Description

FAO Species Fact Sheet, Katsuwonus pelamis (Linnaeus 1758), http://www.fao.org/fishery/species/2494/en

The skipjack is a small to medium size fish with an elongate, round-and-tapered body. Skipjack does not have a swim bladder. The back is dark purplish blue, the lower sides and belly silvery. There are four to six very conspicuous dark bands along the body (appear as dark blotches in live fish). The body lacks scales except for the area behind the pectoral fins and along the lateral line. Skipjack has two dorsal fins separated by a short interspace; the dorsal and anal fins are followed by 7-9 finlets. There is a strong keel on each side of the caudal fin base between two smaller keels. The gill rakers are numerous, 53-63 on the first gill arch (Fishbase; Collette, 2001).

Ecosystem Role

Skipjack are at the top level of the pelagic food chain. Accordingly, fluctuations in their population size may significantly affect the balance of all trophic levels over time (Collette et al 2011). All tuna have the specialised anatomy of a vascular counter-current heat exchanger, that allows them to sustain muscle temperatures above ambient temperature, so enabling them to live in a wide range of temperatures. They regulate their body temperature in response to their muscle temperature changes, moving into cooler or warmer waters as needed. In this way, the feeding habitat of adult fish can be extended to the rich, deep forage layer, or to more productive temperate surface waters (Lehodey et al, 2011).

Habitat and Distribution

Skipjack live in offshore tropical and subtropical waters where they often school in surface waters with birds, drifting objects, sharks and whales (Collette, 2001). They range seasonally to 40°N and 40°S, roughly corresponding to the 20°C surface isotherm (Hoyle et al, 2011; Banks et al, 2011). Skipjack do not occur over shallow coastal shelves (ocean depths less than about 50 m) due to their need for access to warmer and cooler water (see map). [Image SKJ-023][See slide show for skipjack distribution map]

The availability of food, suitable water temperatures and dissolved oxygen levels, determine the distribution and abundance of skipjack. Except for the shallow coastal shelves, skipjack tuna live within a depth range of 0-260 m (Collette & Nauen, 1983) and sea surface temperature range of 17-30 degree C (Lehodey et al, 2011). In general, skipjack are less tolerant of low ambient oxygen concentrations than are yellowfin and bigeye tuna, and increase their swimming speeds when oxygen levels fall below 4 mg/l. Skipjack also spend less than 10% of their time at depths where oxygen levels are below ~ 5.0 mg/l (3.8 ml/l, 75% saturation) (Lehodey et al, 2011).

Skipjack movement is highly variable, being influenced by large-scale oceanographic variability. Skipjack also make long migrations into temperate waters during the summer months (Miyake et al, 2010). Tagging data show that individual skipjack are capable of undertaking long-distance movements of several thousand kilometres; and make significant seasonal movements between the western and eastern equatorial regions (Hoyle et al, 2011).

Growth, Reproduction and Diet

Skipjack growth is rapid compared to that of other large tunas. Females and males mature at 1-2 years, at size 40-45 cm (fork length) and are captured at sizes between 40 and 70 cm (WCPO) or 3.0-3.5 kg (IO) at 1-3 years of age.

If not captured, skipjack can grow to 100 cm fork length (FL), commonly to 80 cm FL. The maximum recorded weight is 34.5 kg (IO) (Collette & Nauen, 1983; Collette, 2001), and 30 kg (WCPO). The maximum reported age estimates vary at least between 8 and 12 years (Collette & Nauen, 1983) but most skipjack do not live beyond 3 to 4 years.

Skipjack are highly fecund and can spawn throughout the year in the tropics, although a study in the Indian Ocean revealed that the gonadosomatic index (GSI) is highest in January, April, July and December, and lowest in June and August (Norungee & Kawol, 2011).

Under favourable conditions, skipjack are serial opportunistic spawners. A skipjack releases its eggs in several portions; the eggs and the larvae are pelagic. The sex ratio does not appear to vary with size for skipjack (Hoyle et al, 2011) and (in the Indian Ocean), males are slightly more common (1.19:1) than are females (Norungee & Kawol, 2011).

Skipjack feed on fishes, crustaceans, cephalopods and molluscs; cannibalism is common (Collette, 2001). The principal predators of skipjack are other large pelagic fishes, including tunas and billfishes (Collette & Nauen, 1983).