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Skipjack tuna fishing: the fish in your can of tuna

Open a can of tuna and there is a good chance there is no tuna in it — not in the zoologist’s sense, anyway. According to English Wikipedia, up to 95 per cent of this fish’s catch is canned; Japanese Wikipedia says more than 80 per cent of the world catch goes into cans, and that 70 to 80 per cent of the raw material of all the world’s canned tuna comes from it. In 2018, English Wikipedia reports, 3.2 million tonnes were landed — the third-largest catch of any marine fishery, after the Peruvian anchoveta and Alaska pollock. This article is about the skipjack tuna, *Katsuwonus pelamis* — German Echter Bonito, Spanish listado, Italian tonnetto striato, French bonite à ventre rayé, Japanese katsuo.

Tuna on the label, not in the system

German Wikipedia puts it plainly: the skipjack is one of the most economically important species sold as “tuna”, although zoologically it does not belong to the true tunas (*Thunnus*). It is the only species in its genus, *Katsuwonus*. Swedish Wikipedia calls it the most common fish sold as canned tuna; Norwegian Wikipedia says canned tuna is usually either skipjack or albacore; Dutch Wikipedia says the cans in Dutch and Belgian supermarkets often contain this species. In France, according to French Wikipedia, it fills most pre-cooked cans as “thon listao”; in Japan the can is called “light tuna” and, according to Japanese Wikipedia, is labelled “katsuo in oil”. German food law requires the scientific name on the pack — if you read “Katsuwonus pelamis”, you are holding a skipjack.

A name from Japan

According to German Wikipedia, the genus name is a Latinisation of the Japanese katsuo; the w comes from the classical spelling カツヲ. Italian and French Wikipedia credit the genus to Kishinouye, 1915; the species was described by Linnaeus in 1758 as *Scomber pelamis*. The species name, says German Wikipedia, comes from the ancient Greek pēlamys — the word for young tuna in their first year. French Wikipedia lists 22 synonyms, from *Thynnus vagans* to *Euthynnus pelamis*. And Japanese Wikipedia derives katsuo itself from katauo, the “hard fish” — and notes dryly that translating it into English as “bonito” is a mistranslation: bonito is the genus *Sarda*, not the katsuo.

Edo and the year’s first katsuo

Off Japan, according to Japanese Wikipedia, the katsuo moves north with the Kuroshio in spring and back south in autumn. The first catch of the year is called hatsugatsuo, “first katsuo” — lean and light; the returning autumn fish is modorigatsuo, and it is fatty (English and Japanese Wikipedia). In old Edo the first katsuo became a mania: in 1812 the kabuki actor Nakamura Utaemon paid three ryō for a single fish, and a satirical verse of the time advised eating the first katsuo even if you had to pawn your wife to pay for it. The fish becomes katsuobushi: boiled, smoked, dried and, for the best grades, matured with kōji mould at least four times over several months — the shavings are the base of the stock dashi. One last curiosity: before insulin could be produced in crystalline form, Japanese Wikipedia says it was for a time extracted from katsuo.

How to recognise it

FishBase describes a fusiform, elongate body, round in cross-section. The first dorsal fin has 14 to 16 spines; the second follows after a gap no wider than the eye and is followed by 7 to 9 finlets, with 7 to 8 behind the anal fin. The pectoral fins are short. It carries scales only on the corselet behind the head and along the lateral line; on each side of the tail base sits a strong keel between two smaller ones. The surest sign is colour: back dark purplish blue, lower sides and belly silvery with 4 to 6 very conspicuous dark longitudinal bands, which in live fish may appear as lines of dark blotches (FishBase). Japanese Wikipedia observes more: when the fish is excited, 4 to 10 vertical bars appear on the belly; when it dies, they fade and the longitudinal stripes show. Similar species — frigate tunas, *Euthynnus affinis* and *Sarda orientalis* — have, the same source says, no belly stripes. It has no swim bladder (German and English Wikipedia) and must keep swimming to stay buoyant. According to English Wikipedia it has the highest share of locomotion muscle of any animal: 68 per cent of its body mass.

