Underwater World

Atlantic Bluefin Tuna

Atlantic Ocean, Mediterranean Sea

At a glance | Vital stats | Curiosities | Closeup

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At a glance

Few fish species are as awe-inspiring as the bluefin tuna with its majestic colours, immense size and ability to swim literally across oceans. Both sport fishermen and commercial fisheries chase after this fish: battling a bluefin tuna is usually an epic experience long remembered by any fisherman. The tuna is a premium source of sushi worldwide and, in fact, market demand for bluefin tuna in general has led to it being over-fished.

Vital stats

Curiosities

Bluefin tuna are warm blooded, which is rare among fish. They can maintain body temperatures higher than the temperature of their surroundings-even during very deep dives. Being warm blooded allows bluefin tuna to visit Canadian waters during summer and fall and take advantage of the rich feeding grounds there.

Closeup

About the bluefin tuna

The bluefin tuna, nicknamed the Olympian of the Oceans, is a highly evolved species. Its streamlined, torpedo-like body makes it an extraordinarily efficient swimmer. While most fish swim by moving their bodies from side to side, the bluefin tuna has a rigid body, channelling energy through tendons to its tail to propel it.

The bluefin's back is dark blue with a greenish-yellow hue; its belly is silver, speckled with spots or bands. This fish has two dorsal fins and a row of small yellow finlets running along its back and underside, all the way to its crescent-shaped tail. While most fish open and close their mouths to breathe, the bluefin tuna keeps its mouth open always, breathing as water flows past its gills.

Bluefin tuna can grow to enormous sizes, the largest of which are called 'giants' and can grow to more than three metres in length and can weigh more than 450 kg. They tend to grow especially fast in their younger years-at a rate of about 30 cm per year.

Bluefin tuna are opportunistic eaters, meaning they eat whatever prey is most available. Young bluefins tend to feed on crustaceans, fish and cephalopods (such as octopus and squid). Adults generally feed on other fish: herring, anchovies, sand lances, sardines, sprat, bluefish and mackerel.

Lifecycle and reproduction

Bluefin tuna return to the place where they were born to reproduce (spawn). The western Atlantic population spawns in the late spring and early summer in the Gulf of Mexico. In the western Gulf, these tuna tend to spawn in small whirlpool areas. Eastern bluefin tuna spawn in the Mediterranean Sea around Spain's Balearic Islands and in the Tyrrhenian Sea west of Italy.

Female bluefin tuna can produce more than 10 million eggs in a single spawning season. While the Atlantic population tends to start reproducing at about age eight (when an individual tuna usually weighs about 140 kg), bluefin tuna in the Mediterranean can begin spawning at age five or younger.

The underwater world of bluefin tuna

The Atlantic bluefin tuna occurs throughout and on both sides of the Atlantic Ocean as well as the Mediterranean Sea. It can be spotted from the Gulf of Mexico to Newfoundland in the West Atlantic, and from the Canary Islands off Africa to Iceland in the East Atlantic. It most often chooses subtropical and temperate waters to live in and can migrate huge distances. Because it is warm-blooded, the bluefin tuna can survive in waters ranging in temperature from one to 31 degrees Celsius, and can dive to depths of 1,000 metres-a huge advantage, as this allows it to move into colder waters and take advantage of abundant food supplies.

Bluefin tuna are at the top of the food chain, thanks to their size. They do, however, occasionally fall victim to sharks and whales. Young tuna are sometimes eaten by other fish in the upper layers of the sea.

Harvesting bluefin tuna

Canadian bluefin tuna fisheries operate in several areas off the Atlantic coast between July and November. In Canadian waters, fishermen use hook and line, electric harpoons and traps to catch them. Bluefin tuna can also be caught as byproducts of other fishing operations during the summer and fall.

Efforts to raise bluefin tuna in captivity-called aquaculture-have proved only moderately successful. Bluefin tuna are also 'ranched': caught and fattened before released for harvest.

Because of their distribution patterns, bluefin tuna are under the jurisdiction of the International Commission for the Conservation of Atlantic Tunas (ICCAT), which recommends management measures for the species. Fisheries and Oceans Canada implements these management measures and applies catch level and minimum size restrictions to the bluefin tuna fishery in Canada.

Revised: October 2009