Fish Facts » Porbeagle Shark
Description
The porbeagle has a heavy spindle-shaped body with a moderately long conical snout. The gill slits are large. The caudal peduncle is strongly keeled, with short secondary keels on the caudal base. The caudal fin is crescent-shaped. The large first dorsal fin has a white free rear tip. The second dorsal and anal fins can pivot. The dorsal and lateral surfaces of the porbeagle are dark blue to gray in color. The first dorsal fin is dark with an abruptly white or gray free rear tip. In Northern Hemisphere porbeagles, the ventral surface of the head and abdomen are white, with the color extending dorsally to the rear of the pectoral bases. In some of the adults of the Southern Hemisphere, the ventral surface of the head is dark and the abdomen is white with dark blotches. The white free rear tip on the dorsal fin distinguishes the porbeagle from both the salmon shark and the white shark.
Habitat
Although the porbeagle is predominantly pelagic (open-ocean), it can be found in both coastal and oceanic waters. The species occurs in both hemispheres and is found at water temperatures from 34o to 64oF (1o-18oC). It prefers cold water, but was once recorded in water of 73oF (23oC). It does not appear to enter freshwater, but has been caught in a brackish estuary of Argentina. It can be found at the surface to a depth of 2,346 ft (715 m). Like other large pelagic sharks, the porbeagle undertakes extensive seasonal migrations. In this species, these migrations appear to be largely longitudinal and are probably temperature-related. Porbeagles appear to segregate by both size and sex. Skewed sex ratios are often noted in catches of this species. Also, in catches in the Bristol Channel, large females were notably absent, meaning mature females may separate from mature males and juveniles for some part of the breeding cycle. This phenomenon has been reported for other pelagic sharks, such as the salmon shark, Lamna ditropis (the porbeagle's Pacific congener), and the blue shark, Prionace glauca, and may be advantageous in limiting copulation to a specific season as well as preventing cannibalism of neonates by adult males.
Diet
Porbeagles are opportunistic feeders. In the northwest Atlantic, their diet consists primarily (90%) of teleosts (bony fish). In the spring, these are mostly pelagic fish, like lancetfish, herring, sauries, and mackerels. In the fall, these are mostly groundfish, like sand lances, lumpsuckers, flounders, hakes, and cod. Cephalopods, like squid, are the second most common prey item.
Fishery
In the past, when population abundances were higher, porbeagles were often considered a nuisance because of the damage they often caused to light fishing gear. Today, there are regulated Norwegian and Canadian fisheries for porbeagles in the north Atlantic. In the southern hemisphere, there is a small regulated Norwegian fishery. The porbeagle is also a significant bycatch species, the second most common in the Norwegian fishery. It is also commonly caught as bycatch by Japanese longliners and probably other fisheries in the southern Indian Ocean. The catch in these fisheries is probably used mostly for fins. Harvested porbeagle flesh is used for human consumption. The porbeagle is also used for its oil, to make fishmeal (fertilizer), and its fins are used for shark-fin soup. The porbeagle is a popular gamefish, although less active than its relatives the shortfin mako and the white shark. There are recreational fisheries in the United Kingdom, Ireland, and the United States, and catch-and-release in Canada.
Size
The porbeagle has a maximum total length of about 12 ft (365 cm) and maximum weight of over 500 lbs (230 kg). Maximum age is likely about 30 years. There is considerable variation in estimates for size at maturity for this species. The most recent studies indicate that females in the northern hemisphere mature at around 7.6 to 8.5 feet (232-259 cm) in total length, and females in the southern hemisphere at around 6.1 to 6.6 feet (185-202 cm). Males in the northern hemisphere mature at around 5.4 to 6.8 feet (165-207 cm). In the northwest Atlantic at least, these sizes correspond to a female age at first maturity of about 13 years and a male age at first maturity of about 8 years.
Range
In the northern hemisphere, it is found only in the Atlantic Ocean. This is an important trait that distinguishes it from its close relative, the salmon shark (Lamna ditropis), which only inhabits the northern Pacific Ocean. In the northern Atlantic, the western boundary of the porbeagle's range is the northwest coast of North America, from New Jersey (and possibly South Carolina) to Canada and Greenland. The eastern portion of its north Atlantic range includes the northwest coast of Africa and the Mediterranean, and proceeds up to waters off Iceland to the north coasts of Norway and Sweden and the northwest coast of Russia. In the southern hemisphere, the porbeagle's distribution is circumglobal, in a band from 30o to 60oS. There is apparently little exchange between porbeagle populations. For example, populations of the northwest Atlantic seem relatively segregated from those of the northeast, and populations in the northern hemisphere are separate from those in the southern hemisphere.
Source(s)
Florida Museum of Natural History
Source(s) on the web
http://www.flmnh.ufl.edu/fish/Gallery/Descript/Porbeagle/Porbeagle.html