  • Size: FishBase gives 110 cm fork length as the maximum and 80 cm as common; the heaviest published weight is 34.5 kg, the oldest reported age 12 years. According to FishBase it matures at around 41 cm.
  • Temperature and latitude: 15 to 30 °C, from 63° North to 47° South, all around the globe (FishBase).

Where it lives and what it eats

  • Range: according to FishBase worldwide in tropical and warm-temperate seas, not in the Black Sea. It occasionally strays north: German Wikipedia says it is sometimes caught in the North Sea in summer, Swedish Wikipedia now and then in the Skagerrak and Kattegat; Italian Wikipedia mentions Great Britain and occasionally Scandinavia.
  • Depth: FishBase gives 0 to 260 metres. According to German and Norwegian Wikipedia it ranges from the surface down to 260 metres by day, but always stays near the surface at night.
  • Schools: up to 50,000 fish (English, Dutch, Finnish and Slovenian Wikipedia). FishBase describes surface schools travelling with birds, drifting objects, sharks and whales, and jumping, feeding and foaming the water. Japanese fishers, says Japanese Wikipedia, distinguish “whale”, “shark” and “bird” schools. Large concentrations form, according to German and Norwegian Wikipedia, at the boundaries between cold and warm water.
  • Diet: fish, crustaceans, cephalopods and molluscs; cannibalism is common (FishBase). According to German Wikipedia it feeds mainly in the early morning and late afternoon.
  • Predators: large pelagic fishes (FishBase); Italian Wikipedia lists sharks, marlin, tunas, dolphinfish and wahoo as well as two species of tern. According to English Wikipedia it is often used as live bait for marlin.

Spawning almost every day

In tropical waters, according to FishBase, reproductively active females spawn almost daily, in several batches and all year round; eggs and larvae drift freely in the water. The further from the equator, the shorter the spawning season — in subtropical waters it runs from spring to early autumn (German Wikipedia), in the Mediterranean only in the warm season (Italian Wikipedia). A female of 41 to 87 cm lays 80,000 to 2 million eggs per season (German Wikipedia). English Wikipedia calls it the most fecund of the main commercial tunas.

How it is caught — and what you should know

Commercially, says German Wikipedia, it is caught near the surface with purse seines or pole and line, and only incidentally on longlines. For anglers, FishBase notes it is also taken by trolling on light tackle — with plugs, spoons, feathers or strip bait; Italian Wikipedia also names trolling as the main sport-fishing method. In the Canary Islands, according to Spanish Wikipedia, it appears from spring to autumn and provides the islands’ largest tuna catch; it is fished with live bait, feathers and trolling lines. On Madeira, says Italian Wikipedia, it is called gaiado and traditionally sun-dried with salt.

⚠️ What follows is a conclusion drawn from its biology, not a sourced statement: a schooling fish that hunts at the surface, travels with birds and feeds mainly in the morning and late afternoon is not searched for at a fixed spot but found — by the birds, the foaming water and the temperature edge where the schools gather.

Three warnings from the sources. FishBase and Dutch Wikipedia report cases of ciguatera poisoning. French Wikipedia warns of histamine poisoning when the cold chain breaks, and of parasites such as Anisakis — the fish should be gutted quickly so the larvae do not migrate into the flesh. And Japanese Wikipedia says it spoils quickly, like mackerel; how it is bled after capture decides freshness, flavour and smell.

And the stock? Since 15 January 2021, the IUCN has listed skipjack as “Least Concern”. FishBase rates its vulnerability to fishing as “moderate” (38 of 100) and its resilience as medium; a stock needs 1.4 to 4.4 years to double. Italian Wikipedia says it is fished very intensively but is not considered overfished — though it was in the past. According to English Wikipedia, the controversy is mainly about method: purse seines set on fish aggregating devices are criticised for bycatch, which is why some cans state the fishing method. Minimum sizes, bag limits and closed seasons are set by each country — check them locally.

