Bigeye tuna

Bigeye tuna
Conservation status
Vulnerable  (IUCN 3.1)[1]
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Actinopterygii
Order:	Perciformes
Family:	Scombridae
Subfamily:	Scombrinae
Tribe:	Thunnini
Genus:	Thunnus
Subgenus:	Thunnus
Species:	T. obesus
Binomial name
Thunnus obesus (Lowe, 1839)
Synonyms[2]
Parathunnus mebachi (Kishinouye, 1915) Thynnus obesus Lowe, 1839

The bigeye tuna, Thunnus obesus, is an important food fish and prized recreational game fish. It is a true tuna of the genus Thunnus, belonging to the wider mackerel family Scombridae.

In Hawaiian, it is one of two species known as ʻahi; the other is yellowfin tuna.^[3]

Bigeye tuna are found in the open waters of all tropical and temperate oceans, but not the Mediterranean Sea.

Anatomy

Bigeye tuna vary up to 250 centimetres (98 in) in length. Its maximum weight probably exceeds 400 pounds (180 kg), with the all-tackle angling record standing at 392 pounds (178 kg). They 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. They display 13 or 14 dorsal spines.

Physiology

The bigeye forages in cold and oxygen-poor subsurface waters. Their blood extracts oxygen efficiently even in oxygen-poor conditions. Their vision functions well in low light conditions. The heart has an unusual ability to function effectively while foraging in cold subsurface water. Nonetheless, they must periodically return to warmer surface waters to warm up.

Life history

Longer-lived than the closely related yellowfin tuna, the bigeye has a lifespan of up to 12 years, with sexual maturity at age four. Spawning takes place in June and July in the northwestern tropical Atlantic, and in January and February in the Gulf of Guinea, which is the only known Atlantic nursery area.

Satellite tagging showed that bigeye tuna often spend prolonged periods diving deep below the surface during the daytime, sometimes reaching 500 metres (1,600 ft). Bigeye have been tracked entering water as cold as 5 °C (41 °F). These movements are thought to be in response to vertical migrations of prey organisms in the deep scattering layer.

Behavior

Vertical Movement

The bigeye tuna shows movement in a vertical fashion, which is different from other species of tuna. In contrast to the bigeye tuna, the yellowfin tuna does also shows vertical movement, but it has been found that yellowfin tuna will not move to depths that differ in temperature by no more than 8 °C from the surface temperature.^[4] On the other hand, the bigeye tuna moves in a vertical fashion to depths that can reach 20 °C colder than the surface temperature.^[4] This movement between different depths is dictated by the temperature of the depth rather than the actual depth in which the bigeye or yellowfin resides. For example, if the depth was only 30 feet below the surface but with a temperature difference more than 8 °C, bigeye tuna are able to travel to that depth while yellowfin tuna are not.

Bigeye tuna are able to move into significantly colder water than yellowfin tuna are due to physiological differences. The bigeye is not as sensitive to the gradient difference as the yellowfin when it comes to temperature differences in different depths.^[5] The available amount of oxygen decreases as water depth increases and bigeye tuna are able to travel down into these oxygen-poor depths that yellowfin tuna cannot reach due to physiological differences between the two species.^[6] Although temperature may seem like the deciding factor in terms of the difference between the yellowfin and the bigeye tuna's depth range, the amount of dissolved oxygen is also important. The bigeye can survive at depths with lower amounts of dissolved oxygen, but the bigeye must travel to depths with greater dissolved oxygen in order to replenish the oxygen within its blood supply. In shallow waters with low amounts of dissolved oxygen, we will only find bigeye and not yellowfin tuna.^[7] The combination of temperature and dissolved oxygen is what determines the maximum depth at which the bigeye can survive.

The bigeye tuna makes its way up to the surface layer even though it can survive at greater depths. They move up and down between the surface layer and deeper waters at regular intervals during the day, while at night they stay near the surface in an example of physiological behavioral thermoregulation.^[8] The bigeye tuna must come up to the surface in order to regulate its body temperature. Water at greater depths is much colder than water at the surface layer, so at night the bigeye tuna stays in the surface layer since the sun cannot raise the water temperature.

The bigeye tuna prefers spending time at different depths depending on the time of day. They spend a majority of the night in the surface layer, but spend most of the day in the deeper waters. One bigeye was reported at a depth of 591.7 meters below surface level, while during the night the average is around 16 meters.^[9] It is most likely that the bigeye travels down to lower depths in order to secure better prey without competition from the yellowfin that occupies the same areas, but cannot dive as deep as the bigeye.

