The Big Tunas

a graphic depicting the various species of tuna

The largest tuna is the Atlantic bluefin (Thunnus thynnus), which can grow to be 1,500 pounds.

(Smithsonian Institution)

Tuna are the predators of the fish world. Large and athletic, with a mouth full of sharp teeth, they are impressive swimmers with large appetites. There are 15 genera and 51 species in the Scombridae, the broader tuna family, which includes the bonitos, Spanish mackerels, and mackerels. However, the eight members of the genus Thunnus are considered the true tunas. Seven of these (plus the related skipjack, Katsuwonus) are called the principal market species sought after by canners and anglers and auctioned at high prices at fish markets. They are sold to consumers in cans, as steaks, and raw at the sushi bar. The appetite for some tuna species is so great that some species are endangered due to overfishing, though conservation efforts seem to be helping in recent years. 

Tunas are also special among fishes. The bluefins, yellowfin, bigeye and albacore are some of the few warm-blooded (or endothermic) fishes and can retain the heat they produce as they swim. Most fish lose a lot of their body heat as the warm blood circulates through the gills, but these tunas have a special counter circulatory system that can both conserve body heat and allow for quick warming. This enables them to travel long distances in cold water, like during the 5,000 mile (8,000 km) long trans-Pacific migration many Pacific bluefin tuna make, or make exceptional dives into deep, frigid water like the southern bluefin. All tunas must continually swim to flush oxygen rich water over their gills, a necessity for a fish with such a high metabolism and muscular body. 

Below, the true tunas of the Thunnus genus are listed and some of their unique attributes are described. 

Bluefins (Thunnus maccoyii, Thunnus orientalis, Thunnus thynnus
Bluefin tunas are the kings of tuna. Reaching average sizes of 5 feet long and 130 pounds, and maximums of about 1,500 pounds, they are the largest of all the tuna. They are also slow to grow compared to all other tuna and have long lifespans of up to 20 years. There are three species of bluefin, each living in a different part of the world. The Atlantic  
(Thunnus thynnus) lives in the northern Atlantic Ocean, the Pacific  
(Thunnus orientalis) in the northern Pacific Ocean, and southern (Thunnus maccoyii), in temperate regions around the globe. Atlantic bluefin are the largest, and also the best divers, reaching depths roughly 3,300 feet deep. 

Bluefin tunas are the most sought-after tuna in sushi restaurants, and for many years the intense fishing pressure caused many bluefin populations to plummet. Scientists from the International Scientific Committee for Tuna and Tuna-like Species (ISC) estimated in a 2016 report that only 2.6 percent of the original, pre-fished Pacific bluefin tuna population remained, and in 2017 the National Oceanic and Atmospheric Administration (NOAA) estimated the population was 25 percent of what it was in the 1950s when the first population data was recorded. Atlantic bluefin were long on the endangered species list following a population crash in the 1970s, and southern bluefin were the hardest hit, earning a critically endangered designation in 1996. As tuna fishing vessels continued to return to port with empty holds, an outcry by scientists in the late 1990s and early 2000s sparked international conservation efforts, which enabled modest population increases. In 2021, the International Union for Conservation of Nature (IUCN) assessments saw the Pacific bluefin move from vulnerable to near threatened, the Atlantic bluefin from endangered to least concern, and the southern bluefin from critically endangered to endangered. Despite the optimistic start to recovery, the populations continue to rely on conservation measures and have a long way to go before they are in the clear.  

Yellowfin (Thunnus albacares
The brilliantly colored yellowfin tuna are not only beautiful, but due to their torpedo shape, extremely athletic swimmers. One of the fastest swimmers in the ocean, they can reach speeds around 45 miles per hour. To maintain such an athletic lifestyle, they are also expert predators with large appetites, eating large bony fish, squid, and other large prey. An average yellowfin tuna will reach sizes close to 60 pounds, but the largest can reach sizes closer to 400 pounds. Often, yellowfin will school with other species, the smaller ones joining smaller fish like skipjack and the larger will even join pods of dolphins. They live in tropical and subtropical oceans around the world and are usually found close to the surface. 

Bigeye (Thunnus obesus
Bigeye spend their days in deep water and then swim to the surface every night, a daily migration also known as diel vertical migration. They make the daily trek to follow their prey. Diving requires lots of oxygen, and a bigeye’s blood can hold more oxygen than the average fish. Spending time in the deep also means living in an environment with little light, a lifestyle that’s meant adapting to low light. Their exceptional vision and larger eyes are the source of its name. Bigeye are a similar size to the yellowfin, and can reach close to 400 pounds. 

Blackfin (Thunnus atlanticus
The blackfin tuna is the smallest of the tunas, maxing out at a length of roughly 40 inches and a weight of roughly 45 pounds. Despite their size, blackfin tuna have a reputation as feisty fighters and are tough to wrangle when caught by anglers. They are highly migratory and can be found in the western Atlantic from the coasts of Cape Cod in Massachusetts to Rio de Janeiro in Brazil. Blackfins often school with skipjack. 

Albacore (Thunnus alalunga
Albacore are one of the smaller species in the genus Thunnus. For a long time, they were one of the main tuna species used for canned tuna.  Tuna labeled white tuna are albacore while those labeled light tuna are mostly yellowfin and skipjack, (Katsuwonus pelamis) a fish in the greater tuna family. Unlike other tunas that eat an array of prey, albacore eat predominantly squid, especially a species known as the odd bobtail squid. They are highly migratory, school with other species of tuna like yellowfin and skipjack and are often found at the surface around floats of sargassum seaweed.  

On menus, yellowfin often appear as ahi tuna, a term that means tuna in the Hawaiian language. Both bigeye and yellowfin, tunas that swim near the Hawaiian coast, are called ahi tuna. In recent years bigeye has become a cheaper alternative to the depleted bluefin tunas and is often used in sushi. It is listed as a species of least concern by the IUCN’s Red list of endangered species. 

Longtail (Thunnus tonggol
The longtail tuna is a coastal tuna that lives in the Indo-Pacific Ocean, and is well known along the shores of Australia and New Zealand.  They even venture into estuaries and river deltas. Once referred to as the northern bluefin to distinguish it from the other tuna species in an overlapping region (the southern bluefin), the name caused confusion with the true bluefins and it has now fallen out of favor. 

September 2022
Tags: Tuna