Atlantic Bluefin Tuna (Thunnus thynnus): Facts and Size

What does an Atlantic bluefin tuna look like?

The bluefin has a massive, torpedo-shaped body that is almost round in cross-section and tapers to a narrow tail base. The back is dark metallic blue to blue-black, and the lower sides and belly are silvery white, marked with faint colorless lines and rows of dots. The first dorsal fin is yellowish or bluish, the second is reddish brown, and a row of small yellow finlets edged in black runs along the top and bottom of the body toward the tail.

The tail is a stiff, crescent-shaped fin, and a strong keel on each side of the tail base helps it slice through the water. The pectoral fins are short, less than 80% of the length of the head, which helps separate the Atlantic bluefin from other tunas. Fins can fold flat into grooves in the body to reduce drag.

Bluefin swim with a rigid body, beating only the tail, a style that saves energy on long journeys. They have to keep moving to breathe, because they rely on water rushing over the gills as they swim. A thin, translucent patch on top of the head, called the pineal window, lets light reach the brain; scientists think it may help the fish sense light levels and perhaps find its way, though its exact role is uncertain.

How big do Atlantic bluefin tuna get?

Atlantic bluefin are the giants of the tuna family. Most adults caught today measure about 1.5–2.5 m (5–8 ft) and weigh roughly 60–300 kg (130–660 lb), and fish over about 150 kg (330 lb) are often called giant bluefin. NOAA Fisheries says the species can reach about 4 m (13 ft) and 900 kg (2,000 lb), although such fish have not been verified in modern times.

The heaviest Atlantic bluefin accepted by the International Game Fish Association weighed 679 kg (1,496 lb). It was caught by Ken Fraser off Aulds Cove, Nova Scotia, Canada, in October 1979, and the record still stands. The story of record fish and how fast bluefin grow is in How big do bluefin tuna get?

Where do Atlantic bluefin tuna live?

Atlantic bluefin live across the North Atlantic and in the Mediterranean Sea. In the west they range from Newfoundland and the Gulf of St. Lawrence south to the Gulf of Mexico and the Caribbean, and in the east from Iceland and Norway south to the Canary Islands. They have disappeared from the Black Sea, where they were once common.

They are fish of the open ocean but often come close to shore to feed, especially off New England, Atlantic Canada, Ireland, Britain and Norway. Tagged bluefin have been recorded in water from about 3 °C (37 °F) to 30 °C (86 °F) and dive to more than 1,000 m (3,300 ft), although they spend most of their time in the upper few hundred meters.

The North Sea was once a major feeding ground. Big bluefin followed herring and mackerel there every summer, and in the 1930s fish of over 320 kg (700 lb) were landed off Scarborough in England. By the 1960s, after heavy fishing of the tuna and their prey, they had all but vanished from northern Europe. Since the mid-2010s, bluefin have returned in growing numbers to Scandinavian, British and Irish waters, helped by stricter catch limits and warmer seas.

Migration and spawning

Atlantic bluefin are among the great ocean travelers. Electronic tags show that some fish cross the entire Atlantic, and individuals from both sides of the ocean mix on shared feeding grounds in the central and North Atlantic. Few fish make journeys as long; others are listed among the longest animal migrations.

Despite this mixing, scientists manage the species as two main stocks that return to separate spawning grounds:

• Western stock: spawns in the Gulf of Mexico from about mid-April to June, in water of about 24–29 °C (75–84 °F).
• Eastern stock: spawns in the Mediterranean from about June to August, especially around the Balearic Islands, Sicily and the eastern Mediterranean.

Since 2016, researchers have also found bluefin larvae in the Slope Sea, an area off the continental shelf of the northeastern United States, suggesting a third spawning area. Most fish appear to return to the region where they hatched to spawn.

A warm-bodied fish

Most fish are the same temperature as the water around them, but the bluefin is a partial exception. Heat made by its powerful red swimming muscles, which lie deep inside the body near the backbone, is kept in by dense networks of fine blood vessels called retia mirabilia. These work as countercurrent heat exchangers: warm blood leaving the muscles heats the cold blood arriving from the gills.

As a result, a bluefin can keep its muscles as much as 20 °C (36 °F) warmer than the surrounding sea, and it also warms its eyes, brain and stomach. Among bony fish, this ability is most highly developed in the bluefin tunas. It lets them hunt in cold northern waters where other tunas cannot follow, as explained in Are bluefin tuna warm-blooded?

What do bluefin tuna eat?

Bluefin tuna are top predators that hunt by sight, often in schools of similar-sized fish. Adults feed mainly on oily schooling fish such as herring, mackerel, sardines, anchovies, sand lance and bluefish, along with squid, crabs and other crustaceans. Young bluefin eat smaller fish, squid and crustaceans.

They eat a great deal. A large, warm, constantly swimming body burns energy fast, so bluefin feed heavily on summer and autumn feeding grounds, building up fat for migration and spawning. Their speed and power come from muscle that can contract strongly for short chases, backed by blood that is unusually rich in hemoglobin.

Growth, reproduction and lifespan

Bluefin grow quickly when young, adding about 30 cm (1 ft) a year at first, then slow down; it takes roughly ten years to reach two thirds of maximum length. Fish from the eastern stock appear to mature at about 4–5 years old, while NOAA describes western bluefin as generally not spawning until they are about 8 years old.

Spawning females release their eggs into open water in batches over several weeks, where they are fertilized by males. NOAA says a female can produce up to 10 million eggs a year. The eggs hatch in about two days, and the tiny larvae are eaten by countless predators, including each other, so only a tiny fraction survive their first year.

NOAA describes Atlantic bluefin as living 20 years or more. Studies that dated growth layers in the ear bones of western Atlantic fish have found bluefin over 30 years old, and some may live to about 40.

Predators and threats

Young bluefin are eaten by bluefish, seabirds, larger tunas and other predators. Adults have few natural enemies apart from orcas, pilot whales and large sharks such as the shortfin mako and the great white shark. A small population of orcas in the Strait of Gibraltar specializes in chasing bluefin tuna.

Overfishing has been by far the greatest threat. Bluefin are among the most valuable fish in the world, mostly sold for sushi and sashimi in Japan. From the 1960s onward, industrial fishing and later the capture of wild fish for fattening in sea pens drove steep declines, made worse by illegal and unreported catches. Pollution and changes in ocean temperature add to the pressure. The 2010 Deepwater Horizon oil spill in the Gulf of Mexico happened during the western stock’s spawning season.

Conservation status

The IUCN Red List moved the Atlantic bluefin tuna from Endangered to Least Concern in 2021, one of the clearest recoveries of a heavily fished species. The improvement followed strict catch limits set by the International Commission for the Conservation of Atlantic Tunas (ICCAT) from the late 2000s, better monitoring and a crackdown on illegal fishing.

The recovery is not complete everywhere. Scientists warn that the western Atlantic stock, which spawns in the Gulf of Mexico, remains far below its size in the 1970s. The full story is in Are bluefin tuna endangered?

Bluefin tuna and people

People have eaten bluefin tuna for thousands of years. Ancient Mediterranean peoples, including the Phoenicians, Greeks and Romans, prized the fish, and some Greek cities put tuna on their coins. Traditional coastal tuna traps, known as almadraba in Spain and tonnara in Italy, operated in the Mediterranean for centuries.

In the 20th century, bluefin went from a cheap catch to a luxury. Demand in Japan, where fatty bluefin belly is a prized delicacy, turned it into one of the most valuable fish on Earth. That value drove the overfishing of the late 20th century, and it is also why careful management matters so much for the species’ future.