The word tuna is derived from a Greek word Thunnus meaning 'to rush'. This torpedo-shaped, streamlined fish is aptly described as 'Ferrari of the ocean'. It is a good fighter and is quite difficult to catch. As tuna is a champion long-distance swimmer. It is a popular game fish too. Certain species of tuna are extensively fished for their meat as there is an increasing demand for canned tuna.
Most fish are cold-blooded. However, tuna are warm-blooded. The organs called 'retia mirabilia' present near their muscles prevent heat loss through gills and help maintain a body temperature higher than that of the surrounding water. Various physical characteristics of these fish help them swim fast, maintain the body temperature, and dive deeper in chilly waters that are low in oxygen.
The subfamily 'Scombrinae' of the family 'Scombrinae' is divided into four tribes, which contain different genera of Mackerels, Spanish Mackerels, Bonitos, and Tunas. Tunas belong to the tribe 'Thunnini' which contains five genera Allothunnus, Auxis, Euthynnus, Katsuwonus, and Thunnus. The genus Thunnus which consists of 'true tunas' is divided into subgenera Thunnus and Neothunnus.
► Thunnus - Thunnus
► Thunnus - Neothunnus
The genus Allothunnus (slender tunas)Species - Allothunnus fallai
It is about one meter (3.3 feet) long. The weight can be around 10 kg. It is found in the southern oceans between latitudes 20° and 50° South. This is an occasionally schooling species. Due to its oily flesh, it is rarely caught. It is commonly seen off the east coast of Tasmania.
The genus Auxis (frigate tunas)Species - Auxis rochei or bullet tuna or bullet mackerel or marusoda
It is found in the Atlantic, Indian, and Pacific oceans at depths of 50 m (164 ft). It is also present in the Mediterranean Sea. The maximum length of this tuna is 50 cm (1.6 ft) and it may weigh 4 lb (1.8 kg). It can be identified with the pattern of zig-zag dark markings on the upper hind body. The color of the body is silver and the fins are dark gray. The species is further divided into subspecies A. rochei rochei and A. rochei eudorax.
Species - Auxis thazard or hirasoda, or frigate tuna
It is divided into subspecies A. thazard thazard, found in tropical and subtropical Atlantic, Indian, and Pacific Oceans, and A. thazard brachydorax, mainly found in Eastern Pacific.
The genus Euthynnus (little tunas)Species - Euthynnus affinis or Mackerel tuna or kawakawa
It is a ray-finned (fins are webs of skin backed by bony or horny spines) bony fish. It can be 2 - 3.3 ft long. It may weigh 30 lb. It usually lives up to 6 years.
Species - Euthynnus alletteratus or little tunny/tuna or false albacore
The length of this fish can be 2.6 - 4 ft. It lives up to 10 years. It weighs up to 36 lb and is a favorite sport fish due to its high speed and exceptional fighting capacity. It is found in the Atlantic and the Mediterranean, and is the most common tuna in the Atlantic Ocean. It can be identified with the help of the worm-like markings on its back. Dark spots between its pectoral and ventral fins are also easily noticeable. Little tuna, packed with oil is an excellent bait for sharks and marlins. It is low in nutrients and is not usually used as a food fish. Certain cases of ciguatera poisoning after consumption of this fish have been reported.
Species - Euthynnus lineatus or black skipjack tuna
This ray-finned fish can be 2 - 2.8 ft long and may weigh up to 26 lb. It is found in the eastern tropical Pacific ocean.
The genus Katsuwonus (skipjack tunas) or aku, or striped tuna, or victor fish.Species - Katsuwonus pelamis
The fish is also known as arctic bonito, mushmouth, oceanic bonito. This medium-sized tuna can be 3 ft long and usually weighs 8-10 kg. It can be identified with the help of the corselet (a circular band of large, thick scales around the body) behind the head. The remaining part of the body has no scales. This tuna is an important game and commercial fish. It is an invariable part of Japanese cuisine.
The genus Thunnus, subgenus ThunnusSpecies - Thunnus alalunga or albacore tuna
Tunas belonging to the genus Thunnus are called 'true tunas'. The albacore is commonly known as albie, pigfish, tombo ahi, binnaga, German bonito, longfin tuna/tunny, etc. This cool water species spawns in warm tropical waters. It is a highly migratory species. It is present in all tropical and temperate oceans and the Mediterranean Sea. It has a very long pectoral fin. This tuna can be about 1.4 m long and may weigh up to 60 kg. It lives for around 12 years. Only albacore tuna is marketed as 'white meat tuna' in the United States. It plays an important role in fish farming. It is a gourmet food and is the most avidly sought-after fish for sports. The status of this tuna species is 'Near Threatened'. According to the information collected by the ISSF Scientific Advisory Committee, the northern and southern Atlantic Ocean stocks of albacore are overfished.
