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Bigeye tuna fishing: the tuna that hunts 500 metres down by day — and is missing from the Mediterranean

There are questions on which the sources about a fish agree, and then there is the bigeye tuna. How big it grows, how old it gets, how deep it dives, whether it goes down by day or by night, when it spawns and how threatened it is — on all of this, FishBase and the Wikipedias in fifteen languages give figures that in part rule each other out. On just one point do FishBase and twelve of these Wikipedias say the same thing: it does not live in the Mediterranean. This article is about the bigeye tuna, *Thunnus obesus* — German Großaugen-Thun, Spanish patudo, French thon obèse or thon aux grands yeux, Italian tonno obeso, Portuguese patudo or atum-patudo, Japanese メバチ (mebachi).

The tuna that goes deep by day

The name already tells you what sets it apart: the eyes. According to the English Wikipedia they are well developed, with a large, spherical lens that works even in low light. It needs them, because it hunts where other tunas cannot follow. According to the English Wikipedia it descends at dawn into deeper, cooler water and returns at dusk to warmer, shallower layers. By day it reaches 300 to 500 metres, where the water can be up to 20 °C colder than at the surface. Exactly how deep depends on the ocean: in the eastern Pacific mostly 200 to 350 metres, around Hawaii 300 to 400 metres, in the Coral Sea 300 to 500 metres. The English Wikipedia reads this as the fish — or its prey — tracking an optimum temperature of 10 to 15 °C. In between, it keeps coming back up to warm itself.

That it can stand this is down to its body. Its blood binds oxygen particularly strongly, so that, according to the English Wikipedia, it still copes in water with 1.0 millilitre of oxygen per litre and regularly visits layers below 1.5 millilitres. Heat exchangers in its blood vessels keep its body warmer than the water; at depth it engages them to hold on to heat, and on the way up it disengages them to warm up quickly. The Japanese Wikipedia puts it more briefly: by day it swims deeper than the other tunas, down to 300 metres, and at night it comes up to the surface. The German Wikipedia also uses this depth to explain its flesh: it is very fatty, as heat insulation.

And what does it hunt down there? According to the English Wikipedia, mainly fish, crustaceans and cephalopods of the upper and middle depth zone; its stomachs held many species that themselves rise and sink every day, and it has been seen, by day and by night, in close association with the scattering layer — the layer of small animals that shows up as a cloud on the echo sounder. The Russian Wikipedia names deep-sea and mid-water fish such as swallowers, lancetfishes, barracudinas and snake mackerels as important prey, plus squid, octopus and shrimp. FishBase notes that it feeds by day and by night.

How to recognise it

A large, robust tuna with a fusiform body, a large head and, of course, large eyes (FishBase, English Wikipedia). The Japanese Wikipedia calls it the tuna with the deepest body — stocky; the Italian one describes a barrel shape, because the greatest body depth lies below the middle of the first dorsal fin. According to FishBase, the pectoral fins are only moderately long in large fish over 110 cm (22 to 31 % of fork length), but very long in smaller ones — as long as in the albacore, to which this series devotes a group of its own. The dorsal fins sit close together; the second is followed by 8 to 10, the anal fin by 7 to 10 finlets. The caudal peduncle is very slender and carries a strong keel between two smaller ones. Two features only show when you gut it: the underside of the liver is striated, and it has a swim bladder (FishBase; according to the Russian Wikipedia, the liver is striated from a length of about 30 cm).

The colour: back metallic dark blue, lower flanks and belly whitish; in live fish an iridescent blue band runs along the side. The first dorsal fin is deep yellow, the second dorsal and the anal fin light yellow, the finlets bright yellow edged with black (FishBase). Careful with juveniles: according to the Japanese Wikipedia they are slimmer than the adults, carry a few white vertical bars and resemble young yellowfin tuna. They swim together, too: according to the German Wikipedia, young bigeye often travel in mixed schools with yellowfin tuna and skipjack; the Norwegian and Swedish Wikipedias say the same.

  • Size: FishBase gives 250 cm total length as the maximum, 180 cm fork length as the common length and 210 kg as the heaviest published weight. The angling record, according to the English Wikipedia, stands at 178 kg; according to the Russian Wikipedia that applies to the Atlantic, while in the Pacific it is 197 kg. According to the Japanese Wikipedia, fish from Japanese waters are smaller than tropical ones; over 2 m is rare there.
  • Age and maturity: FishBase gives 17 years as the highest reported age and a length at maturity of 112.5 cm (range 100 to 125 cm).
  • Temperature: FishBase gives 13 to 29 °C, preferably 17 to 22 °C. How often it turns up where is closely tied to surface temperature and the position of the thermocline.

The Mediterranean: the one point everyone agrees on

FishBase states it without qualification: “Absent in the Mediterranean”. The English, German, French, Spanish, Dutch, Finnish, Swedish, Norwegian, Turkish, Russian and Japanese Wikipedias say the same; the Italian one even writes that it is “completely” absent. The Japanese one adds that off Japan, too, it does not move into the Sea of Japan. Otherwise, according to FishBase, it lives in the tropical and subtropical waters of the Atlantic, Indian Ocean and Pacific, between 52° North and 48° South. According to the Japanese Wikipedia, Atlantic and Indo-Pacific fish are genetically distinct. According to the English Wikipedia, it spawns in the northwestern tropical Atlantic in June and July and in the Gulf of Guinea in January and February — the only known nursery area in the Atlantic.

For anglers in the Mediterranean this means: a tuna that takes the hook there is not a bigeye — but, for example, a bluefin tuna or an albacore, to each of which this series devotes a group of its own. If you want to catch a bigeye, you have to go to the Atlantic or further. The French Wikipedia relates that swordfish fishermen off the east coast of the USA sometimes bring one up on their lines — and that captains prize it for its high value. The Russian Wikipedia calls it a popular target of sport fishing, whereas the Dutch one writes that anglers pursue it little.

