Management Plan for the Bluntnose Sixgill Shark and Tope Shark in Canada [Proposed] 2012: Species Information
Date of Assessment: April 2007
Common Name (population): Bluntnose Sixgill Shark
Scientific Name: Hexanchus griseus
COSEWIC Status: Special Concern
Reason for Designation: This large (maximum reported length 4.8 m), heavy-bodied shark is a benthic species that is widely distributed over continental and insular shelves in temperate and tropical seas throughout the world. In Canadian Pacific waters, it is found in inlets and along the continental shelf and slope typically at depths greater than 91 m (range 0-2500 m). In the absence of information about population structure, it is treated as a single population for assessment purposes. The present population size and abundance trends are not known. The only available abundance index, encounter rates with immature sharks at a shallow site in the Strait of Georgia, has decreased significantly (>90%) in the last five years. This index is not likely representative of the overall abundance trend because only immature sharks are encountered and the site is shallow relative to the preferred depth range. The principal known threat to the species is fishing. This shark has been the focus of at least three directed fisheries in Canadian waters, most recently in the late 1980s and early 1990s. It continues to be caught as bycatch, but survival of released sharks is unknown. Sharks observed by divers sometimes show scars from entanglement in fishing gear. Because of this late age of maturity (18-35 yr for females), it is likely susceptible to overfishing even at low levels of mortality. Little is known about the abundance and movement patterns of this species elsewhere in the world, so the potential for a rescue effect is unknown.
Canadian Occurrence: Pacific Ocean
COSEWIC Status History: Designated Special Concern in April 2007. Assessment based on a new status report.
Date of Assessment: April 2007
Common Name (population): Tope
Scientific Name: Galeorhinus galeus
COSEWIC Status: Special Concern
Reason for Designation: This Pacific coast shark is thought to be highly migratory across its range from Hecate Strait, BC to the Gulf of California. Tope shows no evidence of distinct populations and thus for the purposes of this assessment is considered a single population. It feeds primarily on fish, and in Canada occupies continental shelf waters between western Vancouver Island and Hecate Strait. Maximum length is less than two metres, maximum age is at least 45 years, maturity between 12 and 17 years, and generation time 23 years. Tope is noted for its high concentration of liver Vitamin A, exceeding that of any other north-east Pacific fish species. Demand for vitamin A during World War II led to a large fishery that quickly collapsed due to over-exploitation. More than 800,000 individuals, primarily large adults, were killed for their livers between 1937 and 1949 throughout its migratory range. Tope is rarely seen today in Canadian waters. There is no targeted commercial fishery in Canada, but it continues to be caught as fishery bycatch in Canada and the U.S., and remains the target of small commercial and recreational fisheries in the U.S. Because there is no population estimate for tope, the sustainability of current catches cannot be assessed. The ongoing fishery mortality, the lack of a management plan for Canadian bycatch, and the long generation time and low fecundity of tope suggest cause for concern.
Canadian Occurrence: Pacific Ocean
COSEWIC Status History: Designated Special Concern in April 2007. Assessment based on a new status report.
The Bluntnose Sixgill Shark (Hexanchus griseus) is one of four species belonging to the family Hexanchidae sometimes referred to as cow sharks. The Bluntnose Sixgill Shark is easily recognizable with characteristics not often found in other shark species (Mecklenburg et al. 2002), such as the presence of six gill slits as well as a single dorsal fin (all other shark species found in Canadian Pacific waters, with the exception of the Broadnose Sevengill Shark (Notorynchus cepedianus), have a second dorsal fin). It is dark brown or grey to black dorsally becoming lighter towards its underside. The head is broad and depressed with a blunt snout. The single dorsal fin is located far back on the body and positioned above and in between the pelvic and anal fins on the ventral side. Like many benthic sharks, the caudal fin of the Bluntnose Sixgill Shark has a small lower lobe.
The Bluntnose Sixgill Shark is yolk-sac viviparous (the young hatch within the female’s body before being released), and produce litters estimated to range from 47-108 pups which are 61 to 73 cm in size (Ebert 2002, 2003). This species is sexually dimorphic with females growing larger than males. Maximum length has been reported at 350 cm and 480 cm for males and females respectively. Length at maturity is reported at 421 cm for females and 310 cm for males (Ebert 2002). Age of maturity is estimated to be 11-14 years for males and 18-35 years for females, with an estimated longevity of up to 80 years (Florida Museum of Natural History 2010), although these estimates have not been validated.
Figure 1. Illustration of the Bluntnose Sixgill Shark (DFO, 2011).