Five contradictions, left open

  • How big and how heavy? FishBase: 110 cm fork length, 34.5 kg. English, Dutch, Norwegian and Portuguese Wikipedia: 108 cm. Spanish Wikipedia says 110 cm total length — a different measurement. Japanese: 1 metre and 18 to 20 kg. Slovenian: 100 cm and over 20 kg. Finnish: at most 32 kg.
  • How old? FishBase and German, French and Dutch Wikipedia: 12 years. English: estimates between 8 and 12 years, age being difficult to determine. Japanese: 5 to 6 years.
  • How warm? FishBase: 15 to 30 °C. German: adults at 14.7 to 30 °C. Italian: above 18 °C, adults tolerate no more than 25 °C. Japanese: prefers 19 to 23 °C, Russian 19 to 25 °C. Portuguese: adults prefer an average of about 15 °C. And the larvae? FishBase: 15 to 30 °C — German, Norwegian and Portuguese Wikipedia: at least 25 °C.
  • Is it in the Mediterranean? FishBase excludes only the Black Sea, as does French Wikipedia. Dutch: it occurs among other places in the Mediterranean. German, Spanish, Portuguese and Finnish Wikipedia: it is absent from the eastern Mediterranean. Italian: not common in the Mediterranean. Croatian: only in the west — the Spanish coast and the Tyrrhenian Sea — where it calls it an invasive species, while FishBase treats it as native worldwide. And does it avoid the coast? Spanish Wikipedia: it sometimes comes very close. Italian: rarely.
  • How much, and where does it go? English Wikipedia: 3.2 million tonnes (2018), third place. French: 2.8 million (2006), Finnish 2.8 million (2012), Norwegian 2,523,001 tonnes (2010). Share of the tuna catch: Portuguese Wikipedia about 40 per cent, Russian 60 per cent. Into cans: English up to 95 per cent, Japanese over 80 per cent. And mercury: English Wikipedia “moderate”, Finnish low.

Six names that lead to the wrong fish

  • “Bonito”. In German, English, Spanish and Portuguese it is often simply called “bonito” — but the bonito in the strict sense is the Atlantic bonito *Sarda sarda*, a different fish with its own article in this series. Portuguese Wikipedia explicitly warns that “bonito” and “gaiado” also refer to *Sarda sarda*. The English GBIF list even gives “Atlantic bonito” for the skipjack — the usual name of *Sarda sarda*.
  • “False” and “Unechter Bonito”. German Wikipedia names the Unechter Bonito (“fake bonito”) *Auxis thazard*, which looks similar. “Falscher Bonito” (“false bonito”) is German for the little tunny *Euthynnus alletteratus*. Both are other fish — the “true” in its German name is meant seriously.
  • “Yazılı orkinos”. The Turkish GBIF list gives this name for the skipjack — but the Turkish Wikipedia article under exactly this title describes *Euthynnus alletteratus*, the little tunny. “Çizgili orkinos”, the striped tuna, is less ambiguous.
  • “Palamida” and “pelamide”. GBIF lists “Palamida” in Italian, Albanian and Romanian, “Pelamit” in Maltese and “Pelamide” in Greek — around the Mediterranean these words usually mean *Sarda sarda*. The echo comes from the species name *pelamis*, not from close kinship.
  • “Striped” and “listado”. Its official Spanish name is “listado”, in French trade “listao”. But the English GBIF list also has “striped bonito” — the name usually used for *Sarda orientalis*, a relative of the Atlantic bonito. And the “Gestreifter Thunfisch” (“striped tuna”) on the German GBIF list is no *Thunnus*.
  • “-gatsuo”. In Japanese, many fish carry the word in their name without being katsuo: ソウダガツオ are frigate tunas, ハガツオ is *Sarda orientalis*, マナガツオ a *Pampus* pomfret — and ニシシマガツオ the Atlantic pomfret. In Hawaii it is “aku”, in Indonesia “cakalang”, in the Maldives “kalhubilamas” (English Wikipedia).

That leaves the question every article in this series starts with: when. Temperature, depth and diet are covered above, and they are fixed. What changes every day is everything else — and that is exactly where it is decided whether a morning on the water is worth it. That is what StrikeAhead calculates: it takes the conditions on your water and tells you when the trip is worth it. And because you don’t look for an offshore fish from the shore, StrikeAhead Pathfinder tells you who to go out with: curated guides and skippers, personally vetted before listing — licence and insurance —, with verified catches instead of promo photos, no paid ranking and, for you as an angler, no booking fee.

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