The porbeagle (Lamna nasus) is a medium-sized species of lamnid shark, typically reaching 2.5 meters in length. It is closely related to the salmon shark, its north Pacific ecological equivalent and sister species, and similarly built - with a powerful, stout and fusiform-shaped body, pointed snout, large crescent-shaped caudal fin and strongly keeled caudle peduncle. These adaptations, along with its endothermic physiology and elevated metabolism, facilitate the porbeagle in its highly active and fast-swimming lifestyle. Together with the salmon shark, the porbeagle has the highest capacity for elevating its body temperature among sharks, capable of maintaining core temperatures 10°C warmer than that of the surrounding water. The colouration of the shark is countershaded, with a medium to dark grey dorsal area and a white or light grey underside.
The porbeagle occupies a wide distribution across temperate and cold-temperate waters, typically residing in water temperatures of between 1°C - 18°C. It is both oceanic and coastal, and can be found across the entire epipelagic and mesopelagic water columns (0 - 1,000 meters). Their distribution is amphitemperate and split among the North Atlantic and Southern hemisphere populations; the North Atlantic population occurs predominantly in a band between 30°N - 70°N while the Southern hemisphere populations occur circumglobally between 30°S - 50°S. In the North Pacific, its niche is assumed by the closely related salmon shark.
The porbeagle segregates by size (age) and sex, and local populations tend to be sexually disproportionate. Populations off the coast of Scotland have females outnumbering males four to three while males outnumber females two to one in coastal Spanish populations. Seasonal migrations have been observed in porbeagles from both hemispheres, with extensive migrations of up to 1,500 kilometers being recorded between Newfoundland, Canada and Massachusetts, USA, migrations of over 2000 kilometers being recorded across the Northeast Atlantic from the United Kingdom to Norway, Denmark and Spain, and a migration of over 4,000 kilometers being recorded for one individual from Ireland to Canada - suggesting that mixing does occasionally occur between Northeast and Northwest populations. Little is known about Southern hemisphere migration patterns for porbeagles, other than that they are distinct and separate from the Northern hemisphere populations.
The porbeagle is a late maturing and lowly reproductive species. Mature females are believed to reproduce yearly with typical litter sizes of four, and have a gestation period of around eight to nine months (porbeagles are aplacental viviparous). Following birth, both sexes grow at similar rates until the onset of maturation, which in females occur at an age of 12 - 18 years and in males around an age of 6 - 11 years. The late maturation rate, low fecundity and the high value of its meat and fins have contributed to heavy declines in porbeagle numbers. The sharp collapse of porbeagle stocks in the North Atlantic is a typical example of the “boom and bust” pattern of most shark fisheries, and reflective of its susceptibility and vulnerability to overfishing and exploitation. The IUCN has assessed the porbeagle as Endangered in the Northwest Atlantic, Critically Endangered in the Northeast Atlantic and Mediterranean, and Vulnerable elsewhere. It is currently listed on Annex I of UNCLOS, Annex I of the CMS Migratory Shark Memorandum of Understanding and Appendix II of CMS.