans, and 10 sea turtles. It is thought that bycatch-related mortality is causing population declines in 13 out of the 44 species of marine mammals that are suffering high death rates from human activities. It is also believed that hundreds of thousands of seabirds have been drowned each year by entanglement in pelagic drift-nets.
In 1991, the United Nations passed a resolution that established a moratorium on the use of drift-nets longer than 1.6 mi (2 km), and most fishing nations have met this guideline. However, there is still some continued use of large-scale drift nets. Moreover, the shorter nets that are still legal are continuing to cause extensive and severe bycatch mortality.
Sea turtles, many of which are federally listed as endangered, appear to be particularly vulnerable to being caught and drowned in the large, funnel-shaped trawl nets used to catch shrimp. Scientists have, however, designed simple, selective, turtle excluder devices (TEDs) that can be installed on the nets to allow these animals to escape if caught. The use of TEDs is required in the United States and many other countries, but not by all of the fishing nations.
Purse seining for tuna has also caused an enormous mortality of certain species of dolphins and porpoises. This method of fishing is thought to have killed more than 200-thousand small cetaceans per year since the 1960s, but perhaps about one-half that number since the early 1990s due to improved methods of deployment used in some regions. Purse seining is thought to have severely depleted some populations of marine mammals.
The use of long-lines also results in enormous bycatches of various species of large fishes, such as tuna, swordfish , and sharks , and it also kills many seabirds. Long-lines consist of a fishing line up to 80 mi (130 km) long and baited with thousands of hooks. A study in the Southern Ocean reported that more than 44,000 albatrosses of various species are killed annually by long-line fishing for tuna.
In addition, great lengths of fishing nets are lost each year during storms and other accidents. Because the synthetic materials used to manufacture the nets are extremely resistant to degradation, these so-called "ghost nets" continue to catch and kill fish and other marine mammals for many years.
Clearly, fishery bycatches can cause substantial, non-target mortality that is a grave threat to numerous marine species. It is urgent that effective action be taken to curtail this wasteful environmental impact of commercial fishing as soon as possible.
[Bill Freedman Ph.D. ]
Alverson, D. L., ed. 1994. Global Assessment of Fisheries Bycatch and Discards. Fisheries Technical Paper No. 339. Rome: Food and Agricultural Organization of the United Nations (FAO), 1994.
American Fisheries Society. Fisheries Bycatch: Consequences and Management. Fairbanks, AK: University of Alaska Sea Grant, 1998.
Freedman, B. Environmental Ecology. San Diego, CA: Academic Press, 1995.
The Indiscriminate Slaughter at Sea; Facts About Bycatch and Discards. 1998. World Wildlife Fund. [cited July 9, 2002]. <http://www.panda.org/news/press/archive/news_177f2.htm>.