The Tope Shark (Galeorhinus galeus), also known as the Soupfin Shark, is one of 46 species belonging to the family Triakidae (Houndsharks). The Tope Shark is the only representative from the family Triakidae on Canada’s Pacific coast. The Tope Shark is dark bluish gray on its dorsal side which shades to white on the underside (Mecklenburg et al. 2002). It has two dorsal fins, with the first dorsal fin well ahead of the pelvic fins and the second dorsal fin being about the same size as the anal fin. The caudal fin has a large subterminal lobe which is nearly as long as the lower lobe (Ebert 2003). The snout is long and pointed with a large mouth. The eyes of the Tope Shark are horizontally oval with conspicuous spiracles behind each eye.
The genetic structure of Tope Shark in the Northeast Pacific is unknown, and no studies have been conducted on age and growth of the Tope Shark. Recent molecular studies (Chabot and Allen, 2009) found significant genetic structure within global populations of Tope Shark globally, and little to no gene flow between geographic regions, suggesting that there may be subspecies, or distinct regional species, within this genus. However, further studies are required to validate this, and for the purpose of this management plan, the Tope Shark is assumed to be one species.
Little is known about the breeding behaviour of the Tope Shark. The Tope Shark is yolk-sac viviparous, with the female carrying between 6 and 52 pups released between March and July (Compagno 1984; Ebert 2003), averaging 35-37 cm long (Ripley 1946). The Tope Shark exhibits rapid growth during the first three years followed by steady growth until about 10 years of age, and then slow continued growth through maturity. In the northeast Pacific maximum length of females is 195 cm, for males is 175 cm (Compagno 1984). Tope Shark are slow growing and reach a maximum age of at least 45 years. Age of maturity in females is estimated to be about 13-15 years and males at about 12-17 years (Francis and Mulligan 1998). In eastern Pacific waters, females are mature at 150 cm total length and males are mature at 135 cm.
Figure 2. Illustration of the Tope Shark (DFO, 2011).
Bluntnose Sixgill Sharks are widely distributed throughout temperate and tropical seas around the world (Figure 3). In the north Pacific, they can be found from Japan, south of the Aleutian Islands, to California and Mexico as well as the Hawaiian Islands (Compagno 1984; Mecklenburg et al. 2002). In the south Pacific, they are reported from Australia and New Zealand. In the western Atlantic Ocean, its range is from North Carolina to Florida and from the northern Gulf of Mexico to northern Argentina including Nicaragua, Costa Rica, and Cuba. In the eastern Atlantic, this shark is found from Iceland and Norway to South Africa, including the Mediterranean Sea. Its range in the Indian Ocean includes waters off Madagascar and Mozambique. In 2005, the IUCN Red List assessed Bluntnose Sixgill Shark as globally Near Threatened (Cook & Compagno 2005). There is no information available to estimate global abundance of Bluntnose Sixgill Sharks, though the IUCN Red List indicates the global population trend is decreasing.
Figure 3. Known global distribution of Bluntnose Sixgill Sharks (Cook & Compagno 2005).
The Bluntnose Sixgill Shark is likely well distributed throughout much of Canadian Pacific waters including inlets, continental shelf and slope, and the Strait of Georgia. Recorded observations available in databases are limited to recent commercial catch records (1979-2007) and research surveys for other species and therefore do not fully describe their Canadian range (Figure 4). The trawl fleet captures this species over a wide range of depths (20-1000 m) with the number of captures being proportional to effort with no particular preferred depth range. The hook and line fleet has encountered this species between 20 and 440 m with most observations less than 200 m. Intensive fishing for this species took place in the late 1930s to mid-1940s but otherwise catch has been limited to bycatch. Migratory behaviour on a seasonal and/or latitudinal basis in the northeast Pacific is limited; however a recent study (Andrews et al. 2010) reported that Bluntnose Sixgill Sharks tagged in Puget Sound with acoustic transmitters moved seasonally to the north from winter to spring, and to the south from summer to fall. Further, this study described two of these tagged sharks moved north as far as Queen Charlotte Strait and the north west coast of Vancouver Island, British Columbia, and another shark moved south as far as Pt. Reyes, California. Overall, movement patterns are characterized by a bathymetric migration of mature individuals to shallower, nearshore nursing areas (depths <200m) to give birth (Ebert 2002, 2003). Juveniles appear to utilize coastal waters in inlets along the west coast of Vancouver Island, the Strait of Georgia and Puget Sound, and have extended residency in these relatively small areas (COSEWIC 2007a; Dunbrack and Zielinski 2003; Andrews et al. 2007). Migratory behaviour on a seasonal and/or latitudinal basis is unknown.
