Thunnus is a genus of ocean-dwelling fish in the family Scombridae, all of which are tuna, although other tuna species are found in other genera. The name of the genus is the Latinized form of the Greek θύννος, thýnnos, tuna, the word being first mentioned in Homer. There are eight species:
* Albacore, Thunnus alalunga (Bonnaterre, 1788).
* Bigeye tuna, Thunnus obesus (Lowe, 1839).
* Blackfin tuna, Thunnus atlanticus (Lesson, 1831).
* Pacific bluefin tuna, Thunnus orientalis (Temminck & Schlegel, 1844).
* Northern bluefin tuna, Thunnus thynnus (Linnaeus, 1758).
* Southern bluefin tuna, Thunnus maccoyii (Castelnau, 1872).
* Longtail tuna, Thunnus tonggol (Bleeker, 1851).
* Yellowfin tuna, Thunnus albacares (Bonnaterre, 1788). 
ALBACORE
The albacore, Thunnus alalunga, is a type of tuna in the family Scombridae. This species is also called albacore fish, albacore tuna, longfin, albies, pigfish, tombo ahi, binnaga, Pacific albacore, German bonito (but see bonito), longfin tuna, longfin tunny, or even just tuna. It is the only tuna species which may be marketed as "white meat tuna" in the United States.
It is found in the open waters of all tropical and temperate oceans, and the Mediterranean Sea. Lengths range up to 140 cm and weights up to 45 kg.
The pectoral fins of the albacore are very long, as much as 50% of the total length. The dorsal spines are 8 to 10 in number, and well forward of the rays of the dorsal fin. The anterior spines are much longer, giving a concave outline to the spiny part of the dorsal fin.
Albacore is a prized food, and the albacore fishery is economically significant. Methods of fishing include pole and line, long-line fishing, trolling, and some purse seining. It is also sought after by sport fishers.
The National Scientific Committee (NSC) conducts regularly scheduled stock assessments of Pacific albacore. The 2003 stock assessment found the albacore stocks to be at or near record highs. The North and South Pacific albacore stocks are not over fished. The ISC findings are accepted by the Inter-American Tropical Tuna Commission and the Western Central Pacific Fisheries Commission and employed in the responsible management of Pacific albacore tuna stocks. Regrettably, the same cannot be said for Atlantic stocks of albacore. Assessments of the North and South Atlantic stocks showed them to be vulnerable and critically endangered respectively, due to significant population reductions measured through an index of abundance and considering "actual or potential levels of exploitation".
SeaChoice ranks albacore as a "best choice" for consumers, although notes some "moderate concerns" regarding the management effectiveness (in particular, no definitive survey of the albacore stock of the Indian Ocean fishery has taken place), and "moderate concern" over the fishing stock, especially regarding the North Atlantic albacore population, which the National Marine Fisheries Service (NMFS) considers overfished with overfishing still occurring. The southern Atlantic stock is not considered overfished. The North Pacific and South Pacific albacore stocks are not overfished and not experiencing overfishing.
Consumers are increasingly concerned about the sustainability of fisheries. A number of programs have been developed to help consumers identify and support responsible and sustainable fisheries. Perhaps the most widely accepted of these programs is that of the Marine Stewardship Council (MSC). The MSC declared the U.S. North and South Pacific albacore pole & line and troll fisheries ("pole & troll") as the only certified sustainable tuna fisheries in the world. Products from MSC certified sustainable fisheries are readily identifiable by the MSC's distinctive blue and white "eco-label".
By purchasing products bearing the MSC eco-label, consumers express their support for sustainable fisheries and encourage the use of sound fishing methods that promote the future health and abundance of ocean ecosystems.
In some parts of the world, other species may be called "albacore":
1. Blackfin tuna (albacore) - Thunnus atlanticus;
2. Yellowfin tuna (albacore, autumn albacore, yellowfinned albacore) - Thunnus albacares;
3. Yellowtail amberjack (albacore) - Seriola lalandi;
4. Kawakawa (false albacore) - Euthynnus affinis;
5. Little tunny (false albacore) - Euthynnus alletteratus.
BIGEYE TUNA
The bigeye tuna, Thunnus obesus, is an important food fish and highly sought after recreational game fish. It is a member of the true tunas of the genus Thunnus. Bigeye tuna are found in the open waters of all tropical and temperate oceans, but not the Mediterranean Sea. Its length is between 60 and 250 cm (23 and 93 inches). Maximum weight probably exceeds 400 lb with the all-tackle angling record standing at 392 lbs. Bigeye tuna are large deep-bodied streamlined fish with large heads and eyes. The pectoral fins are very long, reaching back as far as the second dorsal fin. There are 13 or 14 dorsal spines.
A longer-lived fish than the closely related yellowfin tuna, the bigeye tuna is thought to have a lifespan of up to 10 to 12 years, with individuals achieving sexual maturity at the age of four. Spawning has been recorded as taking place in June and July in the northwestern tropical Atlantic and in January and February in the Gulf of Guinea in the eastern Atlantic, which is, so far, the only known nursery area for Atlantic bigeye tuna.
Feed items include both epipelagic and mesopelagic species, with deep diving behaviour during the day thought to be related to the seeking of prey. Satellite tagging has shown that bigeye tuna often spend prolonged periods cruising deep below the surface during the daytime, sometimes making dives as deep as 500 metres. These movements are thought to be in response to the vertical migrations of prey organisms in the deep scattering layer.
Physiological adaptations to foraging in these cold (bigeye tuna have been tracked entering water as cold as 5 degrees Celsius) and oxygen-poor subsurface waters include blood that is highly efficient in extracting oxygen from the water even in oxygen-poor conditions, and vision that is highly adapted for effective function in low light conditions. The heart of bigeye tuna also has an unusual ability to function effectively at the low ambient temperatures encountered while foraging in cold subsurface water. Nonetheless, bigeye tuna must make return trips to warmer surface waters to warm themselves up.
