Atlantic bluefin tuna
The Atlantic bluefin tuna is a species of tuna in the family Scombridae. It is variously known as the northern bluefin tuna, giant bluefin tuna, and formerly as the tunny.
Atlantic bluefins are native to both the western and eastern Atlantic Ocean, as well as the Mediterranean Sea. They have become regionally extinct in the Black Sea. The Atlantic bluefin tuna is a close relative of one of the other two bluefin tuna species, the Pacific bluefin tuna. The southern bluefin tuna, on the other hand, is more closely related to other tuna species such as yellowfin tuna and bigeye tuna, and the similarities between the southern and northern species are due to convergent evolution.
Atlantic bluefin tuna have been recorded at up to in weight, and rival the black marlin, blue marlin, and swordfish as the largest Perciformes. Throughout recorded history, the Atlantic bluefin tuna has been highly prized as a food fish. Besides their commercial value as food, the great size, speed, and power they display as predators has attracted the admiration of fishermen, writers, and scientists.
The Atlantic bluefin tuna has been the foundation of one of the world's most lucrative commercial fisheries. Medium-sized and large individuals are heavily targeted for the Japanese raw-fish market, where all bluefin species are highly prized for sushi and sashimi.
This commercial importance has led to severe overfishing. The International Commission for the Conservation of Atlantic Tunas affirmed in October 2009 that Atlantic bluefin tuna stocks had declined dramatically over the last 40 years, by 72% in the Eastern Atlantic, and by 82% in the Western Atlantic. On 16 October 2009, Monaco formally recommended endangered Atlantic bluefin tuna for an Appendix I CITES listing and international trade ban. In early 2010, European officials, led by the French ecology minister, increased pressure to ban the commercial fishing of bluefin tuna internationally. However, a UN proposal to protect the species from international trade was voted down. Since then, enforcement of regional fishing quotas has led to some increases in population. the Atlantic bluefin tuna was moved from the category of Endangered to the category of Least Concern on the IUCN Red List of Threatened Species. However, many regional populations are still severely depleted, including western stocks which spawn in the Gulf of Mexico.
Most bluefins are captured commercially by professional fishermen using longlines, purse seines, assorted hook-and-line gear, heavy rods and reels, and harpoons. Recreationally, bluefins have been one of the most important big-game species sought by sports fishermen since the 1930s, particularly in the United States, but also in Canada, Spain, France, England, and Italy.
Taxonomy
The Atlantic bluefin tuna was one of the many fish species originally described by Carl Linnaeus in his landmark 1758 10th edition of Systema Naturae, where it was given the binomial name Scomber thynnus.It is most closely related to the Pacific bluefin tuna, and more distantly to the other large tunas of the genus Thunnus – the southern bluefin tuna, the bigeye tuna and the yellowfin tuna. The southern bluefin tuna was traditionally considered the closest relatives of the two northern species, but a recent study has suggested that the morphological similarities are the result of convergent evolution, and many are adaptations for cold water habitats. For many years, the Pacific and Atlantic bluefin tuna species were considered to be the same, or subspecies, and referred to as the "northern bluefin tuna". This name occasionally gives rise to some confusion, as the longtail tuna can in Australia sometimes be known under the name "northern bluefin tuna". This is also true in New Zealand and Fiji.
Bluefin tuna were often referred to as the common tunny, especially in the UK, Australia, and New Zealand. The name "tuna", a derivative of the Spanish atún, was widely adopted in California in the early 1900s, and has since become accepted for all tunas, including the bluefin, throughout the English-speaking world. In some languages, the red color of the bluefin's meat is included in its name, as in atún rojo and tonno rosso, amongst others.
Description
The body of the Atlantic bluefin tuna is rhomboidal in profile and robust. The head is conical and the mouth rather large. The head contains a "pineal window" that allows the fish to navigate over its multiple thousands-of-miles range. Their color is dark blue above and gray below, with a gold coruscation covering the body and bright yellow caudal finlets. Bluefin tuna can be distinguished from other family members by the relatively short length of their pectoral fins. Their livers have a unique characteristic in that they are covered with blood vessels. In other tunas with short pectoral fins, such vessels are either not present or present in small numbers along the edges.Fully mature adult specimens average long and weigh around. The largest recorded specimen taken under International Game Fish Association rules was caught off North Lake, Prince Edward Island, an area renowned for huge Atlantic bluefin, and weighed and was long.The longest contest between man and tuna fish occurred near Liverpool, Nova Scotia in 1934, when six men taking turns fought a tuna for 62 hours. Both the Smithsonian Institution and the U. S. National Marine Fisheries Service have accepted that this species can weigh up to, though further details are lacking. Atlantic bluefin tuna reach maturity relatively quickly. In a survey that included specimens up to in length and in weight, none was believed to be older than 15 years. However, very large specimens may be up to 50 years old.
