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Thresher shark fishing: the shark that hunts with its tail

On 14 April 1923 the researcher W. E. Allen stood on the pier of the Scripps Institution in California and watched a thresher about 2 m long stun a small fish with a single stroke of its tail fin — so the English Wikipedia reports. That fin is its trademark: according to FishBase, its upper lobe is about as long as the rest of the shark, or longer. With it, FishBase writes, it herds and stuns its prey. This article is about the common thresher, *Alopias vulpinus* — in German Gemeiner Fuchshai or Fuchshai, in Italian squalo volpe or pesce volpe, in Spanish pez zorro or tiburón zorro, in French requin-renard commun, in Portuguese tubarão-raposo, in Dutch voshaai, in Danish rævehaj, in Swedish rävhaj, in Norwegian revehai, in Finnish kettuhai, in Russian обыкновенная морская лисица, in Turkish saban balığı, in Greek κοινός αλωπίας, in Japanese マオナガ (maonaga), in Arabic قرش دراس شائع, and according to GBIF in Croatian lisica, in Slovenian morska lisica and in Albanian peshkaqeni dhelpër. According to the English Wikipedia, the International Game Fish Association (IGFA) lists it as a game fish — and that is exactly why this is about recognising it and releasing it.

The shark the Greeks called a fox

According to the German and English Wikipedia, it was described by Pierre Joseph Bonnaterre in 1788, as *Squalus vulpinus* — according to the German Wikipedia, from an animal from the Mediterranean. In 1810 Constantine Rafinesque described a thresher caught off Sicily all over again, as *Alopias macrourus*; under the rules of nomenclature, the older species name stands. The name is ancient: according to FishBase, *Alopias* comes from the Greek alopex, fox, and *vulpinus* is Latin for fox-like. What it alluded to was proverbial cunning: according to FishBase, people in antiquity believed the thresher would swallow a bait, hook and all, until it reached the line, then bite through it and escape. According to the English Wikipedia, Aristotle already wrote that hooked threshers could bite through fishing lines and that they protected their young by swallowing them — and science, the English Wikipedia says, has not confirmed either claim. According to the English Wikipedia it is one of three living thresher species, alongside the bigeye thresher (*Alopias superciliosus*) and the pelagic thresher (*Alopias pelagicus*).

How to recognise it

According to FishBase, a large thresher with relatively small eyes, curved, narrow-tipped pectoral fins and a conspicuous white patch above the pectoral-fin bases: there, the white of the belly reaches upwards. The upper lobe of the tail fin is very long and strap-like, the lower one short but well developed. The back and the underside of the snout are brown, grey, blue-grey or blackish, the flanks lighter, the belly a sharply demarcated white; the pectoral, pelvic and dorsal fins are blackish, sometimes with white dots at the tips.

The white patch is your best clue. According to the English Wikipedia, in the common thresher the white of the belly extends as a band over the bases of the pectoral and pelvic fins, whereas the fins of the pelagic thresher are uniformly coloured; the Japanese Wikipedia names exactly this patch to tell it apart from the very similar ニタリ (nitari, *Alopias pelagicus*). The bigeye thresher gives itself away by its name: FishBase explicitly lists the common thresher’s small eyes as a feature. And there is something else in the English Wikipedia: its red muscle is on average about 2 °C warmer than the water around it — it keeps heat in its body with a heat exchanger, a rete mirabile.

  • Size: FishBase gives 573 cm total length as the maximum, 450 cm as the common length and 348 kg as the heaviest published weight; according to the same source it matures at around 351 cm (226 to 400 cm). Almost half of that length is tail.
  • Young: It is viviparous; once their yolk sac is used up, the embryos, according to FishBase, feed on eggs that the mother keeps producing. According to FishBase, 2 to 4 pups are born, usually 2, each 114 to 150 cm long. According to the English Wikipedia, newborns weigh 5 to 6 kg and gestation lasts nine months.
  • Growth: According to the English Wikipedia, young threshers grow about 50 cm a year, adults only about 10 cm.

How it hunts — and what it eats

According to the English Wikipedia, around 97 % of its diet consists of bony fishes, mostly small schooling fish: sardines, anchovies, mackerel, hakes, bluefish, herring, needlefish and lanternfish. FishBase also lists squid, octopus, pelagic crustaceans and, rarely, seabirds. The German Wikipedia describes how it circles schools of fish and strikes into them with the upper tail lobe to kill or stun fish; according to the English Wikipedia, threshers often hunt in pairs or small groups, slapping the water to drive the school more tightly together.

Not every story holds up. According to the English Wikipedia, Harry Blake-Knox reported that in the winter of 1865, in Dublin Bay, he saw a thresher kill a wounded loon with its tail and swallow it; other experts doubted this, because the tail is not stiff enough for that. For angling, another sentence matters: because the thresher strikes with its tail, the hook often ends up in the tail rather than in the mouth — so write both the English and the German Wikipedia.

Where it lives — and the Adriatic nursery

FishBase: in temperate and tropical seas around the world, between 74° North and 58° South, over continental and island shelves as well as far out in the open ocean — but most common close to land; young animals are often found close inshore and in shallow bays. According to the English Wikipedia, in the eastern Atlantic it ranges from the North Sea and the British Isles to Ghana, including Madeira, the Azores, the Mediterranean and the Black Sea. It follows warm water into higher latitudes; according to the German Wikipedia, European animals migrate north in summer as far as the North Sea and the Skagerrak. According to the English Wikipedia it prefers 16 to 21 °C, occasionally going down to about 9 °C.

