Oceanic Bonito

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).

Perception Channels: tactile ; chemical

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.

US Federal List: no special status

CITES: no special status

IUCN Red List of Threatened Species: no special status

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.

Positive Impacts: food

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.

Animal Foods: fish; insects; mollusks; aquatic crustaceans; other marine invertebrates

Primary Diet: carnivore (Piscivore , Eats non-insect arthropods, Molluscivore )

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.

Biogeographic Regions: indian ocean (Native ); atlantic ocean (Native ); pacific ocean (Native )

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.

Habitat Regions: temperate ; tropical ; saltwater or marine

Aquatic Biomes: pelagic ; coastal

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

Average lifespan
Status: wild: 8-12 years.

Average lifespan
Status: captivity: 12 years.

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.

Range mass: 34.5 (high) kg.

Average mass: 8-10 kg.

Other Physical Features: endothermic ; ectothermic ; homoiothermic; bilateral symmetry

Sexual Dimorphism: sexes alike

Skipjack tuna spawn throughout the year, although they limit spawning from early fall to spring in regions near the equator (Collette and Nauen 1983). Fecundity is related to size. In one study (Stequert and Ramcharrun 1995) it was found that a 44 cm female carrried 80000 eggs while a larger female (75cm long) possessed 1.25 million eggs. Based on these two estimates, it was determined that the relative batch fecundity varies from 40 to 130 eggs/g body weight. These authors estimate four successive spawning periods per year for the skipjack. Stequert and Ramcharrun (1996) also looked at other aspects of reproduction in the skipjack tuna. They found that females mature at 41-42 cm fork-length while males mature at a slightly larger size, 42-43 cm fork-length. Both of these are equivalent to approximately 1.5 years of age. In their study, 70% of the females during any given month had ovaries in the terminal stages of maturation, providing more evidence that reproduction is not allocated to a particular time of year. Exactly how skipjacks reproduce is not known, but the breeding area of this species is thought to be limited to tropical regions of the world's oceans.

Average age at sexual or reproductive maturity (female): 1.5 years.

Average age at sexual or reproductive maturity (male): 1.5 years.

Key Reproductive Features: iteroparous ; seasonal breeding ; gonochoric/gonochoristic/dioecious (sexes separate); sexual ; fertilization (External ); oviparous

Average age at sexual or reproductive maturity (female)
Sex: female: 1095 days.

Catches of skipjack tuna have been steadily increasing since 1950, reaching a peak in 1991 at 1 674 970 t. In 1995, catches for this species have been reported from 15 fishing areas (practically all except the 4 fishing areas covering the Arctic and Antarctic regions). Most of the catches are taken in the 71 (840 449 t), 51 (218 005 t), 61 (177 991 t) and 34 (130 372 t) fishing areas. The reported world catch reported for FAO Statistics in 1996 was 104 551 t. Skipjack tuna is taken at the surface, mostly with purse seines and pole-and-line gear but also incidentally by longlines. Other (artisanal) gear include gillnets, traps, harpoons and beach seines. Tuna pole and line fishing and Tuna purse seining are the most used fishing techniques. The importance of flotsam or manmade aggregation devices has increased greatly in recent years. Furthermore, supporting exploration techniques such as aerial spotting find increasing application in skipjack fisheries and utilization of remote sensing is being tried experimentally. In the pole-and-line/bait boat fishery, availability of suitable bait-fish presently represents one of the major constraints and hence, efforts to culture bait-fishes are receiving more attention. It appears, however, that bait rearing is hardly feasible on large enough scale to support a major fishery. The total catch reported for this species to FAO for 1999 was 1 976 479 t. The countries with the largest catches were Japan (287 326 t) and Indonesia (205 670 t). Skipjack tuna are marketed fresh, frozen and canned. In Japan, they are also dried.

Body fusiform, elongate and rounded. Teeth small and conizal, in a single series; gillrakers numerous, 53 to 63 on first gillrakers. Two dorsal fins separated by a small interspace (not larger than eye), the first with 14 to 16 spines, the second followed by 7 to 9 finlets; pectoral fins short, with 26 or 27 rays; interpelvic process small and bifid; anal fin followed by 7 or 8 finlets. Body scaleless except for the corselet and lateral line. A strong keel on each side of caudal fin base between 2 smaller keels. Swimbladder absent. Vertebrae 41. Colour: back dark purplish blue, lower sides and belly silvery, with 4 to 6 very conspicuous longitudinal dark bands which in live specimens may appear as discontinuous lines of dark blotches.

Cosmopolitan in tropical and warm-temperate waters; absent from the Black Sea.

Maximum fork length is about 108 cm corresponding to a weight of 32.5 to 34.5 kg; common to 80 cm fork length and a weight of 8 to 10 kg. The all-tackle angling record is a 18.93 kg fish with a fork length of 99 cm taken in Mauritius in 1982. Fork length at first maturity is about 45 cm.