Drifting objects, schools and the catch

Normally it rises at night and descends by day. According to the English Wikipedia, that changes as soon as it stays around seamounts, buoys or fish aggregating devices (FADs, anchored or drifting rafts under which fish gather): then it stays at the surface, for periods of about 10 to 30 days. Fisheries exploit this: in the western and central Pacific, according to the English Wikipedia, around 27 % of all tuna caught by purse seiners comes from such FADs. According to the German Wikipedia, this mainly affects the juveniles, which hold under the drifting objects together with the schools of other tunas; adult bigeye are drawn to them far less. The Russian Wikipedia: the vast majority of the purse-seine catch consists of immature fish under 10 kg; the big ones over 30 kg come from the longline.

And on the plate? FishBase: the meat is highly prized and processed into sashimi in Japan. According to the Japanese Wikipedia it is red and soft, with less fatty “toro” than bluefin tuna, and is particularly in demand in spring and summer, when bluefin is considered less tasty. Two sources issue a warning: according to the Russian Wikipedia it accumulates more mercury than yellowfin, skipjack and albacore; according to the Japanese Wikipedia, Japan’s health ministry advises pregnant women to eat no more than about 80 grams a week of it.

Eight contradictions, left standing

  • How deep does it go? FishBase: 0 to 1500 m, usually 0 to 500 m. The Swedish Wikipedia: down to 1900 m, usually 500 m. The Finnish one: for long periods below 500 m. The English one: 300 to 500 m by day. The Russian one: from the surface down to 300 m. The Italian, Dutch and Norwegian ones: 0 to 250 m.
  • Does it go deep by day or by night? The English Wikipedia: it descends at dawn and rises at dusk; the Japanese one: deep by day, at the surface at night. The Slovenian Wikipedia: satellite data showed that it dives mainly at night — because its prey withdraws into the depths at night.
  • How big does it get? FishBase and almost all Wikipedias: 250 cm. The Norwegian Wikipedia: 127 cm maximum length. As for weight: FishBase and the German, Italian, Spanish, Portuguese, Russian and Japanese Wikipedias 210 kg; the English one: “probably exceeds 180 kg”, with 178 kg as the angling record.
  • How old does it get? The German Wikipedia: 5 to 7 years, according to some sources 11. The Italian, Portuguese and Dutch ones: 11. The Slovenian one: 10 to 12. The English one: 16. FishBase and the Swedish one: 17. The Russian one: 16 in the western Pacific, 8 in the Indian Ocean, 9 in the Atlantic.
  • When does it mature? The English Wikipedia: at 2 to 4 years, at 135 cm in the eastern Pacific and 102 to 105 cm in the western Pacific. The Russian one: in its second or third year of life. The German one: at 3 to 4. The Slovenian one: in its fifth year of life.
  • How often does it spawn? FishBase: every one to two days, over several months, around the full moon, in the tropics all year round. The Russian Wikipedia: in batches, at intervals of up to a month, 1.5 to 9 million eggs. The Dutch one: twice a year, 2.9 to 6.3 million eggs.
  • How threatened is it? FishBase: “Vulnerable”, assessed on 15 January 2021; the Japanese Wikipedia: classified as such since 1994. The German one: in the Pacific “massively overfished and threatened with extinction”; the Finnish one: in great danger of dying out. The English one draws distinctions: overfished in the western, central and eastern Pacific, close to it in the Atlantic, not overfished in the Indian Ocean. The Russian one: the Indian Ocean commission considers the stock there stable.
  • Is it any good for the can? FishBase: marketed mainly canned or frozen. The German Wikipedia: in South America, young bigeye are processed into canned tuna. The Russian Wikipedia: its light grey flesh makes it unsuitable for canning.

Five names that lead to the wrong fish

  • “Obesus” — the fat one? *Obesus* is Latin for “fat”, and the Italian (tonno obeso) and French names (thon obèse, also thon ventru, “pot-bellied”) carry that over. According to the Italian Wikipedia, though, the name refers to the barrel shape of the body. And fat? The German Wikipedia calls its flesh very fatty, while the Japanese one says it has less fatty “toro” than bluefin tuna.
  • “Albacora”. The Portuguese Wikipedia and GBIF list the bigeye, among other names, as albacora, GBIF also as alvacora and albacora-bandolim. In English, however, albacore is the name of another tuna (*Thunnus alalunga*), to which this series devotes a group of its own. The unambiguous names are patudo and atum-patudo.
  • “Ahi”. In Hawaii, according to the Finnish Wikipedia, it is called ahi — but that is also the name there for the yellowfin tuna, to which this series likewise devotes a group of its own. So if you order “ahi” in Hawaii, you can’t be sure which tuna you will get.
  • “Atún” and “Atum”. GBIF also lists it simply as atún in Spanish and as atum in Portuguese — that is, under the word for tuna in general. The unambiguous names are patudo, atún ojo grande and atún de ojos grandes.
  • “ダルマ” and the young yellowfin. In Japan, according to the Japanese Wikipedia, young bigeye are called ダルマ (daruma) in many places; regionally it also goes by バチ (bachi), メブト (mebuto) or シビ (shibi). The problem is less the names than the juveniles themselves: they resemble young yellowfin tuna and swim with them in the same school.

That leaves the question every article in this series starts with: when. Depth, prey and hunting style are covered above, and they are fixed. What changes every day is everything else — and that is exactly where it is decided whether a day 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 can’t read the thermocline, drifting objects and blue water from day one, 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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