By Gareth Jordaan, Marine Scientist…
The blue shark, Prionace glauca, is a slim graceful species with a dark blue back, bright blue flanks, and a white demarcated underside. They have a long conical snout, large eyes and no spiracles. Their pectoral fins are long, narrowed, and scythe-shaped situated well in front of the first dorsal fin.
DISTRIBUTION: Possibly one of the widest ranging of all shark species. Occurs worldwide in temperate and tropical oceanic waters.
HABITAT: Oceanic and pelagic, usually off the edge of the continental shelf. Blue sharks occur in depths of 0 – 350 m (deeper in warmer waters) and migrate following major trans-oceanic currents. They are sometimes known to move inshore at night, particularly around oceanic islands or where the continental shelf is narrowest. Blue sharks prefer water temperatures of 12 – 20°C, but may occur in 7 – 25°C. Nursery areas are generally offshore, although a nursery area has been identified around the Cape Point area in South Africa.
BIOLOGY: Blue sharks are viviparous, meaning they give birth to live young. Litter size can vary from 4 to 135 pups (average 15 – 30), which are generally born in spring and summer after a 9 to 12 month gestation period. Pups are born at a length of 35 to 44 cm and tend to stay in the nursery areas until about 130 cm total length. At this size they begin to migrate elsewhere. Males mature at about 4 to 6 years (182 – 281 cm), and females at about 5 to 7 years (220 cm) of age. Mature females can breed annually, or on alternate years. The maximum confirmed length of a blue shark is 380 cm total length.
They feed on relatively small prey items, usually squid or pelagic fish, invertebrates, bottom-dwelling fish and other small sharks. Some have been known to prey on seabirds at the sea surface.
MOVEMENT: These sharks are highly migratory (some recorded to migrate up to 9200 km), with complex movements that are related to prey availability and their reproductive cycles. Juveniles, sub-adults, mature sharks and pregnant females are usually found in separate areas, with adult males and females interacting for a short period of time when mating. They move seasonally to higher latitudes, where prey is more abundant in highly productive areas.
BEHAVIOUR: Blue sharks generally cruise slowly at the surface with the tips of their dorsal and caudal fins out of the water. They are most active in the early evenings and at night when they move inshore. They form large aggregations to feed on shoals of prey. Some individuals have been seen biting floating objects and are known to harass spearfishermen. Blue sharks sometimes circle swimmers, boats and divers for a while before approaching and occasionally bite out of curiosity. This species is becoming more and more popular for offshore dive tourism. They are very inquisitive so expect some sort of interaction whilst diving.
CONSERVATION STATUS: Currently categorised as “Near Threatened” on the IUCN Red List of Threatened Species. This is the most heavily fished shark species in the world, often caught as a bycatch species by pelagic longliners targeting tuna and swordfish.
For more information on blue sharks, visit FishBase.