Home | Category: Commercial and Sport Fishing and Fish / Economics, Fishing and Agriculture / Government, Infrastructure, Economics / Government, Infrastructure, Economics / Government, Infrastructure, Economics / Government, Infrastructure, Economics / Government, Infrastructure, Economics
bluefin tuna Tuna are relatives of billfish such as sailfish and marlin. There are 15 species of tuna. Some sources say there are 21 or even more species. Sometimes this is matter of counting groups in the Atlantic or Pacific as different species. Several are important commercially. Among these are skipjack, yellowfish tuna. albacore tuna, bluefin tuna and little tuna.
Tuna are mostly an open water fish found through temperate and tropical waters around the world. Unlike other fish, tuna are warm blooded. Their body temperatures are as much as 12 degrees C higher than the water they swim in. The need to keep their bodies at a high temperature for the muscles and reactions to be as strong and quick as they are.Tuna change temperature when they are feeding.
Tunas are important food, commercial, and sport fishes. In some parts of the world, such as the Mediterranean and Californian coasts, tunas have been fished locally for a long time. Heavy commercial exploitation of large tunas has led in some cases to depletion of tuna populations. Much of the tuna catch is harvested for canning. As of the 1990s the southern bluefin tuna was listed as as critically endangered and bigeye tuna was listed as vulnerable. Albacore tuna northern bluefin tuna may also be threatened. [Source: World Conservation Union, 2002; Monica Weinheimer, Animal Diversity Web (ADW)]
Websites and Resources: Animal Diversity Web (ADW) animaldiversity.org; “Netting Billions 2020: A Global Tuna Valuation, Pew Charitable Trusts”, October 6, 2020 pewtrusts.org; National Oceanic and Atmospheric Administration (NOAA) noaa.gov; Fishbase fishbase.se ; Encyclopedia of Life eol.org ; Smithsonian Oceans Portal ocean.si.edu/ocean-life-ecosystems
Scombridae — the Mackerel and Tuna Family of Fish
Scombridae— a fish family embracing mackerels, tunas, and bonitos — includes some of the world’s most popular food and sport fishes. The family also boasts some of fastest-swimming of all fishes and some of the largest of all bony fishes. Scombrids’ speed and size are at least in part due to their high-energy metabolism, highly-evolved adaptation to a pelagic (open ocean) environment and their nomadic, constantly hunting existence. Their bodies are designed to maximize swimming efficiency. Tuna even have a vascular heat exchange system that them to swim for prolonged periods in colder water while their bodies remain warm [Source: Monica Weinheimer, Animal Diversity Web (ADW) /=]
Scombridae are mainly open ocean fish. Smaller mackerel often live closer to shore, but other mackerels, tunas, and bonitos range across deep waters, often following wide-ranging and complex migratory patterns. Some groups occur in brackish water, and one normally marine species, Scomberomorus sinensis, has been found in fresh water 300 kilometers up the Mekong River. Many groups within Scombridae tend to remain near the surface and over the continental shelf. /=\
The family Scombridae is comprised of two subfamilies, subdivided into 15 genera and 49 species. Tunas, mackerels, and bonitos can be found worldwide in tropical, subtropical seas, with many species traveling periodically into cool temperate waters. The fossil record for Scombridae dates back to the Lower Tertiary Period (66 million to 56 million years ago) and Lower Eocene Period (56 million to 47.8 million years ago). Due to their great range and extensive use as food fish, scombrids have many common names and are well-known to many groups of people. Bonitos, for example, were described in Captain Cook’s journals. Human influence — primarily overfishing — has caused at least five species to be listed as endangered or vulnerable to extinction.
Tuna Physical Characteristics
Scombrids (tunas, mackerels, and bonitos) are endothermic (use their metabolism to generate heat and regulate body temperature independent of the temperatures around them) and heterothermic (have a body temperature that fluctuates with the surrounding environment).
According to to Animal Diversity Web: Scombrids have streamlined bodies that taper on either end, moderately large mouths, and well-developed teeth. Gill membranes are not attached to the isthmus. Scales are cycloid and usually tiny, and body coloration is metallic, often blue and silver. Bonitos and tunas may have dusky bands and fins. The dorsal fin is composed of 9 to 27 densely packed rays, and the pelvic fins have six rays. [Source: Monica Weinheimer, Animal Diversity Web (ADW) /=]
Scombrids are highly adapted to continuous swimming in the open ocean. Their bodies are an ideal streamlined shape, with the thickest part of the body occurring two-fifths of the way back from the head. Their dorsal fins can slot into grooves to reduce drag, and the caudal fin is stiff and sickle-shaped for powerful propulsion. The five to 12 separate finlets behind the anal and second dorsal fins may allow the tail to push against less turbulence by preventing vortices from forming in water flowing toward the tail. The slender caudal peduncle bears at least two keels that reduce drag and may accelerate water flowing over the tail.
Kenneth Brower wrote in National Geographic: “The true tunas, genus Thunnus, are supercharged fish, streamlined to perfection and jammed with state-of-the-art biological gear. The characteristics that distinguish the true tunas include great size, great range, efficient swimming stroke, warm bodies, large gills, finesse at thermoregulation, rapid oxygen uptake, high hemoglobin concentration, and clever physiology of the heart. All of these reach their apogee in the bluefin.[Source: Kenneth Brower, National Geographic, March 2014]
Tuna High Speed Swimming Ability
Tuna have hydrodynamically-shaped bodies that allow them to swim very fast. Their snouts are pointed like a supersonic plane; their rear part tapers gently; and they have a crescent-shaped tail. The surface of their eyes are contoured so they don’t bulge and interrupt the streamlined surface. The crescent-shaped tail of tuna, marlins, swordfish and sailfish, creates a thrust similar to that of the wings of birds.
