Tuna
Tuna
Tuna are large, fast-swimming bony fish (teleosts of the family Scombridae) found in waters of the world’s oceans. All species of tuna are economically important, usually supporting a large commercial fishery, and sometimes a local sport fishery. The largest species is the bluefin tuna (Thunnus thynnus ), which can achieve a length of 13 ft (4 m) and weigh up to 1,760 lb (800 kg). Populations of bluefin tuna occur in temperate and warm waters throughout the world. Smaller species of tunas include the albacore tuna (T. alalunga ), yellowfin tuna (T. albacares ), skipjack tuna (Katsuwonus pelamis ), bonitos (Sarda spp.), frigate mackerels (Auxis spp.), Spanish mackerels (Scomberomorus spp.), and the wahoo (Acanthocybium spp.).
Biology of tuna
A remarkable aspect of the physiology of tunas in the genus Thunnus is their ability to maintain a body temperature significantly warmer than that of the ambient seawater. For example, the bluefin tuna can maintain a core body temperature of 75-95° F(24-35° C), even in water as cold as 43° F (6° C). However, unlike typical endothermic creatures such as mammals and birds, the body temperatures of tuna are not held constant within a relatively narrow range.
The endothermy of tunas is achieved by conserving the heat generated through normal body metabolism. This is accomplished through the action of an intertwined meshwork of veins and arteries, known as the rete mirable (meaning “wonderful net”), located in the periphery of the body. The rete mirable reclaims much of the heat in the venous blood, and transfers it to arterial blood through the action of a counter-current exchange system. This heat transfer slows down the rate of cooling of the tuna at the body surface, and thereby allows the animal to maintain a warmer core temperature. Higher body temperatures allow tuna to use their muscles more efficiently, and therefore swim more quickly with relatively little expenditure of energy. Higher temperatures of the body core may also contribute to the rapid and efficient digestion and absorption of food.
Tuna are fast swimmers, reaching speeds of up to 56 mph (90 km/h) in the case of the large bluefin tuna. Moreover, tuna are well adapted for cruising great distances at a relatively brisk speed. However, tuna can also accelerate quickly while preying on other fish, or to avoid their own predators. A tuna’s body is relatively elongated and fusiform, that is, tapering at both ends.
Their fins are of a size and position designed to minimize drag, so that maximum speeds can be achieved and maintained with relatively small expenditures of energy. The major source of forward thrust while swimming is side-to-side movements of the caudal, or tail fin. Tunas also have relatively large median fins (especially the dorsal, or top fin), which is an adaptation to minimizing the drag associated with sideways slippage of the body during swimming.
Tuna fisheries
Tuna support large commercial fisheries wherever they are abundant, and are thus economically important fish. However, as with any fishery, stocks of tuna can be easily exhausted through excessive harvesting. Indeed, most if not all stocks of tuna have been significantly degraded by overharvesting. This problem can be illustrated by the case of the fishery for bluefin tuna in the western Atlantic Ocean.
The bluefin tuna is a very large, fast-swimming fish which undertakes regular trans-oceanic migrations. Bluefin tuna are extremely valuable since they are eagerly sought for sale as a delicacy in Japanese sushi restaurants. During the early 1990s, a prime bluefin tuna caught in North America could be sold for $30,000 (U.S. currency) at the wharf, and then for at least $60,000 at an auction in Tokyo. The tuna meat might then be sold for about $350 per pound, as prepared sushi in restaurants. This is equivalent to about $230,000-$385,000 per fish, depending on its weight.
Because bluefin tuna are so enormously valuable, they have been exploited intensively, and their populations are declining rapidly. For example, in 1975 there were an estimated 150,000 bluefin tuna in the western Atlantic Ocean, but by the early 1990s this number had decreased by 90% through excessive harvesting, to only 22,000 animals. This resource collapse occurred even though the fishery was regulated by an international agency, the Atlantic Commission for the Conservation of Atlantic Tunas. The problem was that the managers of the commission consistently ignored the advice of their resource scientists, and set the allowable catches higher than was recommended, or was prudent. In addition, there was substantial unregulated pirate fishing by ships flying the flags of nations that are not members of the commission. As of 1999, this situation has not significantly improved.
KEY TERMS
Counter-current exchange —An exchange of heat or respiratory gases between two fluids moving across each other in different directions. In the rete mirable of tunas, the fluids are contained in veins and arteries, and the transfer of heat occurs across the walls of these vessels.
Endothermy —Refers to animals that maintain their body temperatures within a range substantially warmer than their ambient environment. The source of heat is from internal metabolism.
Overharvesting —The unsustainable exploitation of a potentially renewable, biological resource. In such a case, the harvesting rate exceeds the rate of regeneration, so the quantity of the resource diminishes over time, sometimes to commercial or even biological extinction.
The regulated and non-regulated overfishing were both ultimately caused by greed, and a desire to reap large, short-term profits. This was done without significant regard for the sustainability of the enterprise, or of the natural resource of bluefin tuna.
