Description
The bluefin tuna, one of the largest species of bony fish in the world, is renowned for its size, speed and beauty in both the Atlantic and Pacific Oceans. The known range of the Atlantic bluefin tuna is from Newfoundland to Brazil in the western Atlantic and Norway to central Africa in the east Atlantic. The bluefin attracts intense interest in numerous recreational and commercial fisheries. Bluefin tuna is prized in Asian markets as one of the best fish to eat raw. Its seasonal feeding migration to the Gulf of Maine has become a traditional feature to New England fishing ports. Over seas, feeding and spawning migrations in the Mediterranean and in the Pacific have attracted fisheries for centuries, and resulted in cultural reverence for this giant fish.

Similar to other members of the mackerel family, bluefin tuna has a graceful, streamline appearance. The snout is pointed and the tail region is slender, both leading up to a robust body, hence providing for the "football" name often given to juvenile bluefin. All fins appear streamlined and the dorsal, pelvic and pectoral fins fit into slots in the body to reduce drag. A series of small, yellowish finlets occur from the second dorsal and anal fin to the caudal fin. Bluefin tuna are darkly colored on their dorsal surface with dark, shiny blue tones that can approach black. The dark coloration fades and becomes silvery towards the lateral line. Below the lateral line and the belly are silvery and may have irregular bands and spots that are iridescent white, gray and silver. Dorsal fins are dusky to black and ventral fins are dusky with lighter shades of white, gray, and silver.

Because of their large size, it is usually not a problem distinguishing bluefin tuna from other tuna species off the coast of Massachusetts. Other than large size, bluefin can be separated from other Thunnus species by their higher gill raker count (34-43), shorter pectoral fin, and presence of striations on their liver.

Bluefin tuna are a fast-growing species that can exceed 10 ft in length and weigh over 1,000 pounds. The largest bluefin tuna caught by an angler in Massachusetts waters weighed 1,228 pounds in 1984, and the all-tackle record for the Atlantic is 1,496 lbs. caught in Nova Scotia in 1979. One-year-old bluefin tuna are about 10 pounds by mid-summer and are a sporadic visitor to our shores south of Cape Cod. Ages 2-4 are typically 20-80 pounds and an annual visitor to feeding grounds south of Cape Cod. Until recently, these school tuna were not considered common visitors to the Gulf of Maine, although they were found in high abundance in the Gulf of Maine during the 1940s and 1950s and observations have increased since the 1990s. There is substantial variation to individual growth once bluefin reach five or six years. Giant tuna is a subjective term used for mature bluefin seen in the Gulf of Maine that typically are at least 10 years old and about 300 pounds or higher.

Habits
The bluefin's large size and capacity to visit all the temperate oceans of the world have made it a difficult species to study. There is much we still don't know about bluefin tuna.

Physically, they have the ability to retain metabolic heat, rendering them the closest thing to a warm-blooded fish. Being warmer than the surrounding water allows them conduct physiological processes faster than cold-blooded fish. Food digestion and oxygen transport can occur quicker and more efficiently. And the warmer bluefin can colonize colder regions of the Atlantic in search of prey.

Their ability to stay warm in cool water brings them to the Gulf of Maine each year on a feeding migration. They typically arrive in June and depart in October. They will aggregate and forage on concentrations of small pelagic prey like mackerel, sand lance, sea herring, menhaden and squid. In the absence of large schools of pelagic prey, they will feed on whatever they encounter throughout the water column. Bluefin are a schooling species that usually remain in schools of similar sized cohorts. Jeffreys Ledge, Stellwagen Bank, Cape Cod Bay and the Great South Channel are traditional fishing grounds for giant bluefin. However, movements in the Gulf of Maine and south of Cape Cod are highly variable within each season and year-to-year and certainly depend on forage concentrations.

The movements and spawning habits of bluefin tuna still contain some mystery. Spawning is known to occur in the western Atlantic primarily in the Gulf of Mexico and within the Mediterranean Sea over in the eastern Atlantic. We have long known that many bluefin that spawn in the spring in the Gulf of Mexico will head north to feeding grounds along the U.S. continental shelf. There is a growing body of evidence that indicates western Atlantic bluefin tuna can interact with the spawning and feeding grounds in the eastern Atlantic and Mediterranean Sea. Despite being the same species, the expected age of first spawning is quite different for the two groups of tuna. Western bluefin are thought to mature at about age-8 when they weigh near 250 pounds. Eastern bluefin mature at about age-4 when they weigh less than 80 lbs. Hopefully, ongoing research on reproduction, migrations and stock identification will shed more light on the Atlantic stock composition of bluefin tuna.