Tuna


A tuna is a saltwater fish that belongs to the tribe Thunnini, a subgrouping of the Scombridae family. The Thunnini comprise 15 species across five genera, the sizes of which vary greatly, ranging from the bullet tuna up to the Atlantic bluefin tuna, which averages and is believed to live up to 50 years.
Tuna, opah, and mackerel sharks are the only species of fish that can maintain a body temperature higher than that of the surrounding water. An active and agile predator, the tuna has a sleek, streamlined body, and is among the fastest-swimming pelagic fish—the yellowfin tuna, for example, is capable of speeds of up to.
Found in warm seas, the tuna is commercially fished extensively as a food fish, and is popular as a bluewater game fish. As a result of overfishing, some tuna species, such as the southern bluefin tuna, are threatened with extinction.

Etymology

The term "tuna" comes from Spanish atún < Andalusian Arabic at-tūn, assimilated from al-tūn التون : 'tuna fish' < Middle Latin thunnus. Thunnus is derived from used for the Atlantic bluefin tuna, that name in turn is ultimately derived from θύνω thýnō, meaning "to rush, dart along".
A dated alternative term is "tunny".
In English, tuna has been referred to as Chicken of the Sea. This name persists today in Japan, where tuna as a food can be called シーチキン, literally "sea chicken".

Taxonomy

The Thunnini tribe is a monophyletic clade comprising 15 species in five genera:
The cladogram is a tool for visualizing and comparing the evolutionary relationships between taxa, and is read left-to-right as if on a timeline. The following cladogram illustrates the relationship between the tunas and other tribes of the family Scombridae. For example, the cladogram illustrates that the skipjack tunas are more closely related to the true tunas than are the slender tunas, and that the next nearest relatives of the tunas are the bonitos of the tribe Sardini.


True species

The "true" tunas are those that belong to the genus Thunnus. Until recently, it was thought that there were seven Thunnus species, and that Atlantic bluefin tuna and Pacific bluefin tuna were subspecies of a single species. In 1999, Collette established that based on both molecular and morphological considerations, they are in fact distinct species.
The genus Thunnus is further classified into two subgenera: Thunnus , and Thunnus .

Other species

The Thunnini tribe also includes seven additional species of tuna across four genera. They are:

Biology

Description

The tuna is a sleek, elongated and streamlined fish, adapted for speed. It has two closely spaced but separated dorsal fins on its back; The first fin is "depressible" – it can be laid down, flush, in a groove that runs along its back; it is supported by spines. Seven to ten yellow finlets run from the dorsal fins to the tail, which is lunate – curved like a crescent moon – and tapered to pointy tips. A tuna's pelvic fins are located below the base of the pectoral fins. Both dorsal and pelvic fins retract when the fish is swimming fast.
The tuna's body is countershaded to camouflage itself in deeper water when seen from above, its dorsal side is generally a metallic dark blue while the ventral or under side is silvery, often with an iridescent shine. The caudal peduncle, to which the tail is attached, is quite thin, with three stabilizing horizontal keels on each side.

Physiology

Thunnus are widely but sparsely distributed throughout the oceans of the world, generally in tropical and temperate waters at latitudes ranging between about 45° north and south of the equator. All tunas are able to maintain the temperature of certain parts of their body above the temperature of ambient seawater. For example, bluefin can maintain a core body temperature of, in water as cold as. Unlike other endothermic creatures such as mammals and birds, tuna do not maintain temperature within a relatively narrow range.
Tunas achieve endothermy by conserving the heat generated through normal metabolism. In all tunas, the heart operates at ambient temperature, as it receives cooled blood, and coronary circulation is directly from the gills. The rete mirabile, the intertwining of veins and arteries in the body's periphery, allows nearly all of the metabolic heat from venous blood to be "re-claimed" and transferred to the arterial blood via a counter-current exchange system, thus mitigating the effects of surface cooling. This allows the tuna to elevate the temperatures of the highly-aerobic tissues of the skeletal muscles, eyes and brain, which supports faster swimming speeds and reduced energy expenditure, and which enables them to survive in cooler waters over a wider range of ocean environments than those of other fish.
Also unlike most fish, which have white flesh, the muscle tissue of tuna ranges from pink to dark red. The red myotomal muscles derive their color from myoglobin, an oxygen-binding molecule, which tuna express in quantities far higher than most other fish. The oxygen-rich blood further enables energy delivery to their muscles.
For powerful swimming animals like dolphins and tuna, cavitation may be detrimental, because it limits their maximum swimming speed. Even if they have the power to swim faster, dolphins may have to restrict their speed, because collapsing cavitation bubbles on their tail are too painful. Cavitation also slows tuna, but for a different reason. Unlike dolphins, these fish do not feel the bubbles, because they have bony fins without nerve endings. Nevertheless, they cannot swim faster because the cavitation bubbles create a vapor film around their fins that limits their speed. Lesions have been found on tuna that are consistent with cavitation damage.