In the Mediterranean, the German Wikipedia calls it a common species, especially in the Gulf of Lion and off Spain, where young are born regularly; the Italian speaks of a nursery off the Spanish coast. A study by Licia Finotto, Alberto Barausse and Carlotta Mazzoldi (*Acta Adriatica*, 2016) looked at the northern Adriatic: from 1997 to 2013, an average of 14 threshers a year were landed in the port of Chioggia, mostly between July and October — more than 75 % of them not yet sexually mature, some weighing under 10 kg. The authors see this as a sign of a nursery in the northern Adriatic, and they link the sharks’ appearance to the abundance of their prey, the sardine.

Vulnerable — and what that means on the line

On the hook, according to the English Wikipedia, it is considered — together with the shortfin mako, to which this series devotes a group of its own — one of the strongest fighters among sharks; according to the German Wikipedia it is popular in sport fishing for its leaps out of the water. According to FishBase it is harmless to humans; a few attacks on boats are attributed to it only doubtfully, even though the size of adult animals demands respect.

The reverse view matters more. The IUCN lists it worldwide as Vulnerable (assessed on 6 November 2018, FishBase); it is listed in Appendix II of the CITES convention and of the Bonn Convention (CMS). FishBase rates its resilience as low: a population needs 4.5 to 14 years to double; FishBase rates its vulnerability to fishing as very high (77 out of 100). Off California, according to the English Wikipedia, the stock declined by more than 50 % within three generations; for the Mediterranean, the 2016 Adriatic study describes a marked decline. Which rules apply on your water today is something you check yourself before the trip — with the harbour office, the fisheries authority, the guide. Anyone who hooks a thresher releases it as gently and as quickly as possible in the water.

Seven contradictions, left standing

  • How big does it get? FishBase: at most 573 cm, commonly 450 cm. The English Wikipedia: usually 5 m, confirmed record 5.7 m, possibly 6.1 to 6.5 m. The German: on average 3 to 4 m — and the largest thresher ever caught is said to have measured 7.60 m; the Italian gives the same figure.
  • How heavy? FishBase: 348 kg as the heaviest published weight. The German Wikipedia: 340 kg. The English and the Italian: 510 kg, a female 4.8 m long — plus unconfirmed reports of up to 900 kg (English Wikipedia).
  • How many young? FishBase: 2 to 4, usually 2. The English Wikipedia: in the eastern Pacific 2 to 4 (rarely 6), in the eastern Atlantic 3 to 7 — and in its introduction “typically four”. The German: 1 to 6, on average 4.
  • How old does it get? FishBase gives 25 years as the highest reported age — and on the same page works with a maximum age of 19 years. The English Wikipedia: at least 15 years, estimated up to 45 to 50. The German: 15 to 19 years.
  • How deep? FishBase: 0 to 650 m, usually down to 200 m — and elsewhere: pelagic at 1 to 366 m. The English and the Japanese Wikipedia: down to at least and to about 550 m respectively. The German: over 350 m.
  • When does it mature? FishBase: at around 351 cm (226 to 400 cm). The English Wikipedia (northeastern Pacific): males at 3.3 m, females at 2.6 to 4.5 m. The Japanese: at about 260 cm.
  • Common or rare in the Mediterranean? The German Wikipedia: a common species. The Italian: fairly common. The 2016 Adriatic study: a marked decline in abundance in the Mediterranean. We could not directly consult a separate IUCN assessment for the Mediterranean — so none is given here.

Four names that lead to the wrong fish

  • “Pesce spada”, “pez espada”, “poisson-épée”. GBIF also lists these names regionally for the thresher — otherwise they mean the swordfish (*Xiphias gladius*), to which this series devotes a group of its own. The sabre shape of the tail is also in the Croatian sabljorep, “sabre tail” (GBIF).
  • “Thon blanc” and “loup de mer”. GBIF also lists both French names for the thresher. In France, “thon blanc” otherwise means the albacore (*Thunnus alalunga*), and “loup de mer” the European seabass — both have their own articles in this series.
  • “Морская лисица”. In Russian the thresher is called обыкновенная морская лисица. But if it only says “морская лисица”, the Russian Wikipedia takes that to mean the thornback ray (*Raja clavata*) first.
  • “Onagazame” — not just this shark. In Japanese the common thresher is called マオナガ (maonaga). According to the Japanese Wikipedia, オナガザメ (onagazame) is the name of the whole family and genus — just as “thresher” in English can also mean the bigeye or the pelagic thresher. If you want to be sure, check the scientific name.

That leaves the question every article in this series starts with: when. Habitat, prey and habits are set out above, and they are fixed. Everything else changes every day — and that is exactly where it is decided whether a day on the water is worth it. That is what StrikeAhead works out: it takes the conditions on your water and tells you when the trip is worth it. And because you cannot read sardine shoals, current edges and the open blue from day one — and because a thresher hooked by the tail that is to be released needs experienced hands —, 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. The app tells you when, Pathfinder tells you who with.