By Elizabeth Gibbs
Tuna are finfish belonging to the tribe Thunnini, a sub-grouping of the mackerel family (Scombridae), which also includes the mackerels, bonitos, and the skipjacks. Species of tuna found in Northeast waters include albacore (Thunnus alalunga), yellowfin (Thunnus albacares), blackfin (Thunnus atlanticus), bigeye (Thunnus obesus), northern bluefin (Thunnus thynnus), and skipjack (Katsuwomus pelamis).
Physical Characteristics
The tuna is a streamlined fish, stout in the middle and tapering to points at either end. Two closely spaced dorsal fins rise from its back. The first is depressible–it can be laid down, flush, in a groove along the fish’s back. The second dorsal fin and corresponding anal fin are long and pointed, resembling a sickle. Seven to 10 yellow finlets run between these fins and the tail, which is lunate–curved like a crescent moon–and tapered to pointy tips. The caudal peduncle, to which the tail is attached, is very slender, with three stabilizing keels on each side. The tuna’s dorsal side is generally a metallic dark blue color, while the ventral side, or underside, is silvery or whitish.
Size
Atlantic tunas vary greatly in size, from the skipjack and blackfin, which rarely exceed three feet (90 cm) in length, to the northern bluefin–the world’s largest living bony fish–which can attain a length of over 10 feet (300 cm) and weigh well over half a ton. The largest bluefin ever caught on a rod and reel–in Nova Scotia in 1979–weighed in at 1,496 pounds. Common sizes for the bluefin range from 15 inches to roughly 6.5 feet (40 to 200 cm), but the "giants" of this species, which live in the Atlantic, are longer than 77 inches and weigh over 310 pounds. Yellowfin and bigeye tunas reach a maximum length of about six feet (190 cm), while albacore generally grow no longer than four feet (120 cm).
Reproduction and Longevity
The age of tuna at sexual maturity ranges from about three to five years, depending on the species. A spawning female may release as many as 100,000 eggs per 2.2 pounds (1 kg) of body weight. This means that a 100-kg female would produce as many as 10 million eggs in a spawning season. After about 30 hours, the eggs are ready to hatch, but very few survive to adulthood. Tuna grow rapidly and are long-lived. Bigeye and albacore, for instance, have been estimated to live nine or more years, while bluefin are thought to reach an age of over 30 years.
Distribution
Tuna are considered epipelagic-to-midwater fish, inhabiting the upper and middle layers of ocean water, to a depth of 1,600 feet or more (500 m), depending on size and species. They are found in oceans the world over, except in polar seas. They roam long distances, following extensive north-south and even transoceanic migration patterns. Scientists and international organizations have tracked these patterns by the use of tagging. Fish tagged in the Bahamas have been recovered in Norway and even Uruguay. Others tagged in the Northeast have appeared off the coast of Europe. Tagged bluefin tuna have been known to travel over 4,800 miles (7,700 km) across the Atlantic in just 119 days—an average, assuming the fish swam in a straight line, of over 40 miles (65 km) per day. While yellowfin tuna seem to make less extensive journeys–most have been recaptured within 1,000 miles (1,600 km) of tagging–they have been known to travel over 3,100 miles (5,000 km).
Tuna migrate on a yearly cycle. Northern bluefin, for example, arrive in waters off the coast of the Northeast by June of each year and depart in late autumn. The species may be found as far north as Newfoundland in the summer, and travels as far as 40 degrees south of the equator during the winter. Northern waters provide the fish with rich feeding grounds, where they can grow and store fat as an energy source for migration. They then return to their specific spawning grounds–the Caribbean region in the case of the bluefin–each year. Some researchers suspect that there are two separate populations–eastern Atlantic and western Atlantic–of the northern bluefin, although others believe that there is only one population, which commonly migrates across the Atlantic.
Physiology and Behavior
Tunas have a circulatory and respiratory system that is unique among fish, enabling them to maintain a body temperature slightly higher than the surrounding water. This additional heat, when transmitted to oxygen-rich blood, gives an extra boost to already powerful muscles, permitting the bluefin tuna, for instance, to reach speeds of over 40 miles per hour for short distances. The tuna’s steady, powerful swimming sustains a uniquely high metabolic rate, which permits its extraordinary growth rate. This also places a large oxygen demand on the fish, requiring tuna to swim continuously in order to meet that demand. Tuna must swim at a rate of at least one body length per second to pass enough oxygen over their gills.
Tuna normally travel in small schools ranging from six to as many as 40 fish, all of the same size but often including several species. The very large tuna, over 500 pounds (225 kg), are usually solitary.
Tunas are agile predators, often feeding on smaller, particularly schooling, fishes. The species of prey depends on what is locally available at the time, but common species include herring, menhaden, hake, cod, bluefish, whiting, and mackerel, as well as squid and crustaceans such as shrimp.
Because of their size, large tuna have few predators besides billfish, some sharks, toothed whales such as orcas and pilot whales, and humans.
