Yellowfin Tuna (Thunnus albacares)
Also known as: ahi
Morphology & Biology
Yellowfin tuna (Thunnus albacares) is a highly migratory, pelagic and oceanic species that resides worldwide in tropical and sub-tropical seas. It schools primarily by size, either in monospecific or multi-species groups. Larger fish frequently school with porpoises. Schools of yellowfin tuna are also associated with floating debris and other objects. The species occurs both above and below the thermocline, to a depth of at least 400m. Due to its high oxygen demand, however, it is commonly found in depths up to 250m in the tropics, and in waters with temperatures between 18-31°C.
Size, weight and age of the fish differ depending on their geographical location. While its lifespan lies between four and eight years, a yellowfin tuna reaches maturity after approximately 1.5 – 3 years, having grown to a length of 50 – 100cm. Adults can reach a length of over 200cm and a weight of over 175kg. The average length lies between 50 – 150cm. 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.
Distribution
Tagging data show that trans-Atlantic migrations occur, and yellowfin tuna from the entire Atlantic are considered to be part of a single population. In the eastern Pacific, this species ranges from southern California and the southwest and central eastern parts of the Gulf of California to Peru, including all of the oceanic islands. The primary Atlantic spawning grounds are in the Gulf of Guinea, and to a lesser extent in the Gulf of Mexico.
Spawning occurs throughout the year in the core areas of distribution at sea surface temperatures of 24°C or higher, but peaks are observed in the northern and southern summer months respectively.
Threats
The IUCN lists the yellowfin tuna as nearly threatened. Although fish populations have been fluctuating, a steady decline in numbers has been observed since 1990. In particular, fish populations in the Indian Ocean and in the West – Central Pacific are declining at an alarming rate.
Yellowfin tuna are primarily captured by the purse-seine fisheries. A purse seine is a large wall of netting extended around a school of fishes. This net can reach more than 2km in length and 200m in depth. The seine has floats along the top of the net so that the net stays open. Along the bottom of the net there are lead lines that ensure the bottom of the net sinks. Once a school of fish is located, a fishing boat will encircle the school with the net. The lead line is then tightened to “purse” the net closed at the bottom. This prevents fish from escaping.
Because purse-seine fishery catches a whole school of fish at once, already overfished species are most vulnerable. This method is fishing is a non-selective fishing method, meaning that bycatch of unwanted species is a problem. Secondly, 20% of Yellowfin tuna are also caught by longlines, which have been identified as having one of the highest bycatch rates for many species. This incidental bycatch is considered to be a global threat to long-lived animals such as sharks, sea birds, sea turtles, and marine mammals. Sharks and tuna represent top predators in many ecosystems: their loss, therefore, has been shown to negatively impact several ecosystems. The primary product forms of Yellowfin tuna are canned, fresh whole fish, frozen pre-cooked loins, and raw frozen loins and steaks.
Fisheries & Aquaculture
Due to the tuna’s migratory behaviour, cultivating it in aquacultures is challenging. In general, 99.9% of tuna is caught in the wild and only 0.1% derive from aquacultures. Cultivating yellowfish tuna in farms is currently being researched, but is so far only experimental.
Fun fact
Did you know that fishes are not all necessarily ‘cold blooded’? The yellowfin tuna (along with other tuna fishes) is able to increase its own body temperature a few degrees above the surrounding environment. For this reason tuna are often referred to as being a ‘warm-blooded’ animal. This warmed blood is crucial for strong and fast swimming.