Bigeye tuna are amongst the tuna species most threatened by overfishing. Juvenile bigeye tuna associate closely with floating objects such as logs, buoys and other flotsam, which makes them extremely susceptible to purse seine fishing in conjunction with man-made FADs (Fish Aggregation Devices). Bigeye mature at a later age than other commercially important tuna species such as skipjack and yellowfin tuna, and the removal of large numbers of juvenile bigeye before they reach breeding age is a major concern to fisheries managers, scientists and sport fishermen.
NORTHERN BLUEFIN TUNA
The northern bluefin tuna (Thunnus thynnus), or giant bluefin tuna, is a species of tuna native to both the western and eastern Atlantic Ocean, as well as the Mediterranean Sea and the Black Sea. Although not native to the Pacific Ocean, the species is now commercially cultivated off the Japanese coast. The species was in the past called the common tunny. It is often referred to simply as the "bluefin" or "bluefin tuna", but this name is ambiguous as it is also sometimes used for the southern bluefin tuna (Thunnus maccoyii) and the Pacific bluefin tuna (T. orientalis).
The body of the northern bluefin tuna is cigar-shaped and robust. Northern bluefin tuna can live for 30 years. Due to overfishing of this species, few known specimens grow to a mature age and typical specimens average 2-2.5 m (6.5-7 ft) long and around 350 kg (770 lb) in weight. The species can reach a maximum length of about 4.3 m (14 ft) and 680 kg (1,496 lb), the largest recorded. Northern bluefin tuna can easily be distinguished from other members of the tuna family by the relatively short length of their pectoral fins.
The northern bluefin tuna typically hunts small fish and invertebrates such as sardines, herring, mackerel, squid and crustaceans.
Bluefin tuna are caught by sports fishermen using heavy-duty rod and reels and by commercial fishermen using purse seine gear. 
The northern bluefin tuna is an important source of seafood, providing most of the tuna used in sushi. To supply sushi markets, the bluefin tuna is reported to be fished at 4 times the sustainable rate. As a result, some fisheries of bluefin are considered overfished, and this problem is compounded by the bluefin's slow growth rate and late maturity. The Atlantic population of the species has declined by nearly 90 percent since the 1970s. The species is currently classified as data deficient, unlike the southern bluefin tuna which is critically endangered. Consumers have been recommended to avoid consumption of bluefin tuna until stocks recover.
SOUTHERN BLUEFIN TUNA
The southern bluefin tuna, Thunnus maccoyii, is a tuna of the family Scombridae found in open southern hemisphere waters of all the worlds oceans mainly between 30°S and 50°S, to nearly 60°S. The southern bluefin tuna is a large streamlined fast swimming fish with a long slender caudal peduncle and relatively short dorsal, pectoral and anal fins. At up to 2.5 m (8.2 ft) and weighing up to 400 kg (882 lbs) it is one of the largest bony fishes. The body is completely covered in small scales. The body color is blue-black on the back and silver-white on the flanks and belly, with bright yellow caudal keels in adult specimens. The first dorsal fin color is grey with a yellow tinge, the second dorsal is red-brown, and the finlets are yellow with a darker border.
The southern bluefin tuna is an opportunistic feeder, preying on a wide variety of fishes, crustaceans, cephalopods, salps, and other marine animals.
The southern bluefin tuna is classified as Critically Endangered on the IUCN Red List of Threatened Species. Since the 1950s, when industrial fishing commenced, the total population of southern bluefin tuna has declined by about 92 percent.
In 1994 the then existing voluntary management arrangement between Australia, Japan and New Zealand was formalised when the Convention for the Conservation of Southern Bluefin Tuna came into force. The Convention created the Commission for the Conservation of Southern Bluefin Tuna (CCSBT). Its objective is to ensure, through appropriate management, the conservation and optimum utilisation of the global southern bluefin tuna fishery. Later, South Korea, Taiwan and the Philippines have joined, or are cooperating with, the Commission. The CCSBT is headquartered in Canberra, Australia.
YELLOWFIN TUNA
The yellowfin tuna (Thunnus albacares) is a type of tuna found in open waters of tropical and subtropical seas worldwide. Also known as ahi tuna, from its Hawaiian name ‘ahi, yellowfin is becoming a popular replacement for the severely depleted supplies of Southern bluefin tuna. It is an epipelagic fish ranging in the top 100 m (330 feet) of the water column. Reported sizes have ranged
as high as 239 cm (94 inches) in length and 200 kg (440 lb) in weight. The second dorsal fin and the anal fin are both bright yellow, thus the common name, and can be very long in mature specimens, as are the pectoral fins. The main body is very dark metallic blue, changing to silver on the belly, which has about 20 vertical lines.
Yellowfins tend to school with fishes of the same size, including other species of tuna, and larger fish. They are often seen with dolphins, porpoises, whales and whale sharks. Yellowfins eat other fish, crustaceans, and squid.
Recent studies proved that this species is endangered. Especially in the Mediterranean sea, due to over fishing serving commercial interests regardless of high concern from the scientific and environmental community. The extinction of the "yellowfin (ahi) tuna" could occur in the next 15 years if governments don't take serious measures to protect the specie by lowering the fishing quotas tremendously and by reacting to illegal fishing.
Commercial fisheries catch yellowfin tuna with encircling nets (purse seines) and with longlines. The fish are mainly sold in frozen or canned form, but are also popular as sashimi. Otherwise Yellowfin tuna are a popular sport fish in many parts of their range and are prized for their speed and strength when fought on rod and reel as well as for their table qualities. References http://wikipedia.org
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