Tunas comprise fifteen species in the Thunnini family of saltwater fish. Of those, eight are considered “true tunas”: five species of bluefins and three species of yellowfins, all which belong to the genus Thunnus. (Skipjack, one of the most popular species of canned tuna, aren’t “true tunas.”)
They range in size from the four-pound, foot-long bullet tuna up to the Atlantic bluefin tuna, which weighs an average 550 pounds and measures 6.6 feet long on average. Some can grow to be much larger.
Tunas are apex predators, which means they’re at the top of their food chain. Most eat fish, while some species prefer to feast on cephalopods like squid.
Tunas are fished extensively for food, and their speed and agility means that they are also prized by hunters as big game fish. As a result of overfishing, some species, notably bluefin tuna, have been threatened by extinction.
Physiology and speed
Tunas are known for their sleek, torpedo-shaped bodies designed for speed and endurance. The Atlantic bluefin tuna, for example, can swim up to 43 miles per hour; the yellowfin can swim even faster. Tunas have crescent moon-shaped tails and two dorsal fins on their backs, one of which can be flattened to reduce resistance in the water.
They’re among the only partially warm-blooded fish on Earth. While most fish are entirely cold-blooded and have body temperatures that match the water around them, some species of tunas have evolved the ability to warm up their swimming muscles temporarily, enabling them to swim at high speeds and migrate from warm to cold waters. The bluefin tuna is comfortable in the cold waters off Newfoundland and Iceland, as well as the tropical waters of the Gulf of Mexico and the Mediterranean Sea, where it goes each year to spawn.
Habitat and migration
Tunas are pelagic fish, meaning they live beyond the continental shelf in the open sea—not near coral reefs, the sea floor, or shoreline. The pelagic zone is the largest habitat on Earth, with a volume of 330 million cubic miles.
Tunas generally need to eat a lot and move a lot to sustain themselves. They’re known for long migrations and are capable of traversing great distances. Some geotagged bluefins have been tracked swimming from North American to European waters and back several times a year.
Several species also move vertically through the water column. The southern bluefin regularly moves 8,000 feet down through the ocean in search of prey, and the bigeye tuna (one of the bluefin species known as ahi) migrates more than 1,500 feet vertically in a single day, descending early in the morning to deeper, cooler waters and returning to shallower, warmer waters at dusk.
Many of the longest-distance tuna migrations are linked to spawning. They generally congregate in large groups to reproduce, and females can produce several million eggs at a time. Highly concentrated tuna spawnings are highly vulnerable to commercial fishing.
Tuna fishing and dolphin bycatch
Tuna has long been a staple of the diets of millions of people around the world and is considered to be one of the most commercially valuable fish. Skipjack tuna are the most-fished, by far, accounting for 50 to 60 percent of annual global tuna catches. Along with albacore (a type of bluefin), these species account for the majority of canned tuna. Yellowfin, bigeye, and other species of bluefin tuna are also fished extensively, and primarily used in sushi and sashimi.
One of the consequences of widespread tuna fishing has been dolphin bycatch. In the tropics of the eastern Pacific Ocean, fishermen have long used dolphin pods as living fish finders: spotted, spinner, and other species of dolphins routinely swim with schools of yellowfin tuna, and fishing boats follow with nets to catch the tuna below.
Scientists estimate that four million dolphins have died in the Indian Ocean’s poorly regulated gillnet fisheries since the 1950s. The researchers report that roughly 80,000 dolphins are now killed as bycatch annually.
Efforts to label cans of tuna “dolphin-safe” have generated controversy. (Learn how to identify truly dolphin safe tuna.)
Threats to conservation
One 2019 study found that the amount of tuna taken from the ocean has increased by 1,000 percent over the last 60 years—a rate that some scientists say is unsustainable.
Bluefin tuna have been especially hard-hit. In the 1970s, demand and prices for large bluefins soared worldwide, particularly in Japan. As a result, bluefin stocks, especially of large, breeding-age fish, plummeted, leading the International Union for Conservation of Nature (IUCN) to list several species as threatened by extinction—and the southern bluefin as critically endangered.
But there is hope as conservation efforts have led to curbs on commercial takes. In September 2021, surprising scientists, the IUCN announced that several tuna species have stepped back from the edge of extinction. According to the new data, the Atlantic bluefin tuna, once listed as endangered, qualified for a status of least concern. As do the yellowfin tuna and albacore tuna, which were both considered near-threatened the last time they were assessed.
Additionally, the southern bluefin tuna’s status has improved from critically endangered to endangered, bigeye tuna remains at a status of vulnerable, and skipjack tuna maintains its status of least concern.
SOURCE : nationalgeographic