Species - Thunnus maccoyii or southern bluefin tuna
This is also a highly migratory species of tuna. The length of this tuna can be more than 2 m and the weight can be more than 250 kg. It lives for about 20 years. This torpedo-shaped, powerful tuna is one of the largest bony fish of the world. These tunas have a silvery white body and are found throughout the southern Atlantic, Pacific, and Indian Oceans. They can reach speeds of 70 kilometers per hour. This species is classified as 'Critically Endangered' on the IUCN Red List of Threatened species.
Species - Thunnus obesus or bigeye tuna
It is present throughout Australian waters. It is found in the subtropical and tropical areas of the Atlantic, Indian, and Pacific oceans. Because of a unique physical characteristic (vascular counter-current heat exchangers), this tuna can forage in deeper colder waters and can tolerate oxygen-poor waters of 1.0 ml/L-1. This species can be identified by its plump body, large head, moderately large pectoral fins, and larger eyes (the large spherical lens functions well in low light conditions). It can be about 2 m long and may weigh 150 - 200 kg. These are marketed as fresh or frozen. The average life expectancy of this tuna is 10-16 years. It is an important food fish as well as game fish.
Species - Thunnus orientalis or Pacific bluefin tuna
The status of this tuna is 'vulnerable'. Due to overfishing, their population has declined significantly. Being a valuable commercial species, they are overfished. These tuna can live for more than 20 years. They are commonly found in the North Pacific Ocean. However, they are present in the southern hemisphere too. They are highly migratory and often visit the South Pacific. They spawn to the south of Japan, near Okinawa and Honshu. An adult is about 9.8 ft (3 m) long and may weigh about 990 lb (450 kg). They can live for 15-25 years. Most Pacific and Atlantic bluefin tunas are consumed in Japan. Those that are suitable for sashimi and sushi recipes are sold at very high prices.
Species - Thunnus thynnus or Atlantic bluefin tuna
The Atlantic tuna is the largest of all tuna species. A 4.6 m (15 ft.) long Atlantic bluefin weighing 1,508 lb (684 kg) is the largest tuna recorded. These tuna can dive to depths of 1,000 meters (550 fathoms) in search of food. Although all tunas are warm-blooded, bluefin tunas are excellent in thermoregulation. So, they can forage in freezing waters of the North Atlantic. It is believed that these fish can live up to 50 years. Normally, they live for 15-30 years. These sleek and powerful tunas are overfished. They are valuable ingredients of sushi and sashimi and are sold at very high prices. Although a female can produce up to 30 million eggs, this is declared as an 'endangered' species.
The genus Thunnus, subgenus NeothunnusSpecies - Thunnus albacares or yellowfin tuna
These are found in tropical and subtropical oceans around the world. They are often seen swimming near the surface with other tunas, such as skipjack tuna and bigeye tuna, and also with dolphins. The juveniles grow quickly, and reach a weight of 3.4 kg in about 18 months. Both bigeye and yellowfin tuna are marketed as 'ahi'. A yellowfin tuna can weigh over 400 lb (180 kg) and can live up to 10 years. It has bright yellow dorsal and anal fins, and finlets. The body is dark blue or blackish, and the silvery belly has a yellow tint. With a streamlined body and folded fins, this tuna can swim with a speed of 80 kph.
Species - Thunnus atlanticus or blackfin tuna
This is the smallest tuna in the Thunnus genus. It is found in the western Atlantic Ocean from Cape Cod to Brazil. It can be about 39 inches (100 cm) long and may weigh about 46 pounds (21 kg). It has a black back. The yellow color on the finlets and also on the sides of the body is easily noticeable. This is a short-lived species.
Species - Thunnus tonggol or longtail tuna
This is the second-smallest of all tunas. The main characteristic feature of this species is that it lacks a swim bladder (an air-filled sac which helps the fish to maintain buoyancy). The elongated oval spots on the silvery-white body of this tuna are arranged in horizontal rows. The second dorsal fin and the anal fin is slightly yellowish. The blackish caudal fin also has a yellowish-green tinge. These tuna have silvery white bellies. They are dark blue or black on top (dorsally). They can grow up to 145 centimeters (57 inches), however their common length is about 70 cm. A longtail tuna may weigh 35.9 kilograms (79 lb).
Large predatory fish like tuna, swordfish, and sharks contain the highest levels of mercury, a harmful element that can seriously affect the human nervous system. Mercury levels can be high in larger species like bluefin and albacore. The average total mercury content of an albacore tuna is .14 ± .05 ppm. Pregnant and breastfeeding women, and young children should avoid eating tuna. Processing techniques like canning, freezing, or cooking do not help lower the mercury content. However, canned tuna usually has lower mercury levels than other tuna, because smaller and younger (less than one year old) tuna are used for canning. Albacore and skipjack tunas are popular types of tuna to eat, especially canned.
As tunas swim fast, they need to feed almost continuously. Although they belong to the class of large predatory fish, they themselves have predators who feed on them, for example killer whales, pilot whales, sharks, and of course human beings. It should be kept in mind that as a top predator, tunas help maintain a balance in the ocean environment. Overfishing, in the long run, can destabilize the food chain, and can affect the balance of life in the oceans.