Blue shark (Atlantic and Pacific populations) COSEWIC assessment and status report: chapter 2

Species Information

The blue shark (Prionace glauca) is long and slender with distinctive blue coloration on dorsal and lateral surfaces. They are widespread and highly migratory with some evidence of movement between hemispheres although most tagging studies suggest that blue sharks are largely separated by hemispheres. The North Atlantic and North Pacific populations are considered as two designatable units because they are geographically isolated by the continental landmass of North America: there is no evidence of movement between ocean basins. The population structure is not well defined from a genetic point of view. In French this species is known as requin bleu.

Distribution

Blue sharks are found worldwide in temperate and tropical oceans, most often in the offshore surface waters. In Atlantic Canada they are regularly found in almost all waters with a peak occurrence in the late summer and fall. Similarly, blue sharks are widespread throughout Canada’s Pacific waters with peak occurrences in the late summer and fall. Studies in both Atlantic and Pacific waters indicate large scale latitudinal movements and segregation of the population by sex and size.

Habitat

Blue sharks are most commonly encountered offshore between the surface and 350 m. Water temperature appears to influence their depth and latitudinal distributions as well as size and sex distributions. Canada’s waters (Atlantic and Pacific) provide habitat for primarily subadult (immature) individuals although adult (mature) specimens are occasionally encountered. Loss of habitat is not considered a threat for this species.

Biology

Blue sharks have a 9-12 month gestation period and females produce litters approximately every two years. The average litter size is between 25-50 pups and is positively correlated with female length. Maturity is reached between ages 4-6 and maximum age is between 16-20 years. Their generation time is about 8.1 years. Blue sharks are opportunistic feeders and are reported to eat a wide variety of prey including bony fishes, squids, birds and marine mammal carrion. Adult blue sharks have no known predators; however, subadults and juveniles are taken by both shortfin makos and white sharks as well as by sea lions. Blue sharks are the most heavily fished species of sharks in the world and fishing is the single largest source of adult mortality.

Population Sizes and Trends

Population size and trends of blue sharks in Canada reflect what is happening to populations existing at the scale of hemispheric ocean basins. In Atlantic Canada, on average approximately 600 t of blue shark have recently been killed per year, which is estimated to represent a small fraction of the fishing removals in the North Atlantic. North Atlantic-wide trend assessments are constrained by limited data. Population assessments by an international commission suggest the population is not depleted but the estimates are considered preliminary and extremely uncertain. Two analyses of abundance trends covering large geographical areas indicate either no decline since 1971 or a decline of 60% since 1986. Abundance indices based on catch rates in or near Canadian waters show varying decline rates between near 0 and 53% since the mid-1990s. Biological data indicate a decline in the mean lengths of commercially caught blue sharks in both the Canadian and Japanese fisheries in the northwest Atlantic since 1986.

There are no studies of blue shark trends or abundance in Pacific waters. Canadian fishing vessels occasionally catch blue sharks as bycatch at a level of 20-40 t/yr. The low level of bycatch reflects the methods of fishing rather than the abundance of blue sharks.

Limiting Factors and Threats

Fishing mortality is the single largest threat to blue shark populations worldwide. Pelagic fisheries regularly catch blue sharks as bycatch. In Canada’s Atlantic waters approximately one third of the biomass of animals caught in the Canadian pelagic fishery (tunas and swordfish) is blue shark, but removals in Canada are probably 1% or less of total North Atlantic removals. A review of published catch rates in the North Atlantic indicates a range of 5.1-40 blue sharks per 1000 hooks. Overall fishing effort for pelagic species in the North Atlantic has increased substantially since the mid-1950s implying an increase in catch of blue sharks. Fishing mortality of blue sharks in Canada’s Pacific waters accounts for possibly 0.1% of the fishing mortality in the entire North Pacific.

Special Significance of the Species

This species is one of the most abundant, widespread, fecund, and fast growing shark species worldwide and likely is a significant component of tropical and temperate open ocean ecosystems worldwide. Blue shark has very low value in the marketplace and is often discarded when caught as bycatch. The meat is rarely marketable due to the rapid breakdown of urea in the muscle tissue into ammonia immediately following death thereby tainting the meat. The fins are of low value but may contribute an estimated 50-70% of the international fin market traded through Hong Kong.

Existing Protection

There are no jurisdictions that prohibit the capture of blue sharks. In 2004, ICCAT accepted a resolution to prohibit finning in Atlantic high seas pelagic fisheries. Likewise, in Canada finning is prohibited on both coasts and on the Pacific coast blue sharks cannot be landed in the hook and line fishery. The Atlantic recreational shark fishery is catch and release only. The IUCN assessed the blue shark in 2000 and listed it as lower risk/near threatened (LR/nt).