Some of the smaller species of tuna are caught using a type of net called a purse seine, set around a school of fish. The net initially floats vertically, with one side buoyed at the surface. Once the purse seine is set around a group of fish, the deeper side of the net is closed using a drawstring like apparatus, trapping the fish inside the “purse.” Unfortunately, purse seines also trap other species, including dolphins and porpoises that often associate with schools of tuna in some regions. In fact, in some fishing sectors boat captains deliberately set their nets around groups of these marine mammals, because they know that schools of tuna are generally found beneath them. From the 1960s to the late 1980s, huge numbers of some species of dolphins and porpoises were killed in purse seines set for tuna, perhaps 200,000 of these marine mammals each year. More recently, the dolphin kill rate has declined to about 100,000 per year. This non-target fishing mortality has significantly depleted the populations of dolphins and porpoises in some regions.
Large tuna often have a significant contamination of their flesh with mercury. This commonly occurs to a degree that exceeds the maximum acceptable concentration of mercury in fish intended for human consumption, that is, 0.5 ppm (parts per million, on a fresh weight basis). The contamination of tuna and other large, oceanic fish by mercury is apparently a natural phenomenon. One study found no difference in the mercury concentrations of modern tuna, and animals collected between 1878 and 1909 and stored in a museum.
The IUCN—The World Conservation Union considers the bigeye tuna (Thunnus obesus ) to be vulnerable and the Monterrey Spanish mackerel (Scomberomorus concolor ) to be endangered. The southern bluefin tuna (Thunnus maccoyii ) is listed as critically endangered.
See also Drift net.
Resources
BOOKS
Block , Barbara A., et al., eds. Tuna: Physiology, Ecology and
Evolution. San Diego: Academic Press, 2001.
Freedman, B. Environmental Ecology. San Diego: Academic
Press, 1994.
Harris, C. L. Concepts in Zoology. 2nd ed. New York:
Harper-Collins, 1996.
Nelson, Joseph S. Fishes of the World. 4th ed. New York:
John Wiley & Sons, 2006.
Scott, W. B., and M. G. Scott. Atlantic Fishes of Canada. Toronto: University of Toronto Press, 1988.
PERIODICALS
Safina, C. “Bluefin Tuna in the West Atlantic: Negligent
Management and the Making of an Endangered
Species.” Conservation Biology 7: 229-34.
Bill Freedman
Tuna
Tuna
Tuna are large, fast-swimming bony fish (teleosts of the family Scombridae) found in waters of the world's oceans. All species of tuna are economically important, usually supporting a large commercial fishery, and sometimes a local sport fishery. The largest species is the bluefin tuna (Thunnus thynnus), which can achieve a length of 13 ft (4 m) and weigh up to 1,760 lb (800 kg). Populations of bluefin tuna occur in temperate and warm waters throughout the world. Smaller species of tunas include the albacore tuna (T. alalunga), yellowfin tuna (T. albacares), skipjack tuna (Euthynnus pelamis), bonitos (Sarda spp.), frigate mackerels (Auxis spp.), Spanish mackerels (Scomberomorus spp.), and the wahoo (Acanthocybium spp.).
Biology of tuna
A remarkable aspect of the physiology of tuna in the genus Thunnus is their ability to maintain a body temperature significantly warmer than that of the ambient seawater. For example, the bluefin tuna can maintain a core body temperature of 75-95°F (24-35°C), even in water as cold as 43°F (6°C). However, unlike typical endothermic creatures such as mammals and birds , the body temperatures of tuna are not held constant within a relatively narrow range.
The endothermy of tuna is achieved by conserving the heat generated through normal body metabolism . This is accomplished through the action of an intertwined meshwork of veins and arteries , known as the rete mirable (meaning "wonderful net"), located in the periphery of the body. The rete mirable reclaims much of the heat in the venous blood , and transfers it to arterial blood through the action of a counter-current exchange system. This heat transfer slows down the rate of cooling of the tuna at the body surface, and thereby allows the animal to maintain a warmer core temperature. Higher body temperatures allow tuna to use their muscles more efficiently, and therefore swim more quickly with relatively little expenditure of energy . Higher temperatures of the body core may also contribute to the rapid and efficient digestion and absorption of food.
Tuna are fast swimmers, reaching speeds of up to 56 mph (90 kph) in the case of the large bluefin tuna. Moreover, tuna are well adapted for cruising great distances at a relatively brisk speed. However, tuna can also accelerate quickly while predating on other fish , or to avoid their own predators. A tuna's body is relatively elongated and fusiform, that is, tapering at both ends. Their fins are of a size and position designed to minimize drag, so that maximum speeds can be achieved and maintained with relatively small expenditures of energy. The major source of forward thrust while swimming is side-to-side movements of the caudal, or tail fin. Tunas also have relatively large median fins (especially the dorsal, or top fin), which is an adaptation to minimizing the drag associated with sideways slippage of the body during swimming.
