Blue Shark - Prionace glauca
Status: Near Threatened … About 20 million blue sharks are caught globally each year
Location: One of the most widespread larger animals on the planet, found in all temperate and tropical seas.
Size: Mature at around 2.2m, females are usually bigger, and can grow up to 3.8m in length.
Habitat: Coastal and oceanic seas, ranging from surface waters to 600m in depth.
The blue shark is taken in large numbers (an estimated 20 million sharks annually), mainly as bycatch in long-line and trawl fisheries. After death, the meat is rapidly contaminated with its own ammonia and so is not widely valued, but there is a lucrative market for their fins. Captured blue sharks are usually finned at sea and the carcasses discarded, but EC legislation in the north west Atlantic prohibits this practice. Landed carcasses can be processed for fishmeal, but loopholes in EC legislation mean that many are still discarded at sea. Fisheries catch data assessments suggest a 60% decline of blue shark populations in the north west Atlantic, and declines have been observed elsewhere in the range. Better monitoring of these fisheries is needed to determine Atlantic trends as there are real concerns about the impacts of the removal of so many animals from the ecosystem. Blue sharks are also valued by sport anglers, who usually release captured sharks to fight another day.
Blue sharks have been extensively studied, and through tagging and tracking studies we know that the sharks perform impressive, seasonal migrations across the Atlantic, using trans-oceanic currents to aid their migrations. The Atlantic population is believed to be one stock, seasonally moving between mating areas in the north west Atlantic and pupping areas in the north east Atlantic. A larger nursery area has also been identified in the north central Atlantic.
Courtship can be violent, with mature female sharks often displaying bite marks after mating. During gestation, the pups depend on a yolk sac, but when depleted, the sac fuses with the uterus wall, acting as an exchange of nutrients and waste between pup and mother, similar to a mammalian placenta. Blue shark litters are born after a gestation of about 9-12 months, and can number up to 135 pups, although average at about 35 pups. They are a fast growing species that will mature at around 4-6 years.
Blue sharks have a varied diet which includes small pelagic fish, small sharks, invertebrates, cephalopods, cetaceans and seabirds. They will also shoal to scavenge on carrion such as dead cetaceans. Blue sharks are known to feed throughout the day, being most active in the early evening and at night.
Did you know?
- Blue sharks have been recorded living up to 20 years old.
- Blue shark shoals are usually either all male or all female, with the sexes only mixing during mating.
- Females have skin three times thicker than the males to withstand bites during courtship.
What MCS is doing:
- Working towards better protection of important marine habitats in the UK;
- Campaigning at EU level for more effective regulation of fisheries that can impact blue sharks.