An epipelagic,oceanic specieswith adults distributed roughly within the 15° C isotherm (overall temperature range of recurrence is 14.7° to 30°C), while larvae are mostly restricted to waters with surface temperatures of at least 25°C. Aggregations of this species tend to be associated with convergences, boundaries between cold and warm water masses (i.e. the polar front), upwelling and other hydrographical discontinuities. Depth distribution ranges from the surface to about 260 m during the day, but is limited to near surface waters at night. Skipjack tuna spawn in batches throughout the year in equatorial waters, and from spring to early fall in subtropical waters, with the spawning season becoming shorter as distance from the equator increases. Fecundity increases with size but is highly variable, the number of eggs per season in females of 41 to 87 cm fork length ranging between 80 000 and 2 million. Food items predominantly include fishes, crustaceans and molluscs. Even though Carangidae and Balistidae are part of the diet of skipjack tuna in all oceans, the wide variety of species taken suggest it to be an opportunistic feeder preying on any forage available. The feeding activity peaks in the early morning and in the late afternoon. Cannibalism is common.The principal predators of skipjack are other tunas and billfishes.It is hypothesized that the skipjack tuna in the eastern central Pacific originate in equatorial waters, and that the pre-recruits (up to 35 cm fork length) split into a northern group migrating to the Baja California fishing grounds, and a southern group entering the central and south American fishing areas. Having remained there for several months, both groups return to the equatorial spawning areas. A similar migration pattern has been observed in the northwestern Pacific.Studies of the local movements of skipjack tuna showed that small fish (under 45 cm fork length) made nightly journeys of 25 to 106 km away from a bank but returned in the morning, while big individuals moved around more independently. Skipjack tuna exhibit a strong tendency to school in surface waters. Schools are associated with birds, drifting objects, sharks, whales or other tuna species and may show a characteristic behaviour (jumping, feeding, foaming, etc.).In the absence of reliable age determination methods, estimates of longevity vary at least between 8 and 12 years.

This species is distinguished by the following characters: body fusiform, elongate and rounded; teeth small and conical, in a single series; gill rakers on first gill arch numerous, 53-63; D1 XIV-XVI, dorsal fins separated by a small interspace (not larger than eye), the second followed by 7-9 finlets; anal fin followed by 7-8 finlets; pectoral fins short, with 26-27 rays; 2 flaps (interpelvic process) between pelvic fins; body scaleless except for corselet and lateral line; a strong keel on each side of caudal-fin base between 2 smaller keels. Colour of back dark purplish blue, lower sides and belly silvery, with 4-6 very conspicuous longitudinal dark bands which in live specimens may appear as discontinuous lines of dark blotches (Ref. 9684).

Aggregations of this species tend to be associated with convergence, boundaries between cold and warm water masses (i.e., the polar front), upwelling and other hydrographic discontinuities. Normally inhabit waters with surface temperatures of 20°C to 30°C (Ref. 28951). However, adults are sometimes present in waters as cold as 15°C (Ref. 168, 28950). Stay near the surface at night. School near the surface with birds, drifting objects, whales, sharks, and other tuna species and shows characteristic behaviors like jumping, feeding and foaming. Opportunistic feeders preying on any forage available. The feeding activity peaks in the early morning and in the late afternoon.Skipjack tuna also need a dissolved oxygen level of 2.5 ml per liter of sea water to maintain a minimum swimming speed, and require higher levels when active (Ref. 28952). This requirement generally restricts skipjack tuna to water above the thermocline and in some areas, such as the eastern Pacific, may exclude them from surface waters (Ref. 28952).The warm East Australian Current distributes skipjack tuna larvae into subtropical waters off eastern Australia (Ref. 28956). The larvae are generally limited to the upper 50 m of water and are most common in the sub-surface layers (Ref. 6390). Juvenile skipjack tuna less than 15 cm TL inhabit the same areas as larvae but generally move to cooler waters as they grow to maturity (Ref. 28956).There is little information on migration patterns for skipjack tuna in Australian waters. Skipjack tuna tagged in the Coral Sea, off Norfolk Island and off New South Wales have been recovered in waters off the Solomon Islands, New Caledonia, French Polynesia and New Zealand (Ref. 28959). The recapture in Australian waters of a few fish tagged in New Zealand and Papua New Guinea indicates that there could also be significant movement of skipjack tuna into the Australian Fishing Zone from the greater South Pacific (Ref. 6390).

Dorsal spines (total): 14 - 16; Dorsal soft rays (total): 14 - 15; Analspines: 0; Analsoft rays: 14 - 15; Vertebrae: 41

Oceanodromous. Migrating within oceans typically between spawning and different feeding areas, as tunas do. Migrations should be cyclical and predictable and cover more than 100 km.

In tropical waters, reproductively active female skipjack tuna spawn almost daily.

Lagenocystis Infestation. Parasitic infestations (protozoa, worms, etc.)

