Blue shark - Prionace glauca
WHAT TO LOOK FOR
This shark is BLUE, and it is one of the most beautiful sharks. It is a slim, graceful shark with a long, conical snout, large eyes (no spiracles), and long, narrow scythe-shaped pectoral fins well in front of the first dorsal win with no interdorsal ridge. Mature females often bear bite wounds (mating scars).
This shark has a dark blue back, bright blue flanks and sharp demarcation to a white underside.
At birth, the shark is 1.1 to1.4 ft [35 to 44 cm] in length. Males mature at 6 to 9.2 ft [182 to 281 cm], while females mature at about 7.2 ft [220 cm]. Their maximum length is about 12.5 [380 cm].
Curved, saw-edged, triangular upper teeth.
Oceanic and pelagic, usually off the edge of the continental shelf from 0 to 1148.3 ft [0 to 350 m] (deeper in warmer waters). Migrations often follow major trans-oceanic currents. They occasionally venture inshore at night, particularly around oceanic islands or where the continental shelf is narrow. Their nursery areas are offshore.
The shark is located world-wide in temperate and tropical oceanic waters (temperature 44.6 to 77ºF [7 to 25ºC], preferably 53.6 to 68ºF [12 to 20ºC], latitude 60ºN to 50ºS. Possibly the most wide-ranging of sharks because they live in such a broad range of areas.
General – In European waters, pups remain in offshore nursery areas until they reach about 4.3 ft [130 cm] in length, when they begin to migrate with other sharks of the same age and sex. Males mature at 4-6 years: females at 5-7. Mature females may breed annually, or on alternate years. Their longevity is approximately 20 years.
Prey – Feeds on relatively small prey: usually squid and pelagic fish, but also invertebrates and bottom-dwelling fish and small sharks. They sometimes take seabirds at the surface of the water.
Reproduction – Viviparous, yolk-sac placenta. They have anywhere between 4-135 pups per litter (usually 15-30), which are born in the spring and summer after a 9-12 month gestation period.
Blue sharks cruise slowly at the surface with the tips of their dorsal and tail fins out of the water, and long pectoral fins extended. These sharks are most active in the early evening and at night when they may move inshore. They form large aggregations (where still sufficiently abundant) to feed on shoals of prey or carrion. They are highly migratory with complex movements related to prey availability and reproductive cycles. Blue sharks segregate by age, sex, and reproductive phase: juveniles, sub-adults, mature sharks, and pregnant females are usually found in separate areas, with adult males and females meeting only briefly to mate. The sharks move seasonally to higher latitudes where prey is more abundant in productive oceanic convergence or boundary zones. They have frequent vertical excursions made into deep water or to the thermocline, returning regularly to the surface (possibly to prevent body cooling). Tagging studies have demonstrated that Atlantic Blue Sharks undertake numerous trans-Atlantic migrations, swimming slowly with the major current systems. Pacific blue sharks may migrate up to 5716.6 miles [9200 km].
Blue sharks are the most heavily fished shark in the world; many millions are taken annually, mainly as bycatch. Their meat is low of value, but their large, valuable fins are kept and enter the international shark fin trade (often after the carcasses are discarded at sea). Recent reports of approximately 60-80% decline in catch rates and reductions in sightings frequency, but data are inadequate to assess global population decline. Although often timid, they are potentially harmful due to their dentition, and have been implicated in a few accidents.