Northern Bluefin Tuna
Tagged: animal, animals, cat, fishThunnus thynnus
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Tuna are the long-distance specialists of the fish world, covering several thousand miles a year on their migrations. They are also among the fastest-swimming fish in the world. Some populations are now endangered as a result of the world demand for tuna meat. Tuna are fish built for speed. Every aspect of their body form is suited to maximum performance in the water. Their body is fusiform (pointed at both ends) with a stiff, sickle-shaped caudal (tail) fin perfect for producing maximum thrust. The bluefin tuna also has several features designed to reduce water resistance. Its scales are tiny and lie tightly against the skin, so minimizing friction. Its large eyes are well-bedded within their sockets, so the outer layer lies flush with the skin surface. The two dorsal (back) fins and the single anal (belly) fin fit into grooves when they are folded, while the series of finlets between the fins and the tail allow water to flow between them. The pectoral (chest) and pelvic (hip) fins are small and have a stiff front edge, which prevents them from collapsing when they are extended at high speeds. A striking feature of the tuna’s body is the deep-red color of the muscle tissue. This characteristic is found in the family Scombridaea that includes other high-speed species such as mackerel, bonitos, and their relatives. Red muscle has a rich blood supply that is typical of a species constantly on the move. The blood supplies the high levels of oxygen that the fish need and gives them plenty of stamina. However, tuna would be unable to maintain their constant day-and-night swimming at speed were it not for a further adaptation. Unlike the majority of fish whose internal body temperature matches that of their environment, a tuna’s countercurrent blood circulation allows it to maintain a high internal body temperature whatever the water temperature.
All-Consuming Demand
Bluefin tuna have been fished for about 100 years. Originally only sport fishermen and a few small-scale enterprises supplying fish for human consumption fished the species. But starting in the 1930s-and continuing for the next 30 to 40 years-sport fishing soared in popularity. Then in the 1970s a new commercial dimension was added to the sports angling industry, arising out of the fast-expanding demand for fresh (deep-frozen) tuna meat in Japan. The market for raw tuna provided by sushi and sashimi enthusiasts led to 40 percent of the global tuna catch being sent to the Japanese market. A major factor leading to the rapid expansion was the improvement in air freight and transport that began in the 1970s and made possible transglobal overnight deliveries of fresh-caught tuna. Allied to major changes that had occurred within the commercial fishery-which had also led to ever greater catches-the fishing of large tuna by sport anglers for profit as well as sport led to dramatically declining yields in the space of a few years. Total Atlantic harvests of bluefin tuna plummeted from a peak of 38,600 tons (35,000 tonnes) in 1964 to less than half-18,500 tons (16,800 tonnes)-by 1972. By the early 1980s catches in the western Atlantic had dropped even further to about 6,600 tons (6,000 tonnes). A report by the International Commission for the Conservation of Atlantic Tunas (ICCAT) has estimated that by the early 1990s the population of adult bluefins in the western Atlantic had dropped to just 13 percent of its 1975 level.
Population Conundrum
It is clear that fishing controls need to be introduced to protect the northern bluefin tuna. However, differences of opinion, disputed scientific data, a lack of faith in ICCAT’s ability to enforce quotas, demands for higher-than-stated quotas for the western Atlantic mean that there is no consensus on population levels. The picture is made even more complex by the migration of some stocks across the oceans. Meanwhile, tagging programs, aerial surveys, captive breeding, and genetic analysis are some of the methods being used to establish the status of the bluefin tuna on both sides of the Atlantic. This should pave the way for enforcing realistic fishing controls.
Northern bluefin tuna; (Atlantic bluefin tuna)
Thunnus thynnus
- Size: Length: 15 ft (4.6 m). Weight: up to 1,3201b (600 kg)
- Habitat: Open oceanic waters tunaForm: Fusiform (spindle-shaped), streamlined body. Coloration deep blue above, with purple or green iridescence (colors that shimmer as observer changes position); silvery sides and belly
- Family: Scombridaei
- World population: Disputed: about 40,000 in the western Atlantic (no equivalent data available for the eastern Atlantic)
- Distribution: Atlantic. On eastern side from Norway to Mediterranean Sea, along western African coast to Cape Blanc. On western side from Newfoundland south to Brazil. Seen in central and northwestern Pacific
- Diet: Fish (including herring, mackerel, and whiting); also squid
- Breeding: Spawning occurs in the Gulf of Mexico, the western Atlantic, and in the Mediterranean Sea in the east. Western stocks spawn from mid-April to mid-June; their eastern counterparts breed from June-August. Female can release about 30 million eggs
- Related endangered species: Albacore tuna (Thunnus alalunga); bigeye tuna (T obesus); southern bluefin tuna (T maccoyii); Monterrey Spanish mackerel (Scomberomorus concolor)
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