Tunas have a unique, efficient style of swimming called thunniform in which they keep their body stiff while their thin tail oscillates quickly to rapidly propel them forward. Tuna draw heat from the movement of their own muscles. This gives them lots of energy but it also means they need a lot of oxygen too. Swimming at great speeds requires a great deal of energy and oxygen (See Below).Tuna are covered by specially-modified scales just behind the head that act like the spoilers on race cars. They produce a slight turbulence around the widest part of the body, which reduces drag on the rear of the body. When swimming at high speed, the fins slot into special groove so the flow of water is not obstructed.
To study how tuna achieve their great speeds in a medium 800 times denser than water scientists at MIT invented Robotuna. The robot has a stainless steel spine with eight vertebrae connected with ball-bearing joints that are moved with a pulley and cable system. Another tuna-like robot created at Cambridge, called VCUUV, cost $1 million to make.
Tuna Swimming, Oxygen, Heat and Muscles
According to Animal Diversity Web; Tunas are negatively buoyant and must swim continuously to avoid sinking. In addition, they require constant movement to ventilate the gills. Tuna obtain the oxygen they need by swimming with their mouth's open, forcing jets of water over the large gills. This means they have to swim continuously at relatively high speeds to breath. If they don’t they die.
According to Animal Diversity Web: Through a process called ram gill ventilation, swimming (at speeds no less than 65 centimeters per second) forces water over the gills. Tunas have numerous lamellae (gill membranes) and very thin lamellar walls, and are able to extract more oxygen from the water than any other fish. Tunas have large hearts and blood volumes. They also have a high proportion of the red muscle that permits sustained swimming, buried centrally along the spinal column to conserve heat.
Tunas conserve heat produced by swimming muscles through an arrangement of blood vessels called a rete mirabile (“wonderful net”). These blood vessels act as a countercurrent heat-exchanger. In any fish, when blood cycles through the gills to receive oxygen, it also cools to the temperature of the surrounding water. In tunas, this blood is diverted to vessels near the outside of the body instead of traveling directly through the fish’s core. Before flowing inward, the cool, oxygenated blood passes through a network of small vessels, countercurrent to warm blood leaving the swimming muscles, and heat is transferred to the entering blood. In this way much of the heat generated by swimming muscles is conserved. In waters ranging from seven to 30̊ C, bluefin tuna maintain muscle temperatures between 28 and 33 C. Others keep body temperatures three to seven degrees C warmer than the surrounding water. Some species, such as bigeye tuna, utilize the heat exchanger only when they enter colder water. Endothermy also helps warm parts of the central nervous system, which stabilizes nervous system function in cold water.
Tuna Behavior, Feeding, Mating and Predators
Tuna are motile (move around as opposed to being stationary), nomadic (move from place to place, generally within a well-defined range), migratory (make seasonal movements between regions, such as between breeding and wintering grounds), and social (associates with others of its species; forms social groups). [Source: Monica Weinheimer, Animal Diversity Web (ADW) /=]
Tuna sense using vision, touch and chemicals usually detected with smell. Many are schooling fishes, but some can be found singly. According to Animal Diversity Web: They follow a nomadic lifestyle, sometimes making migrations of huge distances. For some groups, migrations are seasonal and may be determined by water temperature. Scombrids are continuous swimmers. Some species of bonito habitually leap clear of the water when pursuing prey.
Tuna are active predators that feed on a wide range of organisms. The diet of a single species may include crabs, shrimps, squids, crustaceans, the larvae of fishes and invertebrates, and fishes several feet long. Tunas feed on a variety of mid-water and surface fishes, with mackerel providing a favorite meal. Tunas’ ability to maintain elevated body temperatures enables them to swiftly pursue prey in the cold waters of deeper depths and higher latitudes. Migratory tunas have the fastest digestion rates and highest metabolic rates of any fish.
On feeding tuna, David Attenborough wrote: "Tuna usually operate in squadrons. A group has been observed carefully herding a shoal [school], driving it from behind and patrolling its flanks to keep it well packed. When they attack there is wholesale carnage. As they rip through the shoal, snatching the small fish with devastating precision and speed, the shoal itself panics. Fish shoot from the surface of the sea in hundred in an attempt to escape the snapping jaws below, like terrified impala leaping away from a team of rampaging lions.”
Tunas, even large ones, may end up as prey. They are fed upon in the temperate and warm temperate waters by other fast-moving fish such as swordfish, sailfishes and marlins. Most species of shark are too slow to catch tuna.
Tuna are iteroparous (offspring are produced in groups) and engage in seasonal breeding. Many species spawn repeatedly. Females may lay as many as hundreds of thousands of eggs, which float near the surface. Bluefin tunas spawn in tropical waters of the Gulf of Mexico, and spend the rest of the year feeding in temperate regions. Carried by the same current system as adults, tuna larvae and juveniles grow and feed along mature individuals. Tuna grow can grow quickly.
Image Sources: Wikimedia Commons, NOAA, graphics from The Pew Charitable Trusts (“Netting Billions 2020: A Global Tuna Valuation, October 6, 2020 pewtrusts.org)
Text Sources: Animal Diversity Web (ADW) animaldiversity.org; National Oceanic and Atmospheric Administration (NOAA) noaa.gov; Wikipedia, National Geographic, Live Science, BBC, Smithsonian, New York Times, Washington Post, Los Angeles Times, The New Yorker, Reuters, Associated Press, Lonely Planet Guides and various books and other publications.
Last Updated April 2023