Fishing

Commerce

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 ISSF, the most important species for commercial and recreational tuna fisheries are yellowfin, bigeye, bluefin, albacore, and skipjack.
Based on catches from 2007, the report 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 agreed upon 6,000 tonnes; the value of such overfishing would be as much as US$2 billion. Such overfishing has severely damaged bluefin 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". Japan's Fisheries Research Agency counters that Australian and New Zealand tuna fishing companies under-report their total catches of southern bluefin tuna and ignore internationally mandated total allowable catch totals.
In recent years, opening day fish auctions at Tokyo's Tsukiji fish market and Toyosu Market have seen record-setting prices for bluefin tuna, reflecting market demand. In each of 2010, 2011, 2012, 2013 and 2019, new record prices have been set for a single fish – the current record is 333.6 million japanese yen for a bluefin, or a unit price of JP¥ 1,200,000/kg. The opening auction price for 2014 plummeted to less than 5% of the previous year's price, which had drawn complaints for climbing "way out of line". A summary of record-setting auctions are shown in the following table :
In November 2011, a different record was set when a fisherman in Massachusetts caught an tuna. It was captured inadvertently using a dragnet. Due to the laws and restrictions on tuna fishing in the United States, federal authorities impounded the fish because it was not caught with a rod and reel. Because of the tuna's deteriorated condition as a result of the trawl net, the fish sold for just under $5,000.

Methods

Besides for edible purposes, many tuna species are caught frequently as game, often for recreation or for contests in which money is awarded based on weight. Larger specimens are notorious for putting up a fight while hooked, and have been known to injure people who try to catch them, as well as damage their equipment.
In 2005, Nauru, defending its vote from Australian criticism at that year's meeting of the International Whaling Commission, argued that some whale species have the potential to devastate Nauru's tuna stocks, and that Nauru's food security and economy relies heavily on fishing. Despite this, Nauru does not permit whaling in its own waters and does not allow other fishing vessels to take or intentionally interact with marine mammals in its Exclusive Economic Zone. In 2010 and 2011, Nauru supported Australian proposals for a western Pacific-wide ban on tuna purse-seining in the vicinity of marine mammals – a measure which was agreed by the Western and Central Pacific Fisheries Commission at its eighth meeting in March 2012.

Association with dolphins

s swim beside several tuna species. These include yellowfin tuna in the eastern Pacific Ocean, but not albacore. Tuna schools are believed to associate themselves with dolphins for protection against sharks, which are tuna predators.
Commercial fishing vessels used to exploit this association by searching for dolphin pods. Vessels would encircle the pod with nets to catch the tuna beneath. The nets were prone to entangling dolphins, injuring or killing them. Public outcry and new government regulations, which are now monitored by NOAA have led to more dolphin-friendly methods, now generally involving lines rather than nets. There are neither universal independent inspection programs nor verification of dolphin safety, so these protections are not absolute. According to Consumers Union, the resulting lack of accountability means claims of 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, also known as FADs, which attract large populations of other organisms. Measures taken thus far to satisfy the public demand to protect dolphins can be potentially damaging to other species as well.