Tuna fisheries
Tuna support large commercial fisheries wherever they are abundant, and are thus economically important fish. However, as with any fishery, stocks of tuna can be easily exhausted through excessive harvesting. Indeed, most if not all stocks of tuna have been significantly degraded by overharvesting. This problem can be illustrated by the case of the fishery for bluefin tuna in the western Atlantic Ocean.
The bluefin tuna is a very large, fast-swimming fish which undertakes regular trans-oceanic migrations. Bluefin tuna are extremely valuable since they are eagerly sought for sale as a delicacy in Japanese sushi restaurants. During the early 1990s, a prime bluefin tuna caught in North America could be sold for $30,000 (U.S. currency) at the wharf, and then for at least $60,000 at an auction in Tokyo. The tuna meat might then be sold for about $350 per pound, as prepared sushi in restaurants. This is equivalent to about $230,000-$385,000 per fish, depending on its weight.
Because bluefin tuna are so enormously valuable, they have been exploited intensively, and their populations are declining rapidly. For example, in 1975 there were an estimated 150,000 bluefin tuna in the western Atlantic Ocean, but by the early 1990s this number had decreased by 90% through excessive harvesting, to only 22,000 animals. This resource collapse occurred even though the fishery was regulated by an international agency, the Atlantic Commission for the Conservation of Atlantic Tunas. The problem was that the managers of the commission consistently ignored the advice of their resource scientists, and set the allowable catches higher than was recommended, or was prudent. In addition, there was substantial unregulated pirate fishing by ships flying the flags of nations that are not members of the commission. As of 1999, this situation has not significantly improved.
The regulated and non-regulated overfishing were both ultimately caused by greed, and a desire to reap large, short-term profits. This was done without significant regard for the sustainability of the enterprise, or of the natural resource of bluefin tuna.
Some of the smaller species of tuna are caught using a type of net called a purse seine, set around a school of fish. The net initially floats vertically, with one side buoyed at the surface. Once the purse seine is set around a group of fish, the deeper side of the net is closed using a drawstring-like apparatus, trapping the fish inside the "purse." Unfortunately, purse seines also trap other species, including dolphins and porpoises that often associate with schools of tuna in some regions. In fact, in some fishing sectors boat captains deliberately set their nets around groups of these marine mammals, because they know that schools of tuna are generally found beneath them. From the 1960s to the late 1980s, huge numbers of some species of dolphins and porpoises were killed in purse seines set for tuna, perhaps 200,000 of these marine mammals each year. More recently, the dolphin kill rate has declined to about 100,000 per year. This non-target fishing mortality has significantly depleted the populations of dolphins and porpoises in some regions.
Large tuna often have a significant contamination of their flesh with mercury. This commonly occurs to a degree that exceeds the maximum acceptable concentration of mercury in fish intended for human consumption, that is, 0.5 ppm (parts per million, on a fresh weight basis). The contamination of tuna and other large, oceanic fish by mercury is apparently a natural phenomenon. One study found no difference in the mercury concentrations of modern tuna, and animals collected between 1878 and 1909 and stored in a museum.
The World Conservation Union (IUCN) considers the southern bluefin tuna (Thunnus maccoyii), bigeye tuna (Thunnus obesus), and Monterrey Spanish mackerel (Scomberomorus concolor) to be threatened species. The southern bluefin tuna is listed as critically endangered.
See also Drift net.
Resources
books
Freedman, B. Environmental Ecology. San Diego: Academic Press, 1994.
Harris, C. L. Concepts in Zoology. New York: Harper-Collins, 1992.
Nelson, J.S. Fishes of the World. 2nd ed. New York: Wiley, 1984.
Scott, W. B., and M. G. Scott. Atlantic Fishes of Canada. Toronto: University of Toronto Press, 1988.
periodicals
Safina, C. "Bluefin Tuna in the West Atlantic: Negligent Management and the Making of an Endangered Species." Conservation Biology 7: 229-34.
Bill Freedman
KEY TERMS
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .- Counter-current exchange
—An exchange of heat or respiratory gases between two fluids moving across each other in different directions. In the rete mirable of tunas, the fluids are contained in veins and arteries, and the transfer of heat occurs across the walls of these vessels.
- Endothermy
—Refers to animals that maintain their body temperatures within a range substantially warmer than their ambient environment. The source of heat is from internal metabolism.
- Overharvesting
—The unsustainable exploitation of a potentially renewable, biological resource. In such a case, the harvesting rate exceeds the rate of regeneration, so the quantity of the resource diminishes over time, sometimes to commercial or even biological extinction.
tuna
tu·na1 / ˈt(y)oōnə/ • n. (pl. same or tunas ) a large and active predatory schooling fish (Thunnus and other genera) of the mackerel family. Found in warm seas, it is extensively fished commercially and is popular as a game fish. ∎ (also tuna fish) the flesh of this fish as food, usually canned.tu·na2 • n. 1. the edible fruit of a prickly pear cactus. 2. a cactus that produces such fruit, in particular Opuntia tuna of Central America and the Caribbean.