Porbeagle - Distribution and Habitat

Distribution and Habitat

The porbeagle has an almost global amphitemperate distribution, i.e. it is absent from the tropics; in the North Pacific, its niche is assumed by the salmon shark. It is found mostly within 30–70°N and 30–50°S latitudes. In the North Atlantic, the northern limit of its range extends from the Newfoundland Grand Banks off Canada, through southern Greenland, to Scandinavia and Russia; the southern limit of its range extends from New Jersey and Bermuda, through the Azores and Madeira, to Morocco. It is found in the Mediterranean Sea, but not the Black Sea. Normally, North Atlantic sharks only stray as far south as South Carolina and the Gulf of Guinea, but pregnant females from the western North Atlantic population are known to range into the Sargasso Sea, almost as far as Hispaniola, to give birth. In the Southern Hemisphere, the porbeagle apparently occupies a continuous band bound in the south by the Antarctic Convergence, and extending as far north as Chile and Brazil, the Western Cape province of South Africa, Australia to southern Western Australia and southern Queensland, and New Zealand. It is speculated that the porbeagle colonized the Southern Hemisphere during the Quaternary glaciation (beginning c. 2.6 Ma), when the tropical climate zone was much narrower than it is today.

Offshore fishing banks are the favored habitat of the porbeagle, though it can be found anywhere from a depth of 1,360 m (4,460 ft) in oceanic basins to littoral (close to shore) waters less than 1 m (3.3 ft) deep, over the entire water column. There is a single, anomalous record from brackish water, of a juvenile in Mar Chiquita in Argentina. A tracking study off the British Isles has found substantial variation in the short-term movements of this species, both between and within individuals. Vertical movements tended to increase with water depth and corresponding temperature stratification: in shallow, unstratified waters, sharks either showed no pattern in changing depth or made reverse diel movements, spending the day in shallow water and descending at night. In deeper, stratified waters, the sharks performed a regular diel migration, spending the day below the thermocline and rising towards the surface at night. The porbeagle favors water temperatures of 5 to 10 °C (41 to 50 °F), though it has been reported across a temperature range of 1 to 23 °C (34 to 73 °F).

Porbeagle populations in the Northern and Southern Hemispheres appear to be completely separate. There are two stocks in the North Atlantic, east and west, that seldom mix; only one individual is known to have crossed the Atlantic, covering 4,260 km (2,650 mi) from Ireland to Canada. Several discrete stocks are likely present in the Southern Hemisphere as well. This species segregates by size and sex in the North Atlantic, and at least by size in the South Pacific. For example, males outnumber females 2:1 off Spain, females are 30% more numerous than males off Scotland, and immature males are predominant in the Bristol Channel. Older, larger sharks may frequent higher latitudes than younger individuals.

Seasonal migrations have been observed in porbeagles from both hemispheres. In the western North Atlantic, much of the population spends the spring in the deep waters of the Nova Scotia continental shelf, and migrates north a distance of 500–1,000 km (310–620 mi) to spend late summer and fall in the shallow waters of the Newfoundland Grand Banks and the Gulf of St. Lawrence. In December, large mature females migrate south over 2,000 km (1,200 mi) into the Sargasso Sea for pupping, keeping deeper than 600 m (2,000 ft) during the day and 200 m (660 ft) at night so as to stay in the cooler waters beneath the Gulf Stream. In the eastern North Atlantic, porbeagles are believed to spend spring and summer in shallow continental shelf waters, and disperse northwards to overwinter in deeper waters offshore. Migrating sharks may travel upwards of 2,300 km (1,400 mi), though once they reach their destination they tend to remain within a relatively localized area. In the South Pacific, the population shifts north past 30°S latitude into subtropical waters in winter and spring, and retreats south past 35°S latitude in summer, when sharks are frequently sighted off subantarctic islands.