How Big Do Bluefin Tuna Get? Size, Weight and Record Fish

How big is a typical adult bluefin?

The Atlantic bluefin tuna is one of the heaviest bony fish in the sea. Most adults caught today measure roughly 1.5–2.5 m (5–8 ft) from snout to the fork of the tail and weigh about 60–300 kg (130–660 lb). Fish of more than about 150 kg (330 lb) are often called giant bluefin, and fish of 300–450 kg (660–990 lb) are still seen each year on rich feeding grounds such as the Gulf of St. Lawrence and Nova Scotia.

Bluefin get much heavier as they grow longer, because their bodies become deeper and thicker. Scientists use length-weight formulas to estimate the weight of a fish from its length:

These figures are averages. Weight also depends on the season: bluefin are thin after spawning and migrating, and much fatter after a summer of feeding in cold, food-rich northern waters.

The biggest bluefin ever recorded

The heaviest Atlantic bluefin accepted by the International Game Fish Association (IGFA) weighed 679 kg (1,496 lb). It was caught by Ken Fraser off Aulds Cove, Nova Scotia, Canada, on 26 October 1979. More than four decades later, it is still the all-tackle world record for the species and one of the heaviest bony fish ever weighed under game fishing rules.

Atlantic Canada has produced many of the biggest bluefin on record, especially the waters off Nova Scotia and North Lake, Prince Edward Island, where giant fish gather in autumn to feed on herring and mackerel. Northern Europe once had giants too: in the 1930s, bluefin weighing more than 320 kg (700 lb) were landed every season off Scarborough in England.

Larger figures exist but are less certain. NOAA Fisheries says Atlantic bluefin can reach up to 4 m (13 ft) and 900 kg (2,000 lb), and older reports from commercial catches describe fish of similar size, but none of these has the careful documentation of the 1979 record.

How fast do bluefin tuna grow?

Bluefin grow fast when young. Juveniles add about 30 cm (1 ft) a year at first, although this is slower than in some smaller tunas and billfish. Growth then slows steadily, and it takes about ten years to reach two thirds of the maximum length.

As a rough guide, a bluefin is about 1 m (3.3 ft) long at 3 years old, around 1.5 m (4.9 ft) at 6 and about 2 m (6.6 ft) at 10. The largest giants, of 2.5–3 m (8–10 ft), are usually 15–20 years old or more. Growth varies between individual fish, years and the two main stocks.

Fish from the eastern stock, which spawns in the Mediterranean, appear to mature at about 4–5 years old and around 1 m (3.3 ft) long. NOAA describes western bluefin, which spawn in the Gulf of Mexico, as generally not spawning until they are about 8 years old.

How old are the biggest bluefin?

Bluefin are long-lived for fish. Scientists age them by counting yearly growth layers in their otoliths, small ear bones. A 2008 study of western Atlantic bluefin checked these layers against radioactive carbon from nuclear bomb tests in the 1950s and 1960s, which marks the years when the fish were young, and confirmed ages of more than 30 years.

NOAA describes the species as living 20 years or more, and some fish may reach about 40. Because the biggest bluefin are also the oldest, heavy fishing in the late 20th century removed many of the largest fish, and decades are needed for a population to rebuild its giants.

Why do bluefin tuna grow so big?

Several things help bluefin reach such a size. They are partly warm-blooded: heat from their swimming muscles is trapped inside the body by networks of blood vessels, which lets them hunt in cold, rich northern seas that other tunas avoid, as explained in Are bluefin tuna warm-blooded? They migrate long distances to follow seasonal food such as herring, mackerel and squid. And a large body loses heat more slowly and stores more fat for migration and spawning.

Large size has costs. A bluefin must eat a great deal to fuel its warm, constantly swimming body, and big females are the most productive spawners, which is why protecting large fish matters for the species. The story of how the population collapsed and recovered is in Are bluefin tuna endangered?

How bluefin compare with other tunas

There are eight species in the genus Thunnus, and the three bluefins are the biggest. Sizes below are typical maximums reported for each species:

Among other bony fish, only a handful rival the Atlantic bluefin for weight, including the blue and black marlins, the swordfish and the ocean sunfish. Sharks grow larger still; the giants of both groups are ranked in the largest fish in the world.