Bigeye tuna (Thunnus obesus)
|French:||Thon aux Grands Yeux, Thon aux Gros Yeux, Thon Gros Yeux|
|Spanish:||Atún Ojo Grande, Patudo|
|Size||Fork length: up to 180 cm (2)|
Classified as Vulnerable (VU) on the IUCN Red List 2007. The Pacific stock is classified as Endangered (EN) (1).
This large, fast-swimming tuna is an important target for numerous commercial fisheries around the world. The streamlined body of the bigeye tuna is dark metallic blue on the back, white on the undersides, and bears moderately long pectoral fins, used for balancing and breaking. The first dorsal fin is deep yellow, while the second dorsal fin and anal fin are pale yellow, and small fins just behind the dorsal and anal fins, called finlets, are bright yellow edged with black (2) (3). The length of the anal fin and the larger eyes of the bigeye tuna, after which it is named, distinguish this species from the similar yellowfin tuna (4).
Occurs in the Atlantic, Pacific and Indian Oceans, but does not occur in the Mediterranean (2). Bigeye tunas are most commonly found in tropical and subtropical waters, but its distribution does extend into temperate waters. There are considered to be separate stocks in the Eastern Pacific, Western Pacific, Atlantic and Indian Oceans (5).
The bigeye tuna inhabits oceanic waters from the surface down to a depth of 250 metres, in temperatures from 13° to 29° C (2)
Bigeye tunas are excellent swimmers, with a number of remarkable adaptations which make them efficient predators of the ocean. Unlike many other fish, they cannot pump water over their gills, but instead, swim with their mouth open which forces water over their gills. While this is an efficient way of getting water over their gills, it means that if they stop swimming, they will suffocate. The gills of tuna cover a surface area up to 30 times larger than other fish, giving a large surface over which water can flow, enabling about half the oxygen present in the water to be absorbed. The hearts of tuna are also much larger than those of other fish; about ten times as large, relative to the size of the body. This, along with a high blood pressure, creates perfect conditions to rapidly transfer oxygen from the gills to other tissues (6).
As well as adaptations to enhance the amount of oxygen reaching the muscles, a unique system to regulate their body temperature enables tuna to maintain their body temperature above that of the ocean. This system is called the rete mirabile; a counter-current heat exchange system which prevents heat from being lost to the surroundings. This maximises the efficiency of the muscles and maintains good brain and eye function, allowing the bigeye tuna to forage in cold water (6).
Juvenile and small adult bigeye tunas school at the ocean’s surface, sometimes together with yellowfin or skipjack tuna, and often associated with floating objects (2), while adult bigeye tunas are found in deeper water (5). The bigeye tunas migrate between feeding grounds in temperate waters and their spawning grounds in tropical waters; however, they are on the move almost all the time as they search for areas of plentiful food (6). Bigeye tunas feed on a variety of fish, cephalopods and crustaceans; which it searches for in the daytime and nighttime. Bigeye tunas become prey themselves for larger billfish and toothed whales (2).
Mature bigeye tuna spawn at least twice a year; releasing between an incredible 2.9 million and 6.3 million eggs each time. In the eastern Pacific, spawning has been recorded between 10°N and l0° S throughout the year, peaking between April and September in the northern hemisphere and between January and March in the southern hemisphere (2).
The bigeye tuna is an important target for fisheries in many parts of its range, and its flesh attracts high prices. All stocks of the bigeye tuna are now considered fully fished or over-fished, and overfishing is occurring in some areas, notably in the Endangered Pacific stock (5). The biological characteristics of the bigeye tuna, (it is relatively long-lived, late to spawn, and has a low productivity) make it more vulnerable to over-fishing than species such as skipjack tuna and yellowfin tuna. Without swift and effective management actions, populations of bigeye tuna are likely to go the same way as the Southern bluefin tuna (5), classified by the World Conservation Union (IUCN) as Critically Endangered (1).
In the Atlantic, Indian and Pacific oceans there are commissions responsible for the conservation and management of the bigeye tuna; the International Commission for the Conservation of Atlantic Tunas (ICCAT), the Indian Ocean Tuna Commission (IOTC), the Western and Central Pacific Fisheries Commission (WCPFC), and the Inter-American Tropical Tuna Commission (IATTC). However, a report by Traffic International and WWF Australia found that management of bigeye tuna stocks has been slow to respond to scientific advice, and have failed to initiate appropriate management measures. Unless this situation changes, all four bigeye tuna stocks will become overfished (5), and the collapse of bigeye tuna stocks would greatly impact many people reliant on the employment and income of the fishing, processing and trading industries (7). Hopefully more precautionary management measures will soon be implemented, before it is too late for this important and fascinating fish.
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- Anal fin: in fish, an unpaired fin on the under surface of a fish, behind the anus.
- Cephalopods: from the Greek for ‘head-foot’, a class of molluscs that occur only in marine habitats. All species have grasping tentacles, and either an internal or external shell. Includes nautiloids, cuttlefish, squids, octopuses, and extinct ammonites and belemnites.
- Crustaceans: diverse group of arthropods (a phylum of animals with jointed limbs and a hard chitinous exoskeleton) characterised by the possession of two pairs of antennae, one pair of mandibles (parts of the mouthparts used for handling and processing food) and two pairs of maxillae (appendages used in eating, which are located behind the mandibles). Includes crabs, lobsters, shrimps, slaters, woodlice and barnacles.
- Dorsal fin: the unpaired fin found on the back of the body of fish, or the raised structure on the back of most cetaceans.
- Pectoral fins: in fish, the pair of fins that are found one on each side of the body just behind the gills. They are generally used for balancing and braking.
- Spawning: the production or depositing of large quantities of eggs in water.
IUCN Red List (January, 2008)
- Collette, B.B. and Nauen, C.E. (1983) FAO Species Catalogue. Vol. 2: Scombrids of the World. An annotated and illustrated catalogue of tunas, mackerels, bonitos and related species known to date. Food and Agriculture Organization of the United Nations, Rome.
South Atlantic Fishery Management Council (January, 2008)
NSW Department of Primary Industries (January, 2008)
- Lack, M. (2007) With an Eye to the Future: Addressing Failures in the Global Management of Bigeye Tuna. TRAFFIC International and WWF Australia, Australia.
FAO Fisheries and Aquaculture Department: Biological Characteristics of Tunas and Tuna-like Species (January, 2008)
WWF Newsroom: Turning a blind eye to bigeye tuna (January, 2008)