Tuna are carnivorous fish from the family Scombridae, mostly in the genus Thunnus . Tuna are fast swimmers—they have been clocked at 70 kilometres per hour (43 mph)—and include several warm-blooded species. Unlike most fish, which have white flesh, tuna flesh is pink to dark red, which could explain their odd nick-name, "rose of the sea." The red coloring comes from tuna muscle tissue's greater quantities of myoglobin, an oxygen-binding molecule. Some of the larger species, such as the bluefin tuna, can raise their blood temperature above water temperature through muscular activity. This ability enables them to live in cooler waters and to survive in a wide range of ocean environments.
While many stocks are managed sustainably, it is widely accepted that bluefin have been severely overfished, with some stocks at risk of collapse. According to the International Seafood Sustainability Foundation (a global, non-profit partnership between the tuna industry, scientists, and the World Wide Fund for Nature), Indian Ocean yellowfin tuna, Pacific Ocean (eastern & western) bigeye tuna, and North Atlantic albacore tuna are all overfished. In April 2009 all stocks of skipjack tuna—which makes up roughly 60 percent of all tuna fished worldwide—were considered overfished.
Taxonomy
There are over 48 different tuna species. The Thunnus genus includes 8 species:
- Albacore, Thunnus alalunga . 105 centimetres (41 in)
- Yellowfin tuna, Thunnus albacares .
- Blackfin tuna, Thunnus atlanticus .
- Southern bluefin tuna, Thunnus maccoyii .
- Bigeye tuna, Thunnus obesus .
- Pacific bluefin tuna, Thunnus orientalis .
- Northern bluefin tuna, Thunnus thynnus .
- Longtail tuna, Thunnus tonggol .
Species of several other genera (all in the family Scombridae) have common names containing "tuna":
- Slender tuna Allothunnus fallai
- Bullet tuna Auxis rochei
- Terriowipet tuna Auxis tongolis .
- Frigate tuna Auxis thazard
- Kawakawa (little tuna or mackerel tuna) Euthynnus affinis
- Little tunny (little tuna) Euthynnus alletteratus
- Black skipjack tuna Euthynnus lineatus
- Dogtooth tuna Gymnosarda unicolor
- Skipjack tuna Katsuwonus pelamis
- Lineside Tuna, Thunnus lineaus .
Biology
A remarkable aspect of Thunnus physiology is its ability to maintain body temperature above than that of the ambient seawater. For example, bluefin can maintain a core body temperature of 75-95°F (24-35°C), in water as cold as 43 °F (6 °C). However, unlike typical endothermic creatures such as mammals and birds, tuna do not maintain temperature within a relatively narrow range.
Tuna achieve endothermy by conserving the heat generated through normal metabolism. The rete mirabile ("wonderful net") the intertwining of veins and arteries in the body's periphery, transfers heat from arterial blood to venous blood via a counter-current exchange system. This reduces surface cooling, maintaining a warmer core. Higher body temperatures allow more efficient muscle use, supporting higher swimming speed with reduced energy expenditure.
Commercial fishing
Tuna is an important commercial fish. The International Seafood Sustainability Foundation compiled a detailed scientific report on the state of global tuna stocks in 2009, which includes regular updates. According to the report, 'Tunas are widely but sparsely distributed throughout the oceans of the world, generally in tropical and temperate waters between about 45 degrees north and south of the equator. They are grouped taxonomically in the family Scombridae, which includes about 50 species. The most important of these for commercial and recreational fisheries are yellowfin (Thunnus albacares), bigeye (T. obesus), bluefin (T. thynnus, T. orientalis, and T. macoyii), albacore (T. alalunga), and skipjack (Katsuwonus pelamis).
The report further states:
The Australian government alleged in 2006 that Japan had illegally overfished southern bluefin by taking 12,000 to 20,000 tonnes per year instead of the their agreed 6,000 tonnes; the value of such overfishing would be as much as USD $2 billion. Such overfishing has severely damaged stocks. According to the WWF, "Japan's huge appetite for tuna will take the most sought-after stocks to the brink of commercial extinction unless fisheries agree on more rigid quotas".
Fishing methods
- Andalusian method of Almadraba , uses a maze of nets. In Sicily the same method in called Tonnara .
- Fish farming (Cage system)
- Longline fishing
- Purse seines
- Pole and line
- Harpoon gun
- Big game fishing
- Fish aggregating device
The Rybovich family of South Florida eventually constructed a boat in 1946 that relaunched the sport and birthed a new industry. This boat, the Miss Chevy II, was the first sportfishing boat the world had ever seen.
Merritt gained particular notoriety during the 1950s through the 1970s with its 37- and 43-foot custom boats, which together with boats like those being built by Rybovich helped fuel the growth of big game fishing around the world.
Association with whaling
In 2005 Nauru, defending its vote at that year's meeting of the International Whaling Commission, argued that commercial whaling is necessary for preserving tuna stocks and that country's fishing fleet.
Association with dolphins
Dolphins swim beside several tuna species. These include yellowfin tuna in the eastern Pacific Ocean, but not albacore or Mahi Mahi. Tuna schools are believed to associate themselves with dolphins for protection against sharks, which are tuna predators.
Fishing vessels exploit this association by searching for dolphin pods. They encircle the pod with nets to catch the tuna beneath. The nets are prone to entangling dolphins, injuring or killing them. Public outcry has led to more "dolphin friendly" methods, now generally involving lines rather than nets. However, there are neither universal independent inspection programs nor verification of "dolphin safeness", so these protections are not absolute. According to Consumers Union, the resulting lack of accountability means claims that tuna that is "dolphin safe" should be given little credence.
Fishery practices have changed to be dolphin friendly, which has caused greater bycatch including sharks, turtles and other oceanic fish. Fishermen no longer follow dolphins, but concentrate their fisheries around floating objects such as fish aggregation devices that attract large populations of other organisms. The public demand to protect dolphins which are not particularly endangered actually damages endangered species.
Recreational fishing
From the 1950s through the 1970s, bluefin were abundant in the waters of Cuba, Bimini and Cat Cay, a few miles off the Florida coast, and were targeted by recreational fishermen, famously Ernest Hemingway and Habana Joe aboard his 1938 40-foot Wheeler named Pilar . Word spread quickly about the exciting new sport of big-game fishing. Despite the growing popularity of the sport, however, the boats of the day were hardly ideal for fighting the prized fish. Most boats used at the time were converted cabin cruisers, which were relatively slow and hard to maneuver.
Management and conservati
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