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Characteristics, distribution and ecology
Taxonomische classification
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Author: (Bonnaterre, 1788)

Field Marks:
Heavy spindle-shaped body, moderately long conical snout, moderately large bladelike teeth with lateral cusplets, long gill slits, large first dorsal fin with light free rear tip, minute, pivoting second dorsal and anal fins, strong keels on caudal peduncle, short secondary keels on caudal base, crescentic caudal fin, ventral surface of body white.

Diagnostic Features:
Snout moderately long,length about 2 times in distance from eyes to first gill opening; first upper lateral teeth with cusps erect or nearly so. Colour: first dorsal fin with a conspicuous white rear tip; ventral surface of body white without dusky blotches.

Geographical Distribution:
Coastal and oceanic, amphitemperate. Western Atlantic: Newfoundland and Gulf of St. Lawrence to New Jerscy and ? South Carolina (USA); ? southern Brazil to southern Argentina. Eastern Atlantic: Iceland and western Barents Sea to Gibraltar, Mediterranean, Morocco, Madeira, western Cape Province, South Africa. South central Indian Ocean. Western South Pacific: Australia (Western and South Australia, Tasmania), New Zealand. Eastern South Pacific: Chile. Subantarctic waters off South Georgia and Kerguelen Islands.

Habitat and Biology:
A common littoral and epipelagic shark, most abundant on the continental offshore fishing banks but also found far from land in ocean basins and occasionally close inshore. This shark prefers cold water, less than 18UC, and does not occur in equatorial seas. The porbeagle is described as active and strong-swimming in pursuit of prey, but when hooked is relatively sluggish and inactive in comparison to the shortfin mako (Isurus oxyrinchus), and does not engage in spectacular leaps like that species.

The porbeagle is found at the surface down to the bottom, singly and in schools and feeding aggregations, and has been caught at depths down to at least 366 m. Porbeagles may come inshore and to the surface in summer, but will stay in winter offshore and beneath the surface. Fisheries catches in Europe indicate that the porbeagle has populational segregation by size (age) and sex.

Porbeagles breed on both sides of the North Atlantic, off the Atlantic coast of Europe and the British Isles, where females have embryos duringmost of the year except July through September, and off North America from Massachusetts to Maine, where females can be found with young at all times of year. Young are apparently born in the spring off Europe and late summer off North America. Mating in European waters occurs in late summer, and breeding there probably occurs every year. Breeding populations presumably exist elsewhere in the range of this species, but details are lacking.

The porbeagle is ovoviviparous and a uterine cannibal, with litters of 1 to 5 young. The fetuses grow enormously by feeding on fertilized eggs, and develop grotesquely expanded abdomens and branchial regions. In European waters the gestation period has been estimated at about 8 months.

The porbeagle may take 5 or more years to reach maturity, and can live to an age of 20 to 30 or more years. Vertebral rings on this species have been demonstrated to be annual, from correlation with length-frequency data. Prior to the intensive fishery that greatly reduced the numbers of this shark in European waters, the annual mortality for the species was an estimated 18% under low human exploitation and probably minimal predation pressure from other species.

This shark is a voracious feeder on small pelagic schooling fishes, including mackerels, pilchards and herring, various gadoids including cod, hakes, haddock, cusk, and whiting, and John dories, dogfishies and tope sharks (Squalus and Galeorhinus), and squids. It is regarded as potentially dangerous to people because of its size and activity, but has never or very seldom has been indicted in an attack on people or boats (unlike its close relatives the shortfin mako and great white sharks). An unconfirmed attack by this species as 'mackerel shark' has been reported, but it could have resulted from confusion with the great white shark.

Size:
Maximum total length 300+ cm, possibly to 370 cm but most smaller, adult males at 219 to 262 cm, adult females 152 to at least 219 cm and possibly to 370 cm; size at birth between 60 and 75 cm.

Interest to Fisheries:
This species has been heavily fished and utilized in the North Atlantic and Ehe Mediterranean by a number of countries, including Norway, Denmark, the Faroer Islands, the United Kingdom, France, previously Spain, with an estimated total of 1530 metric tons landed in 1981 (FAO Yearbook of Fishery Statistics, 1983). A considerable fishery by Japanese longliners exists in the southern central Indian Ocean. It is used fresh and dried salted for human consumption; for oil and fishmeal; and for fins for shark-fin soup. The species is primarily caught with pelagic longlines; also pelagic and bottom trawls, handlines and gillnets. Stocks in the North Atlantic show signs of serious over-fishing in the form of greatly declining catches (3226 metric tons in 1978, FAO Yearbook of Fishery Statistics, 1983).

Type material:
Holotype: Unknown. Type Locality: Probably British waters.

Porbeagle (Lamna nasus)