Lamna nasus
Beaumaris shark
(Also: Blue dog; Mackerel shark; Porbeagle; Porbeagle shark)

Porbeagles (Lamna nasus) can be found in the northern hemisphere of the Atlantic Ocean, from South Carolina in the United States to as far north as the southern edge of Svalbard, Norway. Porbeagles are also found in the Mediterranean Sea and from Morocco to the arctic waters off the northwestern coast Russia. In the southern Hemisphere, they can be found in a band between 30 and 60 degrees south, primarily along the southern, eastern and western coasts of South America, Africa, and Australia. (Leonard, 2011; Roman, 2010)

Porbeagles, a species of mackerel shark, are primarily found in the pelagic and littoral zones and prefer cold, offshore fishing banks. They seldom come inshore and are commonly found swimming along Reykjanes Ridge in the North Atlantic. They have also been found in brackish waters off the coast of Argentina. They often remain in deeper waters during winter and occasionally come inshore during summer. Although they have been documented in waters as deep as 1360 m, average depth of porbeagle inhabited waters is 715 m. Preferred water temperatures range from 1°C to 18°C. (Leonard, 2011; Martin, 2003; Roman, 2010)

While the dorsal and lateral surface of porbeagles is blue-gray, the venter is typically white. Pups may have dark patches on the ventral surface of the pectoral fins and near the underside of the gill slits. The tip of the dorsal fin is completely blue-gray, which differentiates them from salmon sharks and white sharks. Porbeagles have heavy fusiform bodies with large gill slits. They have crescent-shaped tails, long conical snouts and the anal and second dorsal fins can pivot. Their teeth have lateral cusplets and are large and bladelike with smooth edges, unlike those of white sharks, which are often serrated. Like most sharks, adult porbeagles have dermal denticles, however, pups often lack them. Their maximum length and weight are approximately 365 cm and 230 kg, respectively, females are slightly larger than males. Female in the northern hemisphere range from 232 to 259 cm in length, and females in the southern hemisphere range from 185 to 202 cm in length. Males do not exhibit geographic variation in size and range from 165 to 207 cm in length. Pups range from 60 to 75 cm in length at birth. The average weight of porbeagles is 135 kg. Unlike most fish, porbeagles are endothermic. (Martin, 2003; Roman, 2010)

Porbeagle embryos are oophagous, feeding on eggs produced by the ovaries. Females retain eggs in a brooding chamber, where embryos develop while receiving nourishment from the yolk sac. Embryos develop in egg cases, also known as mermaid's purses. Once development is complete, young hatch while inside the mother's uterus and parturition is complete. Gestation lasts 8 to 9 months. Upon birth, pups are 60 to 75 cm long and weigh about 9 kg. Most males become reproductively mature by 1.5 to 2.0 m in length and 130 kg in mass, or by 8 years of age. Females reach maturity by 2.0 to 2.5 m in length and 200 kg in mass, or by 13 years of age. ("Reproduction, embryonic development, and growth of the porbeagle shark, Lamna nasus, in the southwest Pacific Ocean", 2000; Stevens, et al., 2011)

Porbeagles are polygynandrous and mate from September to November. There are two mating grounds for western North Atlantic populations; one of which is off the coast of Newfoundland and the other is on Georges Bank in the Gulf of Maine. Prior to mating, males often bite their mate's pectoral fins and flanks in order to remain stable during copulation. Porbeagles breed once a year. (Martin, 2003)

Porbeagles are ovoviviparous (retaining eggs within the body until they hatch) and breed from September to November. Although North Atlantic populations breed throughout the eastern and western regions of their habitat, most copulation sightings have occurred along the north eastern coast of the United States, in the Gulf of Maine and off the coast of Newfoundland, Canada. Porbeagles are oophagous, as developing embryos feed on fertilized eggs to get nutrients once the yolk sac is depleted. Females give birth to between 1 and 5 pups during late winter and spring, with an average of 4 pups per litter. Pups are usually 60 to 75 cm long at birth and do not exceed 5 kg. Gestation lasts 8 to 9 months. Most males are reproductively mature by age 8, and most females are reproductively mature by age 13. (Campana, 2003; Roman, 2010)

