Thunnus
Bigeye tuna, Thunnus obesus
Tunas ( Thunnus) (including tuna ) are a genus of large predatory fish that is found in all tropical, subtropical and temperate seas. They are among the most important food fish, are of great economic importance to fishing and some of the eight species are now under threat from overfishing.
Features
Tuna fish have an elongated, spindle-shaped, laterally flattened little body whose focus is in the front part of the fuselage. You can reach body lengths of about 1 meter to 4.5 meters and can choose between 20 kg and 700 kg heavy. The body is covered with very small scales, the front body surrounds a corset larger scales. The head is pointed cone. The mouth is terminal, deeply divided and not vorstülpbar. The small, conical teeth in each jaw sitting in a row. On the first branchial arch is found 19-43 gill rakers. The two relatively small dorsal fins are close together. The longer first supported 11 to 14 spines, the second shorter 12 to 16 soft rays. In the first dorsal fin, the first spines are very much higher than that, so that the upper edge of the fin is the last concave, the second crescent- shaped, and as high as the first or higher. Between the second dorsal fin and the tail fin are 7-10 Flössel. The anal fin is the second dorsal fin over nearly symmetric and has the same shape as this one. It is supported 11-16 fin rays. On the anal fin followed by 7 to 10 Flössel. Species vary in length of the pectoral fins. With 30 to 36 fin rays in the pectorals, they have more than any other species of Scombridae. In the ventral fins there is a two-part Interpelvic outgrowth. The caudal peduncle is slender and has on each side to stabilize a strong, bony side keel between two smaller keels. The caudal fin of the tuna is crescent shaped. The first dorsal fin, the pelvic fins and the anal fin can be placed during fast swimming in trough-shaped depressions. Most species have a swim bladder, the volume can be greatly changed and for juveniles is still very small. The number of vertebrae is 39 tunas are generally bluish, belly and gray or silvery and show to the sides of the body no dark mottling. The back is dark blue and without the typical in many other Scombriden stripe pattern. The Flössel behind the dorsal and anal fin are usually yellow and have black borders in some species.
Tunas have a very high metabolic rate and are in addition to the swordfish ( Xiphias gladius ) are the only bony fish with a partially endothermic metabolism. One of the core muscles connected, working in counter-current principle vascular system ( rete mirabile ) directs the heat generated by the activity of large muscle mass inside the body, so that the body temperature and the temperature of the brain and eyes ( retina and optic nerve ) 9-12 ° C may be greater than the temperature of the water. The indispensable for their high metabolism intensive gas exchange through the gills is achieved by the continuous swimming and fast swimming behavior of Thune. The ability to increase the body temperature increases with the size and the albacore tuna, the yellowfin tuna and the bigeye tuna very well developed and widely in the bluefin tuna.
Way of life
Tuna fish live in all tropical, subtropical and temperate seas and are missing in the polar seas. They are living in schools or shoals of prey fish that mainly occupy the upper 500 meters of the water column and can maintain high speeds for extended periods. You undertake long migrations over the open ocean and diel vertical migrations. They wander especially their prey behind, their walks again have regions with high plankton density to the target. The groups consist only of individuals of the same size, small tunas form larger flocks than large ones. Very large specimens can live as a loner. Tuna fish feed on smaller fish, including herring and related with the tuna mackerel, squid and pelagic crustaceans. Is now also a scuba diving for some tuna species detected ( bigeye tuna up to 1839 meters (2.5 ° C) and bluefin tuna up to 1000 meters (3 ° C)) where they have the different daytime subducting at greater depths, fish, cephalopods and crustaceans eat. Because of their size, they have little predators and are eaten mainly of swordfish -like, sharks and toothed whales.
Tunas move while swimming hardly her hull, but set to propel only the rapidly oscillating, very stiff, crescent-shaped tail fin a, assigned to the trunk muscles their strength by means of tendons plates. This thunniforme, probably with the partial endothermy developed swimming behavior is almost unique among teleosts and may be found only in the swordfish. Tropical tuna spawn throughout the year, living in temperate regions hiking and move into the spawning season in warmer regions, while they spend the rest of the year in cooler, chuck richer areas. Eggs and larvae are pelagic. Large females can give millions of eggs ( up to 10 million during the bluefin tuna ).
Species
There are eight species in two subgenera, Neothunnus consisting of four especially tropical species, and Thunnus whose four species occur in temperate latitudes due to better heat retention.
Outer systematics
Tuna fish belong to the family of mackerels and tunas ( Scombridae ), where they form with four others, species-poor genera monophyletic tribe Thunnini. The following cladogram showing the systematic position of tunas within the Scombridae:
Subgenus Thunnus
Subgenus Neothunnus
Katsuwonus
Allothunnus
Sardini ( genus Sarda and relatives )
Scomberomorini ( Wahoo, genera Grammatorcynus & Scomberomorus )
Scombrini ( mackerel and relatives)
Latticed mackerel ( Gasterochisma melampus )
Fishing
Tunas are for their superb, red, fat meat to the most important food fish, are of great economic importance to fisheries. The catch increased according to the FAO of about 400,000 tonnes in 1955 to over 2 million tonnes since 1997. One of the greatest catchers nations include Japan, where raw meat tuna for sushi and sashimi is used, the U.S. and South Korea. Tuna are caught with longlines, purse seines and drift nets. The latter is illegal in EU waters and in many other fields, because it involves a considerable extent to unwanted by-catch of dolphins and sharks. In the Mediterranean Sea to catch tuna with very large anchored float traps the Tonnaras be called. Tuna meat is marketed as fresh fish or frozen. Canned tuna in canned meat comes primarily from bonito.
In some countries, such as Malta, are tuna, which have previously been caught as juveniles, fattened in fish cages in the sea.
The management of global tuna stocks in the hands of international and regional fisheries commissions in the Atlantic and the Mediterranean, the International Commission for the Protection of Tuna ( International Commission for the Conservation of Atlantic Tuna, ICCAT) and the Inter - American Tropical Tuna Commission ( IATTC ) of the stocks in the eastern Pacific.
Around three quarters of export of fish caught in the Mediterranean Thunfiische currently go to Japan.
Fossil record
The tuna parted probably from the other genera of the Thunnini in the Paleocene. The first tuna lived in the Tethys. The earliest fossils of tunas are from the Eocene. Finds there are from North America, Africa and Europe, for example, from the Italian Monte Bolca (Thunnus lanceolatus ).