The yellowfin tuna ( Thunnus albacares ) is a species of tuna found in pelagic waters of tropical and subtropical oceans worldwide.
Yellowfin is often marketed as ahi , from its Hawaiian name ʻ ahi although the name ʻ ahi in Hawaiian also refers to the closely related bigeye tuna. Although the species name albacares might suggest otherwise, the fish usually known as albacore is a different species of tuna, Thunnus alalunga . The yellowfin tuna is sometimes referred to as albacora by French and Portuguese fishermen.
Description
The yellowfin tuna is one of the largest tuna species, reaching weights of over 300 pounds (136 kg), but is significantly smaller than the Atlantic and Pacific bluefin tunas that can reach over 1,000 pounds (454 kg) and slightly smaller than the bigeye tuna and the southern bluefin tuna. Reported sizes in the literature have ranged as high as 239 centimeters (94.1 in) in length and 200 kilograms (441 lb) in weight. The International Game Fish Association (IGFA) record for this species stands at 388 pounds (176 kg). Angler Kurt Wiesenhutter boated this fish in 1977 near San Benedicto Island in the Pacific waters of Mexico. Two larger fish weighing 395 lb and 399.6 lb were boated in 1992 and 1993 respectively. These remarkable fish stand as the largest rod and reel yellowfin tuna captures thus far.
The second dorsal fin and the anal fin, as well as the finlets between those fins and the tail, are bright yellow, giving this fish its common name. The second dorsal and anal fins can be very long in mature specimens, reaching almost as far back as the tail and giving the appearance of sickles or scimitars. The pectoral fins are also longer than the related bluefin tuna, but not as long as those of the albacore. The main body is very dark metallic blue, changing to silver on the belly, which has about 20 vertical lines.
Habitat
Yellowfin tuna are epipelagic fish that inhabit the mixed surface layer of the ocean above the thermocline. Sonic tracking has found that although yellowfin tuna, unlike the related bigeye tuna, mostly range in the top 100 meters (328 ft) of the water column and penetrate the thermocline relatively infrequently, they are capable of diving to considerable depths. An individual tagged in the Indian Ocean with an archival tag spent 85% of its time in depths shallower than 75 meters (246 ft) but was recorded as having made three dives to 578 m, 982 m and an incredible 1,160 meters (3,806 ft).
Life history
Deeper diving and cruising seems to happen more often in the daytime, changing to shallower swimming at night, probably in response to the vertical movement of prey items in the deep scattering layer. They are normally a schooling fish and stay in their immediate school.
Although mainly found in deep offshore waters, yellowfin tuna may approach shore when suitable conditions exist. Mid-ocean islands such as the Hawaiian archipelago, other island groups in the Western Pacific, Caribbean and Indian Ocean, as well as the volcanic islands of the Atlantic such as Ascension Island often harbor yellowfin feeding on the baitfish these spots concentrate close to the shoreline. Yellowfin may venture well inshore of the continental shelf when water temperature and clarity are suitable and food is abundant.
Yellowfin tuna often travel in schools with similarly sized companions. They sometimes school with other tuna species and mixed schools of small yellowfin and skipjack tuna, in particular, are commonplace. They are often associated with various species of dolphins or porpoises, as well as with larger marine creatures such as whales and whale sharks. They also associate with drifting flotsam such as logs and pallets, and sonic-tagging indicates that some follow moving vessels. Hawaiian yellowfin associate with anchored fish aggregation devices(FADs) and with certain sections of the 50-fathom curve.
Diet and predation
Yellowfin tuna prey include other fish, pelagic crustaceans, and squid. Like all tunas their body shape is evolved for speed, enabling them to pursue and capture fast-moving baitfish such as flying fish, saury and mackerel. Schooling species such as myctophids or lanternfish and similar pelagic driftfish, anchovies and sardines are frequently taken. Large yellowfin prey on smaller members of the tuna family such as frigate mackerel and skipjack tuna.