There is no information regarding parental care in porbeagles. In general, however, shark mothers provide a great deal of care to their pups. Sharks mature more slowly than other many other vertebrates, which increases demand for parental investment if young are to survive to reproductive maturity. Parental care by male sharks is uncommon and has not been documented in porbeagles. (Roman, 2010)

Porbeagles live between 25 and 46 years in the wild. (Martin, 2003)

Porbeagles have been documented as being both solitary and gregarious. They are very active, and occasionally exhibit playful behavior with conspecifics or as individuals. Porbeagles often ram floating objects with their snout and chase one another during play. They have been sighted jumping completely out of the water while pursuing prey, a behavior only seen in a few sharks. (Humphries, et al., 2009; Martin, 2003; Roman, 2010)

There is no information available concerning the average home-range size of porbeagles. (Campana and Joyce, 2003)

Male porbeagles communicate with females by biting their pectoral fins, letting the female know he wants to mate. Porbeagles are one of a few shark species that exhibit playful behavior. They have been observed alone, and in groups, playing with floating objects. Like other sharks, porbeagles possess Ampullae of Lorenzini, a series of electroreceptors that allows sharks to detect electrical fields. This electrosensory system likely helps them detect electrical pulses created via muscle contractions of prey and has been hypothesized to help them detect minor changes in temperature and pressure in the local environment. Porbeagles also perceive their local environment via the lateral line system common in most fish. The lateral line system helps them detect movement and vibrations in the surrounding environment. In conjunction with the Ampullae of Lorenzini, the lateral line system has helped sharks become extremely efficient and adept predators. (Martin, 2003)

Porbeagles prey upon a variety of pelagic fishes including lancetfish, herring, sardines or pilchards, sauries, and mackerel. Secondary prey includes cusk, cod, redfish (i.e., Sebastes, Lutjanus, Trachichthyidae and Berycidae), haddock, squid, lumpsuckers, sandlances, flounders (Achiridae and Bothidae), and shellfish. Occasionally they may feed on other sharks such as tope and spiny dogfish. Porbeagles have also been documented consuming sea snails and sea urchins, which suggests they may forage at the bottom of the water column. Porbeagles have not been documented eating marine mammals. About 90% of the diet of porbeagles resident to the northwest Atlantic consists of bony fishes. Although they are considered to be opportunistic feeders, porbeagles prefer pelagic fish in the spring and groundfish in the fall. (Campana, 2003; Martin, 2003; Roman, 2010)

There is no information available regarding predators specific to porbeagles. White sharks and orcas have been suggested as possible predators, however, there is no evidence to support this idea. (Leonard, 2011)

Porbeagles are important primary and secondary consumers of a number of marine animals and are considered a keystone species in their environment. As a result, porbeagles help maintain and regulate the abundance and distribution of numerous species, and thus help mediate the balance of their ecosystem. Little else is know of the potential ecosystem roles filled by porbeagles. (Martin, 2003)

Porbeagles are commonly sold in fish markets across the globe. Specifically, their fins are used for shark-fin soup. Porbeagles are also used for liver oil and to make fishmeal. In the 1960's, fisheries caught over nine million pounds of porbeagle annually. Porbeagles are also considered an important game fish throughout their geographic range. (Martin, 2003; Roman, 2010)

The porbeagle Lamna nasus is a relatively large species and could be concidered dangerous as a result. The International Shark Attack File (ISAF) has only three probeagle attacks on record, however, these reports are not entirely reliable as porbeagles are often misidentified as white sharks. (Roman, 2010)

Porbeagles are classified as vulnerable on the IUCN's Red List of Threatened Species. The blogal population of porbeagles is currently in decline and major threats vary by location. From 1998 to 1999, only 15 specimens were caught in the Mediterranean Sea as a result of by-catch and it appears that by-catch is their biggest threat in this region. In the northwest Atlantic, overfishing has lead to their decline, and in the southern oceans, it appears that by-catch on longlines has lead to significant declines throughout the southern limits of their geographic range. (Stevens, et al., 2011)