Katsuwonus pelamis
skipjack tuna

Skipjack tuna, Katsuwonus pelamis, are marine fishes found in most waters all over the world but are rarely seen in the North Sea and have never been caught in the Black Sea. Skipjacks are confined to waters with temperatures above 15 degrees C.

Distribution map. (Muus and Nielsen, 1999)

An epipelagic fish, skipjacks are distributed in water with temperatures ranging from 14.7 and 30 C. Larvae are mostly restricted to areas with temperatures of at least 25 C. Skipjacks tend to be associated with regions of upwelling, or areas where cold, nutrient-rich waters are brought up from the bottom of the ocean to the surface, as well as regions where cold and warm water mix. These are areas with high productivity. Rarely are they found at depths greater than 260 m. (Collette and Nauen, 1983)

Katsuwonus pelamis have a typical tuna fish shape, possessing a fusiform, elongate body. They possess two dorsal fins, the first of which consists of 14-16 spines. The second dorsal fin is set directly behind the first with a small space separating the two. Following the second dorsal fin are 7-9 finlets. The anal fin is also followed by about 7-8 finlets. The pectoral fins are short and consist of 26 or 27 rays. Skipjacks are scaleless, except along the lateral line and the corselet. Dark purplish-blue dorsally, skipjacks camouflage themselves from predators below by maintaining a silvery hue both laterally and ventrally. There are also 4-6 dark bands along the side of the fish extending to the tail which, in live fish, may appear as a discontinuous lines of dark blotches. The maximum length is about 108 cm fork-length with a maximum weight of 32.5 to 34.5 kg. However, the more common maximum size is 80 cm fork-length and 8-10 kg in weight. Teeth are small and conical in shape. Skipjacks also lack a swim bladder. (Collette and Nauen, 1983)

The maximum age of skipjack tunas is not known but is estimated to be around 8-12 years.

Skipjack tuna are schooling migratory fishes (Collette and Nauen 1983). They tend to school with each other, other tuna, whales or sharks. They also tend to shoal under objects floating on the surface of the water (World Wide Fund For Nature 1996). Skipjacks are thought to have a north to south migratory seasonal pattern but there is still some question as to whether or not these tuna migrate with a purpose or use advective movements (Gauldie and Sharp 1996). Joseph et al. (1988) found that Pacific populations are more migratory than populations in the Atlantic, although they do suggest that this may be due to the fact that Pacific populations have been studied more. Small fish (less than 45cm fork-length) will make nightly journeys of 25 to 106 km away from banks, but will return to safety by daylight. Larger fish move more independently and are not restricted to the banks, although they do spend most of their time within the continental shelf (Collette and Nauen 2000). Katsuwonus pelamis exhibit a variety of characteristic behaviors, including jumping, feeding and foaming while in schools (Fishbase 2000). Breeding behavior is not known. ("Species Summary for Katsuwonus pelamis Skipjack tuna", 10-12-2000; Collette and Nauen, 1983; Gauldie and Sharp, June 1996; Joseph, Klawe, and Murphy, 1988; World Wide Fund For Nature, 1996)

Katsuwonus pelamis feed predominantly on fishes, crustaceans and mollusks. The wide variety of food items consumed suggests that the skipjack is a highly opportunistic feeder. Feeding activities peak in the early morning and again in the late afternoon (Collette and Nauen 1983). Blackburn and Serventy (1981) found that the major food items in stomachs of skipjacks in Australian waters were euphausids, with various fishes and squid making up a smaller percentage of the stomach contents. Ankenbrandt (1985) also studied the food habits of skipjack tuna. She found that Euphausia similis had the highest % IRI (index of relative importance) while the gonostomid Maurolicus muelleri made up the highest MVRM (mean volumetric ratio measurement) during all time periods. Other fish like mackerel (Scomber) and Thyrsitops lepidoides were also common. Crustaceans other than E. similis occurred frequently but were not a major part of the total volume of the stomach. Cephalopods occurred infrequently. Skipjacks were also found to consume pteropods, siphonophores, and beetles. There is still some debate as to whether or not this species is cannibalistic. Ankenbrandt (1985) did not find evidence for cannibalism, but Collette and Nauen (1983) list skipjacks as cannibalistic. This discrepancy could be due to the fact that, as opportunistic feeders, skipjacks will consume their young only when they are prevalent. (Ankenbrandt, 1985; Blackburn and Serventy, 1981; Collette and Nauen, 1983)

Katsuwonus pelamis has become more important in the tuna fishing industry in recent years. In 1950, less than 300,000 metric tons were taken. In 1991, 1,674,970 metric tons were caught. This level has not been reached since (Collette and Nauen 2000). Currently, Katsuwonus pelamis comprise 40 percent by weight of the world tuna catch despite being the smallest of the tunas subject to the large-scale commercial fishing operations. Americans alone consume more than 400000 metric tons of tuna (all species) each year, and it is doubtful that this number will decrease any time soon (World Wide Fund For Nature 1996).

The tendency of skipjacks to group underneath objects floating on the surface of the water is taken advantage of by fisheries, which use Fish Aggregating Devices to attract them (World Wide Fund For Nature 1996). However, they are usually captured at the surface using purse seines or pole-and-line gear.

Skipjacks are currently not threatened, although catches fluctuate widely from year to year, providing scientists with little information as to how long the populations can withstand increased fishing pressure (World Wide Fund For Nature 1996). Scientists have been studying both natural and fishing mortality within these populations, as well as other tuna populations, to obtain a better understanding of their biology. In one such study, conducted in the western Pacific tuna fishery, natural mortality (M) and fishing mortality (F) were studied over a range of sizes for skipjack and other tunas. Skipjacks displayed a u-shaped natural mortality rate, where smaller size classes had higher mortality rates than those in the middle. At size classes above 70cm, the mortality rate increased yet again, indicating the natural age at which these fish die. Fishing mortality was high for the smallest subclasses studied (21-30 cm, 31-40 cm) and decreased steadily for the larger size categories. If these numbers are correct, the high M values under natural conditions for young skipjack tuna would dampen the effect of the high F for this age group. In other words, the high mortality rate sustained by skipjacks at a young age due to fishing would not necessarily alter the population numbers as these fish tend to have low survival rates anyway (Hampton 2000).

There are several groups dedicated to the study and protection of tuna. One such group, the Inter-American Tropical Tuna Commission (IATTC), works with fisheries and governments of member nations to protect these species. The IATTC was established in the 1950s and consists of two main programs, the Tuna-Billfish and the Tuna-Dolphin programs. Both of these programs are designed to investigate yellowfin and skipjack tuna in the eastern Pacific, examine the effects of natural and human activities on the populations of these fishes, and recommend action to member governments to maintain populations at maximum sustained catches (Joseph 1994). This commission (and others like it) is operating to understand more fully the biology of skipjack tunas, and, as a result, make better management decisions. Due to its global distribution, this is a difficult species to manage effectively.

Other common names for skipjacks include skipjack tuna, striped tuna, bonito, and striped bonito. Also, there are several sources that still use an old species name for skipjacks, Euthynnus pelamis (Collette and Nauen 2000).