Scientific Classification of Bluefin Tuna
Information garnered from scientific studies on the bluefin tuna reveal that this fish is well adapted for survival in a diverse range of conditions, and thus has a wide distribution range across the world. However, bluefin tuna actually comprise of three different species, inhabiting three distinct geographical ranges.
They are classified as follows:
Taxon: Thunnus spp (includes eight distinct species as listed below)
There are a total of eight species under the genus Thunnus, Albacore (Thunnus alalunga), Yellowfin tuna (Thunnus albacares), Blackfin tuna (Thunnus Atlanticus), Bigeye tuna (Thunnus obesus), Longtail tuna (Thunnus tonggol), and three species of Bluefin tuna, namely the Atlantic or Northern (Thunnus thynnus), the Pacific (Thunnus orientalis), and the Southern bluefin tuna (Thunnus maccoyii).
The bluefin tuna is a pelagic fish, that inhabits open ocean waters, bays and gulfs, and seasonally visits waters of the coastal shelf. Large schools often aggregate around oceanic fronts where upwelling occurs, bringing with it increased nutrients supporting fish and other prey species in abundance. It is predominantly found in surface and sub-surface waters, but telemetry studies have shown that these tuna frequently dive to depths of between 500-1000m when foraging. It is thought that bluefin tuna may also dive to deeper depths as a physiological response to reduce body temperature in the cool, deeper waters.
Distribution & Range
Bluefin tuna have an extremely wide distribution and can be found worldwide in water temperatures ranging from 3°C to 30°C.
The Atlantic bluefin tuna (Thunnus thynnus) is highly migratory and is found in the subtropical waters of the northeast, northwest, eastern central, western central and southwest Atlantic, the eastern central and northwest Pacific, as well as the Mediterranean and Black Sea. Its range stretches from 72°N-58°S and 99°W-42°E. Although it was initially thought that the North Pacific and Mediterranean populations were separated, scientists now believe that this is in fact the same population, and that part of the North Atlantic population migrates to the Mediterranean to spawn, while part of the population migrates to the Gulf of Mexico to spawn.
There is also a sub-population of Atlantic bluefin tuna found off the coast of South Africa.
The Southern bluefin tuna (Thunnus maccoyii) is a highly migratory species found predominantly in temperate and cold waters (5°C – 20°C ) between 30°S to 60°S, but moves northwards up to 10°S to the warm tropical waters (20°C – 30°C) off the west coast of Australia to spawn. Its distribution includes the southeast and southwest Atlantic, the eastern and western Indian Ocean, and the southwest Pacific. Its range stretches from 8°S – 60°S, circumferencing the globe from east to west 180 degrees.
The Pacific bluefin tuna (Thunnus Orientalis) is found predominantly in the northern Pacific, but ventures south for three months of the year between spring and summer. Its distribution includes the northeast, northwest, eastern central, western central, southwest and southeast Pacific.
Bluefin tuna have a streamlined torpedo shaped body adapted for swimming at fast speeds to allow them to cover vast distances in search of food, and can attain speeds of 45 miles (70 kilometers) per hour. They are warm-blooded, possessing a system of blood vessels that enables heat generated through exertion to be used for thermoregulation, enabling them to keep their body temperature higher than the temperature of the surrounding water. They are thus unrestricted by the limitations of temperature as an environmental constraint, and are able to inhabit both cold temperate waters, as well as sub-tropical waters and waters of the warm tropics.
The bluefin tuna is a ferocious opportunistic predator that feeds on a variety of small fish, squid, crabs and other marine invertebrates. They group together in schools of similar size fish, often teaming up with other Thunnus species presumably to hunt schools of smaller fish. While the bluefin tuna larvae feed on zooplankton, the juveniles feed on squid, crabs and small fish, and adult bluefin tuna feed primarily on schooling fish such as anchovy, sardine, herring, sand lance, and mackerel.
Lifespan, Size & Weight
Bluefin tuna are a long-living fish with a lifespan of between 15 – 20 years. The average size of a mature adult is 6.56 ft (2 m), but this is slightly less for the Southern bluefin tuna, which has an average length of 5.25 ft (1.6 m). However, they can grow much larger than this, with the maximum recorded size of an Atlantic bluefin tuna being 15.03 ft (4.58 m), and the maximum weight recorded being 1010 lb (684.0 kg), while the maximum recorded length for a Southern bluefin tuna is 8.04 ft (2.45 m) and for Pacific bluefin tuna is 9.84 ft (3 m).
This species of fish are oviparous, the females lay eggs during spawning, which are fertilized by the males externally. The fertilized eggs are dispersed over wide distances by ocean currents. The tiny larvae are just 0.12 inches (3 millimeters) long upon hatching, but develop at a rate of 0.04 inches (1 millimeter) per day. Depending on the species, female tuna reach sexual maturity between 5-8 years, however, egg production increases with age/size. While a 5 year old female Atlantic bluefin tuna produces an average of five million eggs, a 15-20 year old female produces up to 45 million eggs! While it is thought that females spawn every year, more research is needed to confirm this.
Natural predators of bluefin tuna include large sharks and killer whales, while larvae and juveniles are preyed on by other fish as well as seabirds, seals and sea lions. However, overfishing of bluefin tuna by humans (as seen in the next article), has had a drastic impact on their populations worldwide, with some stocks being fished out to the point of extinction.
Although the bluefin tuna is well adapted to surviving in a diverse range of conditions resulting in it having a wide distribution range across our oceans, these biological adaptations, such as high growth rates and fast swimming ability, make it a popular target for human predators for both food and sport. The pressure placed on bluefin tuna by human demand has led to most populations declining dramatically, with some populations teetering on the brink of extinction. This is further exacerbated by the fact that as a large, relatively long-lived fish they tend to reach sexual maturity later in life, and if captured before breeding age, would not have had the opportunity to reproduce and add to the population. When large numbers of young juvenile fish of pre-breeding age are removed from the gene-pool in this manner it can have drastic consequences to the long-term survival of the species. Lets hope we can use the facts and information on bluefin tuna that scientists have gathered to help us to better manage and conserve the species in order to save it from extinction.
Collette, B.B. 1995, (Ref 9340) FishBase, https://www.fishbase.org/summary/SpeciesSummary.php?genusname=Thunnus&speciesname=orientalis
Collette, BrB. 1999. (Ref 33246) FishBase, https://www.fishbase.org/summary/SpeciesSummary.php?genusname=Thunnus&speciesname=thynnus
Collette, B.B. and C.E. Nauen. 1983. (Ref 168) FishBase, https://www.fishbase.org/summary/SpeciesSummary.php?genusname=Thunnus&speciesname=maccoyii
Froese, R. and D. Pauly. Editors. 2011. FishBase. World Wide Web electronic publication. https://www.fishbase.org, version (02/2011).
Fromentin, J-M. & Powers, J.E. "Atlantic bluefin tuna: population dynamics, ecology, fisheries and management." Fish and Fisheries. Blackwell Publishing, Dec 2005; 6(4):281-306
Image Credit, Atlantic Bluefin Tuna Thunnus thynnus: Wikimedia Commons/OpenCage
Image Credit, Bluefin Tuna: Wikimedia Commons/National Oceanic & Atmospheric Administration (NOAA)
Image Credit, Fishing for Bluefin Tuna: Wikimedia Commons/Kotarokurosawa