Fishery
Among the world’s most valuable commercial species, tuna are fished in over 70 countries worldwide, and marketed in fresh, frozen, or canned form. The most important commercial species caught in the Northeast are yellowfin, bigeye and bluefin tunas.
Japan and the United States are the largest consumers of tuna, using about 36 percent and 31 percent, respectively, of the world’s catch, although U.S. consumption of canned tuna has declined somewhat since the "dolphin safe" controversy beginning in 1990.
Methods of Catch
The majority of tuna are harvested using one of three methods: pole and line, longlining, and purse seining. The method used varies locally and among species. Pole and line fishing from baitboats is the technique used for over 40 percent of the world’s catch. Longlining, as the name implies, involves the setting of a line at the surface that stretches for as many as 80 miles (130 km). Baited hooks, set at depths of 180 to 500 feet (55 to 150 m), are suspended at intervals along the line. Purse seines are large nets–as much as a mile (1.6 km) long and 600 feet (185 m) deep–that are set in a circle around a school of fish, then drawn closed at the bottom, like a draw-string purse. All three of these are among the methods used in the Northeast.
The Dolphin-Tuna Issue
In recent years, the consumption of tuna became an issue of conscience as well as taste, as attention has focused on tuna fishing-associated deaths of dolphins. Yellowfin tuna have been caught using the highly efficient method of "dolphin fishing." Yellowfins swim closely with several species of dolphin. When the dolphins surface to breathe, fishing boats set their nets around both tuna and dolphin, sometimes drowning the dolphins. International efforts to cut dolphin kills during tuna seining reduced the number of dolphins killed from over 400,000 per year in the 1960s to 2,500 in 1996. On August 15, 1997, President Clinton signed a bill that redefines "dolphin safe" tuna. Prior to this law, "dolphin safe" meant that tuna was not taken in association with dolphins. As a result of the law, "dolphin safe" now means that no dolphins were harmed during the catch. Labelling changes took place in 1999.
Bycatch Reduction
Dolphins are not the only marine animals that have been harmed in the process of catching tuna. Other animals, such as whales, sharks and sea turtles, have been adversely affected by tuna and other pelagic fishing. Take reduction plans mandated by the amended Marine Mammal Protection Act are designed to reduce bycatch significantly.
Sportfishing
Sportfishing for tuna is also popular, especially on the East Coast, where giant bluefin are much in demand although increasingly rare. The peak season for the sport fishery in this region is late August through October. The annual Rhode Island Tuna Tournament, incorporated in 1958, became one of the world’s largest such events in the late 1960s and early 1970s. It continues today, but with a 50 percent drop in participation from its heyday due to a reduction in the numbers of tuna to be found off the Rhode Island coast. Increasingly, emphasis is being placed on "tag and release" fishing, both in the tournament and in sportfishing for tuna in general.
Conservation and Management
Conservation and management efforts are the primary responsibility of international organizations such as the International Commission for the Conservation of Atlantic Tunas and the Inter-American Tropical Tuna Commission, which set catch quotas on tuna catch and operate research and conservation programs. In the United States, the National Marine Fisheries Service (NMFS) produced the Final Fishery Management Plan for Atlantic Tunas, Swordfish, and Sharks in 1999 to regulate the U.S. tuna fishery.
The bluefin has been under management, including catch quotas, in the western Atlantic since the 1980s as a means for rebuilding its stocks, yet there is much controversy over whether the quotas are sufficient to stem a continued drop in numbers. To better understand the dynamics of the Atlantic bluefin tuna fishery in the United States, the NMFS has instituted an automated catch reporting system for recreationally caught bluefins, as well as a large pelagic survey. These efforts will help monitor quotas.
Additional Reading:
Bigelow, H.B. and W.C. Schroeder. Fishes of the Gulf of Maine. United States Department of the Interior, Fish and Wildlife Service, Fishery bulletin 74, Vol. 53. Washington, D.C.: 1953.
Freeman, B.L. and L.A. Walford. Anglers’ Guide to the United States Atlantic Coast: Fish, Fishing Grounds and Fishing Facilities, Section II, Nantucket Shoals to Long Island Sound. Seattle, Wash.: 1974. For sale by the Government Printing Office, Washington, D.C.
Joseph, J., W. Klawe and P. Murphy. Tuna and Billfish: Fish without a Country. La Jolla, California: Inter-American Tropical Tuna Commission. 1988.
Miyake, Makoto. Field Manual for Statistical Sampling of Atlantic Tunas and Tuna-like Fishes. International Commission for the Conservation of Atlantic Tunas, Madrid, Spain. 1990.
NMFS website. http://www.nmfs.noaa.gov
National Oceanic and Atmospheric Administration. Final Fishery Mangement Plan for Atlantic Tuna, Swordfish, and Sharks. Prepared by: Highly Migratory Species Management Division, Silver Spring, Maryland.
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