Bluntnose sixgill shark (Hexanchus griseus) COSEWIC assessment and status report: chapter 6
Biology
Current knowledge regarding life history parameters of Pacific elasmobranchs has been summarized in an online life history matrix assembled by the Pacific Shark Research Centre at Moss Landing Marine Laboratories (http://psrc.mlml.calstate.edu/lht.php). The matrix includes up-to-date information on taxonomy, geographic range, age and growth, longevity, reproduction, demography, trophic interactions, habitat utilization, genetics, recruitment, mortality, and behavior of 102 species. This matrix was drawn upon as an authoritative summary of the current state of bluntnose sixgill shark knowledge in the northeast Pacific.
Life cycle and reproduction
Mating and courtship is believed to take place in deepwater (Ebert 2003). Although courtship and mating have not been observed, it is believed that the male grabs the female near the gills, pectoral fins and flanks as evidenced by the seasonal appearance of scars on the females (Florida Museum of Natural History 2006). 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- to 24- month gestation (Ebert 1990). The number of pups carried by females is known from only three credible accounts of litter size of 47, 51 and 70 pups and 61 to 73 centimetres (cm) in size (Ebert 2002; Ebert 2003). An unverified report of a single specimen with 108 pups exists from a fisherman in France at the turn of the 20th century (Bigelow and Schroeder 1948). The bluntnose sixgill shark is one of the most fecund species of elasmobranch.
Age and growth information is constrained by difficulty in age determination and the lack of large mature specimens (Ebert 1986; McFarlane et al. 2002). McFarlane et al. (2002) estimate that immature bluntnose sixgill shark grow at approximately 25 cm per year. In the same study length-at-age of immature females (N=8) is reported from 130 cm at 4 years to 340 cm at age 10. This species is sexually dimorphic with females growing larger than males. Length at maturity has been reported for females at between 450-482 cm (Springer and Waller 1969), 421 cm (Ebert 1986) and 420 cm (Ebert 2002). A female 405 cm long carrying five pups was found washed up on shore in the Strait of Georgia (Comox, BC) in 2001 (King pers. comm. 2006). For males length at maturity is known primarily from South African waters at 310 cm (Ebert 2002). Mature animals are rarely found with only one mature female recorded from northeast Pacific waters (Ebert 1986). A single tagging study undertaken along the west coast of Vancouver Island in 1994 captured no mature females (N=118) while 20% of the male population were near or at the length of first maturity (N=96) (Figure 8).
Age at maturity is unknown due to the lack of mature specimens. Age of maturity and longevity is widely reported on a variety of websites at 11-14 years for males and 18-35 years for females and an estimated longevity of up to 80 years, but these values have not been confirmed through valid aging studies or published in the primary literature (Florida Museum of Natural History 2006). Maximum reported length is a minimum of 482 cm; however, lengths above 500 cm should not be unexpected (Compagno 1984; Ebert 2002). Claims of specimens over 800 cm are considered false.
A length-weight relationship for female bluntnose sixgill sharks from California waters was found to be: Wkg= -37.5 + (6.64x10-2)(TLcm)+(-3.11x10-5)(TL cm2)+(1X10-8)(TLcm3) (Ebert 1986). A recent study using juveniles captured in Puget Sound found the allometric length weight relationship to be Wkg = (7.80X10-7) * TLcm3.38 (Gallucci et al. 2005).
The generation time for bluntnose sixgill shark is not known nor have there been estimates of natural mortality.
Herbivory/predation
Predators of adult bluntnose sixgill sharks have not been recorded. Their deepwater existence and large size likely limits their potential to be preyed upon. Their large litter size suggests that natural mortality on young bluntnose sixgill sharks is likely quite high. Other elasmobranchs are thought to consume the juveniles; however, there is only a single record from a prickly shark (Echinorhinus cooke) (Varoujean 1972).