Didymocystoides Infestation (intestine). Parasitic infestations (protozoa, worms, etc.)

Didymocystoides Infestation (stomach). Parasitic infestations (protozoa, worms, etc.)

Kollikeria Disease. Parasitic infestations (protozoa, worms, etc.)

Oesophagocystis Infestation. Parasitic infestations (protozoa, worms, etc.)

Coeliodidymocystis Disease. Parasitic infestations (protozoa, worms, etc.)

Lobatozoum Infestation. Parasitic infestations (protozoa, worms, etc.)

Didymoproblema Infestation. Parasitic infestations (protozoa, worms, etc.)

Didymocylindrus Infestation 2. Parasitic infestations (protozoa, worms, etc.)

Didymocylindrus Infestation 1. Parasitic infestations (protozoa, worms, etc.)

Caligus Infestation 2. Parasitic infestations (protozoa, worms, etc.)

Found in offshore waters; larvae restricted to waters with surface temperatures of 15°C to 30°C (Ref. 6390). Exhibit a strong tendency to school in surface waters with birds, drifting objects, sharks, whales and may show a characteristic behavior like jumping, feeding, foaming, etc. Feed on fishes, crustaceans, cephalopods and mollusks; cannibalism is common. Spawn throughout the year in the tropics, eggs released in several portions (Ref. 35388). Eggs and larvae are pelagic (Ref. 6769). Preyed upon by large pelagic fishes (Ref. 6885). Also taken by trolling on light tackle using plugs, spoons, feathers, or strip bait (Ref. 9684). Marketed fresh, frozen or canned (Ref. 9340, 9684 ); also dried-salted and smoked (Ref. 9987).

fisheries: highly commercial; gamefish: yes; price category: high; price reliability: reliable: based on ex-vessel price for this species

廣泛分布於全世界溫帶及熱帶海域。台灣各地均有產。

為各沿岸國相當重要之經濟性魚種，約佔全世界鯖類總產量的40%，全世界年產量可達70-80萬公噸。一般漁法以定置網、一支釣、延繩釣或流刺網捕獲。煎食、煮味噌湯或作生魚片皆宜，亦常被加工成製品，如柴魚或罐頭等。

體紡錘形，橫切面近圓形，背緣和腹緣圓鈍；尾柄細短，兩側中央各具一發達之隆起脊，另有二小隆起脊位於尾鰭基部。頭中大，稍側扁。吻尖，大於眼徑。眼較小，位近頭的背緣。口中大，端位，斜裂；上下頜等長，上下頜齒絨毛狀；鋤骨、腭骨和舌上均無齒。第一鰓弓上之鰓耙數為53-63。體在胸甲部被圓鱗，其餘皆裸露無鱗；左右腹鰭間具2大鱗瓣；側線完全，沿背側延伸，在第二背鰭起點下方呈波形彎曲，伸達尾鰭基。第一背鰭具硬棘XV-XVIII，與第二背鰭起點距離近，其後具7-9個離鰭；臀鰭與第二背鰭同形；尾鰭新月形。體背紫藍色，腹部銀白，體側具4-5條暗色縱帶。全世界僅1屬1種。

大洋性洄游魚類。喜棲於水色澄清水域。天氣晴朗，水溫上昇時，常聚集於上層水域。游泳快速，以魚類及甲殼類為食。

The skipjack tuna (Katsuwonus pelamis) is a medium-sized perciform fish in the tuna family, Scombridae. It is otherwise known as the balaya (Sri Lanka), bakulan/kayu (North Borneo), tongkol/aya (Malay Peninsula/Indonesia), aku (Hawaii), cakalang (Indonesia), katsuo, arctic bonito, mushmouth, oceanic bonito, striped tuna or victor fish. It grows up to 1 m (3 ft) in length. It is a cosmopolitan pelagic fish found in tropical and warm-temperate waters. It is a very important species for fisheries.

Found in offshore waters with temperatures ranging from 14.7 to 30°C while larvae are mostly restricted to waters with surface temperatures of at least 25°C. Exhibit a strong tendency to school in surface waters with birds, drifting objects, sharks, whales and may show a characteristic behaviour like jumping, feeding, foaming, etc. Feeds on fishes, crustaceans, cephalopods and molluscs. Spawns in batches all year long in tropical waters and from spring to early fall in subtropical waters. It is thought to live for at least 8-12 yrs. Also taken by trolling on light tackle using plugs, spoons, feathers, or strip bait (Ref. 9684). Marketed fresh, frozen or canned (Ref. 9340); also utilized dried-salted and smoked (Ref. 9987).

Feed on fishes, crustaceans, cephalopods and mollusks; cannibalism is common

cosmopolitan in tropical and warm-temperate waters, as far north as 40.87°N

Found in offshore waters; larvae restricted to waters with surface temperatures of 15°C to 30°C.

nektonic

Known from seamounts and knolls