Hairtail

Trichiurus lepturus, the largehead hairtail, is a long ribbon-shaped fish of the family Trichiuridae ^[1]. Cutlassfishes of the group are bentho-pelagic and move up toward the surface ^[2]. This species is recorded mostly along tropical and temperate coasts of the Atlantic, Pacific and Indian Oceans ^[2]. Cutlassfishes as a group yield more than 1.2 million tonnes a year worldwide, which makes them an important fishing resource ^[2]. Its global category on the IUCN Red List is Least Concern ^[3].

It is a very elongated, strongly flattened fish shaped like a ribbon that tapers to a fine point, and the tip is often broken ^[4]. The head is large, with ridges above the eyes, and the mouth is large and armed with fang-like teeth ^[4]. It has no scales, no pelvic fins and no tail fin; the dorsal fin is long and continuous, and the anal fin is reduced to 2 spines ^[4]. Live fish are steel blue with silvery reflections, and shortly after death the colour fades to a uniform silvery grey ^[4].

Adults commonly measure 50 to 100 cm and the maximum is about 200 cm ^[4]. In a sample of 104 fish from the tropical eastern Atlantic, with a mean total length of 60.3 cm, the length from the snout to the anus was close to one third of the total length ^[5].

Similar species

• Trichiurus nanhaiensis: For a long time it was misidentified as Trichiurus lepturus because the two look alike; when fresh, its dorsal fin is yellowish green, and DNA barcoding allows unambiguous species assignment ^[2].
• Trichiurus japonicus: It was once treated as a subspecies of Trichiurus lepturus in the north-western Pacific, but morphometric and genetic data show that the 2 are distinct species ^[2].

It is recorded along tropical and temperate coasts of the Atlantic, Pacific and Indian Oceans ^[2]. A genetic study of the 16S gene found 4 groups across the world: Indo-West Pacific, south-eastern Atlantic, western Atlantic and north-eastern Pacific, with 95.17 % of the genetic variance lying among them ^[2]. The authors suggest that the rise of the Isthmus of Panama, 3.5 to 3 million years ago, could have separated the western Atlantic group from the eastern Pacific one ^[2].

It is a bentho-pelagic fish: it lives over the continental shelf and slope down to at least 350 m, and sometimes enters shallow coastal water over muddy bottoms ^[4]. In the western Atlantic it ranges from Cape Cod (40° N) to the La Plata River (37° S), and larvae are usually found where surface water is warmer than 21 °C ^[6]. Off southern Brazil, juveniles live in coastal waters, while subadults and adults range from the coast to the edge of the shelf, often in warm water and around thermal fronts ^[6].

Off southern Brazil, the length at which half the fish were mature was estimated at 63.9 cm in males and 69.3 cm in females; all males over 105 cm and all females over 110 cm were mature ^[6]. It appears to spawn several times; batch fecundity ranged from 3,917 eggs in a 70 cm female to about 154,000 in a 141 cm female, although the number of batches per season was not established ^[6].

On the shelf (10 to 100 m deep) the cycle has 3 phases: gonads develop from July to October, spawning runs from November to February, and fish recover from March to June ^[6]. At the shelf edge, spawning takes place at least in autumn and winter ^[6]. Females seem to stay on the shelf in the cold season to feed, while adult males move northward to warmer water or offshore ^[6].

Off north-western Africa, an analysis of 103 stomachs showed that small pelagic crustaceans called mysids were in 92.2 % of them and made up 99.7 % of the prey items by number ^[5]. Fish were found in 52.4 % of the stomachs and were 53.2 % of the prey weight, and 1.9 % of the stomachs held other cutlassfishes, a sign of cannibalism ^[5]. The diet shifts with size, from krill to fish, and larger fish carry more parasites that mature in fish-eating hosts ^[5].

It feeds on a wide range of prey sizes, both pelagic and benthic, and in southern Brazil it is absent from cold water but abundant in the front where cold and warm waters meet ^[6]. Cutlassfishes are bentho-pelagic and perform a daily vertical migration between the seabed and the surface, reported to follow their prey ^[2].

Its global category on the IUCN Red List is Least Concern ^[3].

Even so, global and regional cutlassfish catches have fallen sharply, probably as a result of long-term fishing ^[2]. In each of 4 species studied in the north-western Pacific, including this one, the genetic data point to a single well-mixed stock, which would be 1 management unit ^[2]. Worldwide, in contrast, 4 genetic groups exist, and the authors call for more samples from the coasts of the Americas to understand them ^[2].

Not medical advice. For a sting, a poisoning or an allergy, seek medical care.

Its parasites include nematodes with zoonotic potential, such as Anisakis pegreffii, which can cause anisakiasis, with digestive or allergic symptoms in people ^[5]. A study cited there found Anisakis pegreffii in 84.4 % of adults from Chinese waters, and in Brazil the prevalence of anisakid larvae ranged from 73.3 to 100 % through the seasons ^[5].

The risk of swallowing a living larva from a properly gutted fish is judged to be rather small, because the body cavity covers only about one third of the body length and larvae are mostly in the belly flaps, which are not usually eaten ^[5].

Cutlassfishes yield more than 1.2 million tonnes a year worldwide, and they are especially popular in the coastal fisheries of China, Japan, Korea and Taiwan ^[2]. Of the species of this group in the north-western Pacific, only 3, among them Trichiurus lepturus, have commercial value, since the others have little flesh ^[2].

In the Mediterranean region it is taken in semi-industrial fisheries in Spain and by small-scale and recreational fishers, with sliding seines, trawls and bottom longlines ^[4].