While the bigeye tuna has been observed to move between different depths, they spend a majority of their time in the surface layer where other fish such as the yellowfin are present. Bigeye tuna also move up and down depths in schools with other bigeye tuna.^[10]

It is possible that the bigeye tuna is able to hold more oxygen at greater depths due to the high affinity for oxygen by hemoglobin in its blood. The bigeye tuna has the highest affinity for oxygen in its blood in the Thunnus genus. This higher affinity for oxygen means that the bigeye tuna is able to hold more oxygen in the blood in comparison to other fish in the same family. There is also a correlation between higher water temperatures and more effective oxygen binding in the blood.^[11]

Horizontal Movement

Horizontal movement describes how fish move and what they do when they are in the surface layer. When the bigeye is in the surface layer, it is found to aggregate at points. This is how schools of fish are created for the bigeye tuna.^[10] The aggregation points are most likely to be floating objects such as buoys, seamounts, or other random objects, but islands can also act as aggregation points.^[12]

Diet

Bigeye tuna are carnivorous, with an experiment finding that their stomachs are usually full of crustaceans and cephalopods.^[13] Because the bigeye tuna and yellowfin tuna have similar ranges and habitats worldwide, they also have similar diets. However, there is a slight difference in that the bigeye tuna can dive much deeper and prey on different sources that the yellowfin does not have access to. Bigeye tuna prey mostly on cephalopods in deeper waters.^[13]

Feed items include both epipelagic and mesopelagic species, with deep diving behaviour during the day thought to be related to the seeking of prey.

Fishery

Bigeye tuna caught with three-pole one-line rig.

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 susceptible to purse seine fishing in conjunction with man-made fish aggregation devices. The removal of large numbers of juvenile bigeye, before they reach breeding age, is a major concern to fisheries managers, scientists and sport fishermen. Most seafood sustainability guides encourage consumption of other types of tuna.

In 2010, Greenpeace International has added the bigeye tuna to its seafood red list. "The Greenpeace International seafood red list is a list of fish that are commonly sold in supermarkets around the world, and which have a very high risk of being sourced from unsustainable fisheries."^[14]

Western and central Pacific

The central and western Pacific provides about 54 percent of the world's tuna, amounting to about 1.3 million tons annually.^[15] For the first time ever, NOAA closed the western and central Pacific bigeye fishery to the Hawaii-based longline fishing fleet for the final three days of 2009, having reached the internationally-agreed catch limit of 3,673 metric tons (3,615 long tons). This limit is 30% lower than that of earlier years and will also apply to 2010. The ban does not apply to yellowfin tuna and other fishes or bigeye in the eastern Pacific.^[16]

The United States and the eight island nations that are part of the so-called Nauru Agreement are negotiating an extension of the 1987 Multilateral Treaty, which allows the United States 40 vessels with no limit on fishing days. That treaty expires on 2013. The island nations want the U.S. to reduce the number of days its boats fish each year and lower quotas by 20-30%. Since 2004, quotas on longline fishing have reduced bigeye fishing by 10 percent.^[15]

Hawaii's longline fishing vessels virtually stopped 2010 bigeye fishing in the central and western Pacific as of Nov. 22 after reaching their quota. They continued fishing in nontraditional waters in the eastern Pacific, where trips are longer and tuna may be scarce.^[15]

The Western Pacific Regional Fishery Management Council acts as a policy adviser to the U.S. Secretary of Commerce. The council supports a reduction in the use of purse seine nets at fish aggregation devices—open-ocean buoys—because too many juvenile bigeyes are taken before they become sexually mature and can propagate.^[15]

At a conference in April, Nauru countries banned purse seine fishing by vessels under license to the group in some high seas around their nations as of January. 1. That agreement covers an estimated 25 percent of the world's tuna catch.^[15]

The "Nauru countries" are the Federated States of Micronesia, Kiribati, the Republic of the Marshall Islands, Nauru, the Republic of Palau, Papua New Guinea, Solomon Islands and Tuvalu. Other commission members include Australia, China, Canada, the Cook Islands, the European Union, Fiji, France, Japan, South Korea, New Zealand, Niue, the Philippines, Samoa, Taiwan, Tonga, the United States and Vanuatu.^[15][17]

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  Fishermen with bigeye tuna
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  On ice

Notes

References

Wikimedia Commons has media related to Thunnus obesus.

Wikispecies has information related to: Thunnus obesus

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