Bluntnose Sixgill Shark

Hexanchus griseus

SARA Status
No Status
NS
Special Concern
SC
Threatened
TH
Endangered
EN
Extirpated
EX

SARA Status

  • No Status NS
  • Special Concern SC
  • Threatened TH
  • Endangered EN
  • Extirpated EX
COSEWIC Status
Not at Risk
NR
Special Concern
SC
Threatened
TH
Endangered
EN
Extirpated
EX

COSEWIC Status

  • Not at Risk NR
  • Special Concern SC
  • Threatened TH
  • Endangered EN
  • Extirpated EX

Description

The bluntnose sixgill shark, with a maximum reported length 4.8 m, is the largest predatory shark regularly encountered in Canada’s Pacific waters. It is one of four species belonging to the family Hexanchidae, sometimes called cow sharks. The name, sixgill, refers to the presence of six gill slits whereas most other shark species have only five. It is easily distinguished from other sharks as it has only a single dorsal fin, compared to two in all other shark species normally found on Canada’s Pacific coast. Its colour is a dark brown or grey to black on their back dorsal side with the colour becoming lighter towards their underside. Its head is broad and depressed with a blunt snout and its eyes are bright green.

Distribution and Population

The bluntnose sixgill shark is one of widest ranging shark species in the world. It is widely distributed over continental and insular shelves in temperate and tropical seas across the planet. Bluntnose sixgill sharks are likely well distributed throughout much of Canada’s Pacific waters including inlets, continental shelf and slope and the Strait of Georgia.

There are presently no reliable indicators for understanding bluntnose sixgill shark population in Canadian waters. The historic population size for bluntnose sixgill shark in the northeast Pacific was estimated based on genetic techniques to be about 8000, but this estimate has wide uncertainty and cannot be used to estimate current abundance. Encounter rates with immature bluntnose sixgill sharks at a shallow site in the Strait of Georgia have decreased by more than 90% over the last five years based on video surveillance and anecdotal diving records. It is unlikely the decline monitored at this site is related to by-catch mortality, but may be a distributional change related to major changes in environmental conditions such as increased water temperatures in the Strait.

In Atlantic Canada this species has only been sighted twice: two immature individuals from Nova Scotia in 1989 and 1990, suggesting the species is rare in these waters, or only occurs as a vagrant.

Habitat

Although the bluntnose sixgill shark is found from the surface to depths of 2500 m, it is primarily a deepwater species found in waters below 91 m. The species is mostly found over the outer continental and insular shelves. Young bluntnose sixgill sharks are thought to remain in shallower waters of the continental shelf and uppermost slope until they reach adolescence, at which time they move further down the slope and into deeper water. In Canada’s Pacific waters immature bluntnose sixgill sharks regularly make forays into shallow waters in some locales allowing the opportunity for scuba divers to observe them.

Biology

Mating and courtship is believed to take place in deepwater. Bluntnose sixgill sharks are ovoviviparous, meaning the young hatch within the female’s body before being released. Females have a two-year reproductive cycle with an estimated 12-24 month gestation. The number of pups carried by females is known from only three credible accounts ranging from 47-70 pups and 61-73 cm in size. With this species females grow larger than males. Length at maturity has been reported for females to be 421-482 cm. For males length at maturity is 310 cm. Mature animals are rarely found with only one mature female recorded from northeast Pacific waters. Age of maturity is widely reported at 11-14 years for males and 18-35 years for females as is an estimated longevity of up to 80 years, but these values have not been confirmed through valid aging studies.

The bluntnose sixgill shark is a generalist feeder primarily foraging nocturnally on a wide variety of prey items. Overall, movement patterns include a migration of mature individuals to shallower nursing areas to give birth. Juveniles appear to use shallower coastal waters and have extended residency in relatively small areas. Migratory behaviour on a seasonal and/or latitudinal basis has not been recorded.

Threats

Fishing presents the only known proximate (or imminent) threat to bluntnose sixgill shark populations in Canada. Recent observer data indicates that this species is caught regularly as bycatch by fisheries pursuing halibut and spiny dogfish.

There is no current bluntnose sixgill shark fishery, although the species has been the focus of at least three known directed fisheries in Canadian waters. The first occurred in the early 1920s with a focus on skins used to make shark leathers. The second took place between 1937 and 1946 with a focus on the shark livers for vitamin A. The third commercial fishery for bluntnose sixgill sharks commenced under an experimental basis in the late 1980s to early 1990s but was terminated due to conservation concerns

Protection

The IUCN has assessed the bluntnose sixgill shark as lower risk/near threatened (LR/nt) (Shark Specialist Group 2000).

Retention and selling of bluntnose sixgill shark captured by hook and line fisheries, both commercial and recreational, in British Columbia is prohibited. In Puget Sound waters, there has been a permanent closure on the recreational and commercial take of Sixgill Sharks since 2001. Beginning in April 2006 all commercial hook and line fisheries operating in Canada’s Pacific waters became subjected to 100% at-sea monitoring coverage in the form of observers and electronic monitoring (video surveillance). Monitoring will allow for highly reliable catch estimates of non-target species including bluntnose sixgill sharks in the future. (Current observer coverage currently does not identify sharks at the species level.)

Species at Risk (SARA) Public Registry Profile.

Bluntnose Sixgill Shark

Illustration of Bluntnose Sixgill Shark.

Bluntnose Sixgill Shark

Scientific name: Hexanchus griseus
Taxonomy: Fishes (marine)
SARA Status: Special Concern (2009)
COSEWIC Status: Special Concern (2007)
Region: Pacific Ocean

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