Fish Facts: Striped tuna
STRIPED tuna (Katsuwonus pelamis), also known as skipjack tuna or skippies, are a crucial part of the pelagic food chain of the worlds oceans. They can be found in offshore waters throughout tropical and subtropical seas worldwide. These pretty little tuna are dark blue or purple on the back and silvery on their lower sides, with three to five dark longitudinal stripes on their lower flanks. Skippies can be found throughout Australia's oceanic waters between the surface and the thermocline, and are common wherever water temperatures exceed 20°C. Like other tunas, they are warm blooded and larger specimens (which are more efficient at maintaining their body temperature) are often found in waters as cold as 15°C, usually near convergence zones where warm bodies of water mix with cooler ones.
One interesting fact about skippies is they have a very high metabolic rate. This means they need high levels of dissolved oxygen in the surrounding water in order to survive, and indeed water oxygen level is one of the most important oceanic features that influences their distribution (surpassed only by water temperature and food availability). Because they lack a swimbladder and are ram ventilators, skipjack tuna have to continue swimming at a reasonably fast minimum swimming speed at all times in order to survive and keep their position in the water column. Like several other tuna species, they also have an effective circulatory heat exchanger in their red muscle bundles which allows them to maintain their body temperature a few degrees celcius above that of the surrounding water. This combination of warm body, high metabolic rate and high swimming speeds means they require a high calorific intake, which is why skipjack possess a voracious appetite and explains why they are seldom found far from schools of baitfish, squid, and other sources of food. Indeed they are considered opportunistic predators that will feed on virtually any forage species that is available, including smaller skipjack tuna.
In keeping with their fast lifestyle, growth of skipjack is also rapid. Approximate sizes at age are:
- 1 year, 31 cm and 0.5 kg
- 2 years, 51 cm and 2.7 kg
- 3 years 64 cm and 5.8 kg
Maximum size is around 110 cm and 34 kg at their 7-12 years maximum age, making them a relatively shortlived tuna species. Females mature in their second year at between 40 and 50cm. Around Australia, skippies spawn mainly in equatorial waters in the Coral Sea off north Queensland, and also off north western Australia. In these areas spawning occurs throughout the year and almost daily. Some spawning also occurs in sub-tropical waters, but in these areas it is restricted to the warmer months. Females spawn between 100,000 and two million eggs at each spawning event. The East Australian and Leeuwin Currents distribute the eggs and larvae south into subtropical waters.
Schools of juvenile skipjack tend to be found aggregating around floating objects, including not only drifting flotsam, but sharks, whales, flocks of birds or schools of other tuna species. Most active surface feeding occurs mainly at dawn and dusk, but recent data suggests this species may also feed below the surface at night. Tagging studies of the daily movements of skipjack showed that juveniles often made nightly journeys of up to 100km away from an aggregation site, but returned in the morning, while bigger individuals moved around more independently. The latter is consistent with observations that larger fish tend to occur in smaller schools or groups of a few fish.
Because of their high fecundity, fast growth and early maturity, skipjack tuna stocks have been able to withstand higher fishing pressure than other tuna species. This means that stocks in most areas remain in a reasonable state despite heavy commercial fishing pressure (FAO data suggests over 3 million tonnes of skipjack are taken annually by purse seine or pole and line fisheries). It is also thought that the significant reductions in the numbers of natural predators such as marlins, sharks and larger tunas, mainly due to commercial fishing, has allowed skipjack populations to remain relatively unthreatened at this point in time.