Dispersal/migration
Knowledge on dispersal and migration of bluntnose sixgill shark in the northeast Pacific is limited to localized studies. The only study in British Columbia waters involved the tagging of ~214 individuals in 1994. There was only one recovery from this program, a 170 cm female released near Tofino and recaptured by a fisherman in 2000 also near Tofino estimated to be 275-300 cm (McFarlane personal communication (pers. comm.) 2006).
The Seattle Aquarium has conducted a small research project in Puget Sound involving in situ tagging and video observation of bluntnose sixgill sharks from a single station (Larson and Christiansen 2003; Larson et al. 2005). Since 2001, 45 sharks have been tagged with a visible Floy VM69 stainless-steel dart tag of which 16 have been resighted at least once, with one individual being seen on four separate observation periods while at large for 699 days (Christiansen pers. comm. 2006). In 2005 seven sharks were tagged with radio transmitters. Preliminary findings of both the visible and radio tagging program suggest that movement is limited with at least some of the sharks remaining nearby the tagging location (Christiansen pers. comm. 2006). In general the sharks appear to be more abundant from July through to September.
The Washington Department of Fish and Wildlife are also involved with a four-year tagging program in Puget Sound (2003-2006) (Bargmann pers. comm. 2006). Approximately 200 sharks have been captured to date of which all have been juveniles (Gallucci et al. 2005). Captured animals have been tagged with conventional tags and an additional 20 juvenile sharks had radio transmitters placed on them in 2005. Early unpublished information suggests that the juvenile animals have a restrictive home range (Bargmann pers. comm. 2006). Survey effort in deeper Puget Sound waters to capture mature individuals has been unsuccessful.
A third ongoing study at the Flora Islets (49°30.9′N, 124°34.5′W ) in the Strait of Georgia, British Columbia takes place from a single observation area where bluntnose sixgill sharks become seasonally abundant (Dunbrack and Zielinski 2003). Using video surveillance technology Dunbrack and Zielinski (2003) found that sharks at their study site were present only from May 28 to October 16 with a peak between mid-June and mid-July. All of the observed sharks have been immature animals (Dunbrack pers. comm. 2006). Dunbrack suspects that the Strait of Georgia and other near-shore areas along the British Columbia coast are primarily nursery areas and that mature animals are primarily found offshore in deeper waters (Dunbrack pers. comm. 2006). However, a mature female (length = 405 cm; carrying 5 pups) was found beached near the Comox area in the Strait of Georgia in 2001 indicating that some bluntnose sixgill sharks in the Strait of Georgia may be mature (King pers comm. 2006). Dunbrack’s proposed general movement pattern is consistent with that proposed by Ebert (2003) for other areas. Based on scarring patterns, Dunbrack (pers. comm. 2006) has observed some residency over a period of a week. Typically the animals are present for a few days at which time they are replaced by new animals.
There have been no long-term tracking studies on bluntnose sixgill shark. 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. In the Strait of Georgia and Puget Sound, the sharks become more abundant in shallow waters during summer months.
Interspecific interactions
The bluntnose sixgill shark is a generalist feeder primarily foraging nocturnally on a wide variety of prey items including cephalopods, crustaceans, several species of bony fish (e.g. hake, herring, flounders, cod, mackerels, and rockfish), sharks and rays and on the carrion of marine mammals including porpoises, dolphins, and sea lions (Compagno 1984; Ebert 1986; Ebert 1994; Ebert 2003). A study on the west coast of Vancouver Island examined stomach contents of 56 juvenile bluntnose sixgill sharks. Of these 48 were empty, seven contained salmon, and one contained squid (Benson et al. 2001).
Adaptability
The bluntnose sixgill shark is one of widest ranging shark species in the world (Compagno 1984; Ebert 1986). Their body form closely resembles fossil forms dating back 200 million years (Florida Museum of Natural History 2006). This species can likely adapt to natural fluctuations in the environment such as changes in prey type and availability. It is unknown how well the bluntnose sixgill shark is able to adapt to human-caused changes to the environment or changes to population structure due to fishing mortality.
Page details
- Date modified: