Bluntnose sixgill shark (Hexanchus griseus) COSEWIC assessment and status report: chapter 2

Executive Summary

Bluntnose Sixgill Shark
Hexanchus Griseus

Species information

The bluntnose sixgill shark (Hexanchus griseus) is one of four species belonging to the family Hexanchidae sometimes referred to as cow sharks. The name, sixgill, refers to the presence of six gill slits whereas most other shark species have only five. The population structure of bluntnose sixgill sharks in Canada’s Pacific waters is unknown. For the purposes of this report, bluntnose sixgill sharks are considered as one designatable unit throughout Canadian waters.

Distribution

Bluntnose sixgill sharks are widely distributed throughout temperate and tropical seas around the world. 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 two records from Atlantic Canadian waters.

Habitat

The bluntnose sixgill shark is considered to be primarily a deepwater benthic species found in waters below 91 metres (m), but is known to occur from the surface to depths of 2500 m. The species is primarily found over the outer continental and insular shelves as well as upper slopes associated with areas of upwelling and high biological productivity. 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.

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 centimetres (cm) in size. Age and growth information is constrained by difficulty in age determination and the lack of large mature specimens. This species is sexually dimorphic with females growing 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. Maximum reported length is 482 cm. The generation time for bluntnose sixgill shark is not known nor have there been estimates of natural mortality. Predators of adult bluntnose sixgill sharks have not been recorded. 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 utilize shallower coastal waters and have extended residency in relatively small areas. Migratory behaviour on a seasonal and/or latitudinal basis has not been recorded.

Interactions with fisheries

The bluntnose sixgill shark 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. Present-day bycatch of bluntnose sixgill sharks in British Columbia is poorly known. Recent observer data indicates that this species is caught regularly by fisheries pursuing halibut and spiny dogfish.

Population sizes and trends

There are presently no reliable indicators for understanding bluntnose sixgill shark status in Canadian waters. The long-term effective population size for 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 theStrait of Georgiahave decreased significantly (>90%) over the last five years based on video surveillance and anecdotal diving records.

Limiting factors and threats

Fishing presents the only known proximate threat to bluntnose sixgill shark populations in Canada. Intensive fishing for this species took place in the late 1930s to mid-1940s but at present the catch is composed of non-utilized bycatch. The overall impact that present-day fishing has on the population depends on the size of the population, the fishing mortality, and the demographics of the bycatch itself (age, size and sex), all of which are largely unknown.

Special significance of the species

The bluntnose sixgill shark is the largest predatory shark regularly encountered in Canada’s Pacific waters. 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.

Existing protection

The IUCN has assessed the bluntnose sixgill shark as lower risk/near threatened (LR/nt) (Shark Specialist Group 2000). In Canada this species receives no formal protection.

COSEWIC History

The Committee on the Status of Endangered Wildlife in Canada (COSEWIC) was created in 1977 as a result of a recommendation at the Federal-Provincial Wildlife Conference held in 1976. It arose from the need for a single, official, scientifically sound, national listing of wildlife species at risk. In 1978, COSEWIC designated its first species and produced its first list of Canadian species at risk. Species designated at meetings of the full committee are added to the list. On June 5, 2003, the Species at Risk Act (SARA) was proclaimed. SARA establishes COSEWIC as an advisory body ensuring that species will continue to be assessed under a rigorous and independent scientific process.

COSEWIC Mandate

The Committee on the Status of Endangered Wildlife in Canada (COSEWIC) assesses the national status of wild species, subspecies, varieties, or other designatable units that are considered to be at risk in Canada. Designations are made on native species for the following taxonomic groups: mammals, birds, reptiles, amphibians, fishes, arthropods, molluscs, vascular plants, mosses, and lichens.

COSEWIC Membership

COSEWIC comprises members from each provincial and territorial government wildlife agency, four federal entities (Canadian Wildlife Service, Parks Canada Agency, Department of Fisheries and Oceans, and the Federal Biodiversity Information Partnership, chaired by the Canadian Museum of Nature), three non-government science members and the co-chairs of the species specialist subcommittees and the Aboriginal Traditional Knowledge subcommittee. The Committee meets to consider status reports on candidate species.

Definitions

Wildlife Species
A species, subspecies, variety, or geographically or genetically distinct population of animal, plant or other organism, other than a bacterium or virus, that is wild by nature and is either native to Canada or has extended its range into Canada without human intervention and has been present in Canada for at least 50 years.

Extinct (X)
A wildlife species that no longer exists.

Extirpated (XT)
A wildlife species no longer existing in the wild in Canada, but occurring elsewhere.

Endangered (E)
A wildlife species facing imminent extirpation or extinction.

Threatened (T)
A wildlife species likely to become endangered if limiting factors are not reversed.

Special Concern (SC)Footnotea
A wildlife species that may become a threatened or an endangered species because of a combination of biological characteristics and identified threats.

Not at Risk (NAR)Footnoteb
A wildlife species that has been evaluated and found to be not at risk of extinction given the current circumstances.

Data Deficient (DD)Footnotec
A category that applies when the available information is insufficient (a) to resolve a species' eligibility for assessment or (b) to permit an assessment of the species' risk of extinction.

Canadian Wildlife Service

The Canadian Wildlife Service, Environment Canada, provides full administrative and financial support to the COSEWIC Secretariat.

Footnotes

Footnote a

Formerly described as "Vulnerable" from 1990 to 1999, or "Rare" prior to 1990.

Footnote b

Formerly described as "Not In Any Category", or "No Designation Required."

Footnote c

Formerly described as "Indeterminate" from 1994 to 1999 or "ISIBD" (insufficient scientific information on which to base a designation) prior to 1994. Definition of the (DD) category revised in 2006.

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