Blue shark

Prionace glauca

Blue shark (Prionace glauca) tagged off Southern California. Image credit Walter Heim.

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Distribution: Epipelagic, oceanic and circumglobal in warm seas worldwide; sometimes occurring near the coast where the shelf narrows or is cut by submarine canyons close to shore. This is probably the most wide-ranging of all sharks, being found throughout tropical and temperate seas from 60°N to 50°S latitude. In the Pacific, it is present in greatest abundance between 20°N and 50°N, where it shows strong fluctuations in seasonal abundance related to population shifts northward in summer and southward in winter. There is considerable sexual segregation in populations with females more abundant at higher latitudes than males.

In the North Pacific, seasonal migrations occur between latitudes 20°N and 50°N, with northward movements extending into the Gulf of Alaska as waters warm during the summer months and southward movements occurring during the winter months. In U.S. West Coast waters, mature females are thought to start their northward journey in early spring as warm water moves northward, while juveniles of both sexes follow closely; large males start later and tend to stay further offshore. Blue sharks tagged off southern California have been recaptured to the south off Baja California and Acapulco, Mexico; northward to off Oregon, and westward to off the Hawaiian Islands and Midway in the central Pacific, indicating a wide ranging stock that may overlap with the population fished by longliners in the central Pacific Ocean. The species is known to undertake extensive trans-oceanic migrations in the Atlantic.

Growth and Development: Viviparous; bearing live young, fully formed. While in-utero the young are nourished by a yolk sac, then from the bloodstream by a maternal placental connection. Litters average about 30, with maximum litter size reported at 135; gestation period is about 9-12 months. Off California, parturition reportedly occurs in early spring, and mating occurs during late spring to early winter. The pattern of young pups in the observed California-Oregon drift net catch suggest the nursery habitat may extend northward to off the Columbia River mouth, and primarily offshore of the 100 fm isobath. Maximum reported size is 396 cm TL (~13 ft) but blue sharks taken off the U.S. West Coast average much smaller are seldom over 260 cm TL or 8.5 ft. For Pacific blue shark, size and age at 50% maturity in males is 203 cm TL or 6.6 ft. and 4-5 years old, and in females at 186 cm TL or 6.1 ft and 5-6 years old. Maximum age is estimated to be at least 20 years.

Feeding: In coastal waters off the U.S. West Coast, blue sharks reportedly feed on anchovy, mackerel, hake, dogfish, squid and pelagic crustaceans including euphausiids. Elsewhere also known to feed also on small sharks and seabirds. May feed more actively at night, with highest activity in the early evening.

Conservation and Management: The blue shark is one of the most abundant sharks with the most wide-ranging distribution worldwide.  As such, the species is also taken by the millions annually, both as incidental and as target catch, as a market for blue shark exists in some countries. The ISC Shark Working Group reports that the North Pacific blue shark stock is not overfished and overfishing is not occurring.