The bluefin possesses enormous muscular strength, which it channels through a pair of tendons to its lunate-shaped caudal fin for propulsion. In contrast to many other fish, the body stays rigid while the tail flicks back and forth, increasing stroke efficiency. It also has a very efficient circulatory system. It possesses one of the highest blood-hemoglobin concentrations among fish, which allows it to efficiently deliver oxygen to its tissues; this is combined with an exceptionally thin blood-water barrier to ensure rapid oxygen uptake.
To keep its core muscles warm, which are used for power and steady swimming, the Atlantic bluefin uses countercurrent exchange to prevent heat from being lost to the surrounding water. Heat in the venous blood is efficiently transferred to the cool, oxygenated arterial blood entering a rete mirabile. While all members of the tuna family are warm-blooded, the ability to thermoregulate is more highly developed in bluefin tuna than in any other fish. This allows them to seek food in the rich but chilly waters of the North Atlantic.
Biology and ecology
Bluefins dive to depths of. The Atlantic bluefin tuna typically hunts small fish such as sardines, herring, mackerel, and eels, and invertebrates such as squid and crustaceans. They exhibit opportunistic hunting in schools of fish organized by size. Their white skeletal muscle allows for large contractions which aids burst swimming to ensure prey capture.The species is host to over 70 parasites although none have been yet described as causing harm to the species. The tetraphyllidean tapeworm Pelichnibothrium speciosum is one parasite of the species. As the tapeworm's definite host is the blue shark, which does not generally seem to feed on tuna, the Atlantic bluefin tuna likely is a dead-end host for P. speciosum.
Atlantic bluefin tuna are eaten by a wide variety of predators. When they are newly hatched, they are eaten by other fishes that specialize on eating plankton, as well as by other plankton organisms like jellyfish. They may even be eaten by their own, with some populations of larvae demonstrating cannibalism. At that life stage, their numbers are reduced dramatically. Therefore, adult bluefin tuna spawn at specific locations and time of the moon cycle to minimize predation on their offspring. Those that survive face a steady increase in the size of their predators. Adult Atlantic Bluefin are not eaten by anything other than the very largest billfishes, toothed whales, and some open ocean shark species.
Life history
Bluefin tuna are oviparous, congregating together in large groups to spawn. Over several days, a female releases large numbers of eggs into the water where they are fertilized externally by male sperm. Female bluefins have been estimated to produce a mean of 128.5 eggs per gram of body weight, or up to 40 million eggs at a time. Eggs hatch into larvae two days after fertilization and become cannibalistic quarter-inch long fish by the end of a week. About 40% of larvae survive their first week, and about 0.1% the first year. Surviving bluefin tend to group together in schools according to size.Atlantic bluefin tuna were traditionally known to spawn in two widely separated areas. Pop-up satellite tracking results generally confirm the belief held by many scientists and fishermen that although bluefin that were spawned in each area may forage widely across the Atlantic, the vast majority return to their natal area to spawn.
The spawning ground of the eastern stock of Atlantic bluefin is in the western Mediterranean, particularly in the area of the Balearic Islands. The spawning ground of the western stock is in the Gulf of Mexico. Because Atlantic bluefins group together in large concentrations to spawn, they are highly vulnerable to commercial fishing while spawning. This is particularly so in the Mediterranean, where the groups of spawning bluefins can be spotted from the air by light aircraft and purse seines directed to set around the schools.
In 2016, researchers suggested that a third spawning area exists in the Slope Sea, an area to the north and west of the Northeastern United States Continental Shelf. Subsequent research indicates that comparable concentrations of bluefin larvae are found in the Slope Sea and in the Gulf of Mexico.
A number of behavioral differences have been observed between the eastern and western populations, some of which may reflect environmental conditions. For example, bluefin in the Gulf of Mexico spawn between mid-April and mid-June, when the surface water temperature is between and, while bluefin in the Mediterranean spawn between June and August, when water is between and. In the Gulf of Mexico bluefin appear to correct for higher surface temperatures by diving, going deeper than when entering the Gulf and staying deeper than to spawn.
The western and eastern populations have been thought to mature at different ages. Bluefins born in the east are thought to reach maturity a year or two earlier than those spawned in the west. It has also been suggested that these apparent differences may reflect not-well-understood complexities of migration patterns and additional spawning areas such as the Slope Sea.