In turn, yellowfin are preyed upon when young by other pelagic hunters, including larger tuna, seabirds and predatory fishes such as wahoo, shark and billfish. As they increase in size and speed, yellowfin become able to escape most of their predators. Adults are threatened only by the largest and fastest hunters, such as toothed whales, particularly the false killer whale, pelagic sharks such as the mako and great white, and large blue marlin and black marlin. Industrial tuna fisheries represent by far their most threatening predator.
The commercial fishery
Modern commercial fisheries catch yellowfin tuna with encircling nets (purse seines), and by industrial longlines.
Pole and line
Formerly, much of the commercial catch was made by pole and line fishing, using live bait such as anchovy to attract schools of tuna close to the fishing vessel that were then taken with baited jigs on sturdy bamboo or fiberglass poles or on handlines. This fishery, which targeted skipjack and occasionally albacore as well as yellowfin, for canning, reached its heyday between World War I and the 1950s before declining. The most well-known fleet of pole and line boats sailed from San Diego in California and exploited abundant stocks in Mexican waters, as well as further south to Panama, Costa Rica and the Galapagos Islands. Interviews with fishery participants as well as video footage preserve the memory of this fishery and the boats and men that pursued it.
Pole and line fishing is still carried out today in areas such as the Maldives, Ghana, and by a small number of boats fishing out of the Canary Islands, Madeira and the Azores. Few pole and line boats now specifically target yellowfin and the contribution that these fisheries make to the total commercial catch is incidental to the total take. In the Maldives, for instance, the catch is a mix of skipjack tuna and small yellowfin that often associate with them.
Purse seining
Purse seining largely took over commercial tuna fisheries in the 1960s and 1970s. Today, purse seines account for more of the commercial catch than any other method. The purse seine fishery primarily operates in the Pacific Ocean, in the historic tuna grounds of the San Diego tuna fleet in the eastern Pacific, and in the islands of the western Pacific, where many US tuna canneries relocated in the 1980s; but significant purse-seine catches are also made in the Indian Ocean and in the tropical Atlantic Ocean, especially in the Gulf of Guinea by French and Spanish vessels.
Purse seine vessels locate tuna via onboard lookouts, as was done in the pole and line fishery, but they also employ sophisticated onboard electronics, sea-surface temperature and other satellite data, and from helicopters overhead. Once a school is located, the net is set around it. A single set may yield 100 tonnes (98 LT; 110 ST). Modern tuna seiners have a capacity of up to 2,000 metric tons (2,000 LT; 2,200 ST) reach speeds of over 17 knots (31 km/h; 20 mph) and carry multiple spotting helicopters.
Purse seining for yellowfin tuna became highly controversial in the late 1970s when it became apparent that the eastern Pacific fishery was killing many spinner dolphin, pantropical spotted dolphin and other cetaceans (often referred to as 'porpoise' by the tuna fleet) that accompany the fish. This association has been long-recognized by commercial tuna fishermen.
Since the introduction of "dolphin-friendly" labeling, an increasing number of purse seine sets are now made on "free schools" unassociated with dolphins, as well as schools that associate with floating objects—another long-understood association that has grown in importance in tuna fisheries. The latter practice in particular has a major ecological impact because of the high proportion of bycatch, including manta rays, sea turtles, pelagic sharks, billfish and other threatened marine species taken by setting nets around logs and other floating objects. Such tuna are often significantly smaller than the larger adult tuna associated with dolphins. The removal of huge numbers of juvenile yellowfin and bigeye tuna that have yet to reach breeding age has major potential consequences for tuna stocks worldwide.
Longline
Most of the commercial catch is canned, but the sashimi marketplace adds significant demand for high-quality fish. This market is primarily supplied by industrial tuna longline vessels.
Industrial longlining was primarily perfected by Japanese fishermen who expanded into new grounds in the Western Pacific, Indian and Atlantic oceans in the late 1950s and early 1960s. Longlining has since been adopted by other fishermen, most notably South Korea, Taiwan, and the United States.
Tuna longlining targets larger sashimi-grade
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