There are presently no reliable indicators for understanding Bluntnose Sixgill Shark abundance in Canadian Pacific waters. An abundance estimate based on genetic techniques suggests a minimum breeding population in the northeast Pacific at ~7900 individuals (Larson et al.2005). This estimate is considered unlikely to be accurate (COSEWIC 2007a) due to small sample size. A second index of abundance measured encounter rates of immature Bluntnose Sixgill Sharks at a single shallow site (40m) in the Strait of Georgia (Dunbrack and Zielinkski 2003). This index suggested a decline of at least 90% in the abundance of immature Sixgill sharks over five years. However, this index does not represent overall abundance since it is limited to immature sharks at a single shallow site (40m) relative to the species preferred depth range. Further, individual sharks are typically not identifiable and thus the index may not record abundance but rather behaviour at the site. Due to the use of one surveillance site which is atypical in nature (i.e., that Bluntnose Sixgill Sharks can be observed regularly in shallow waters), interpretations made from this observation trend must be viewed with caution. It is unlikely, even under the assumption that mortality to Bluntnose Sixgill Sharks has increased, that this mortality would be enough to account for the suggested rate of decline from this site. Other plausible explanations include a change in environmental conditions, such as water temperature, that may influence the bathymetric distribution of the sharks. In 2004 the temperature at 10 meters was the second highest annual temperature recorded since 1970, and at bottom depths (395 m) was the warmest on record (DFO 2006). This warm trend persisted through to 2007, and then declined in 2008 (Beamish et al. 2010). It is possible that these observed differences in temperatures extended northwards to the Flora Islet site thereby influencing the video encounter rates of Bluntnose Sixgill Sharks at Flora Islet.
Figure 4. Distribution of catches of Bluntnose Sixgill Shark (Hexanchus griseus) off the west coast of Canada from 1979 to 2007. Positional data of catches retrieved from fisheries and research databases at the Pacific Biological Station (GFCatch; PacHarvTrawl; PacHarvHL; PacHar3; GFBio).
The Tope Shark occurs in temperate and subtropical seas between 68°N - 55°S latitude (Figure 5). Tope Shark are found in the eastern Pacific from northern British Columbia (no records from Alaska) to the Gulf of California as well as waters off Peru and Chile. Migration of this species in eastern Pacific waters is poorly understood; limited tagging of this species in eastern Pacific waters (Ripley 1946; Herald and Ripley 1951) has shown mixing across the range from southern California to British Columbia. Given the high mobility of Tope shark, interchange is probable, at a minimum, between waters off British Columbia, the western U.S. and Baja Peninsula, Mexico. Tope Shark are distributed in the southwestern Pacific Ocean in waters off Australia and New Zealand. In the western Atlantic Ocean, its range is limited from southern Brazil to Argentina while in the eastern Atlantic it can be found from Iceland to South Africa, including the Mediterranean Sea. In the western Indian Ocean region, the Tope Shark can be found in waters off South Africa (Compagno 1984). In 2006, the IUCN Red List assessed Tope Shark as globally vulnerable, and within the Eastern North Pacific as Least Concern (Walker et al. 2006). There is no information available to estimate global abundance of Tope Shark.
Figure 5. Known global distribution of Tope Shark (red shaded areas) (Walker et al. 2006).
In Canadian Pacific waters, records for Tope Shark occur primarily from continental shelf waters along Vancouver Island, Queen Charlotte Sound, and Hecate Strait. There are no known research or commercial fishing records of Tope Shark being taken from the Strait of Georgia (Figure 6). Based on commercial trawl data between 1996 and 2005, 95% of catches of Tope shark (n=109 sets with Tope Shark) fall between the depths of 47-285 m. The area between these two depths off Canada’s west coast is ~73,600 km2 which can be considered the extent of probable occurrence in Canadian Pacific waters. Movement patterns of Tope Sharks in the northeast Pacific are poorly understood. There appears to be both bathymetric and latitudinal movements that vary by both sex and season. Off the west coast of North America males are dominant in northern latitudes and females dominant in southern latitudes (Ripley 1946). In recent research surveys (2002-2009) in Canadian Pacific waters 84% of Tope shark captured (n=19) were male (King pers. comm. 2011). Tagging studies in other areas of the world suggest that at least some component of the population undergoes extended migrations and that these sharks are capable of traveling long distances over a short period of time (COSEWIC 2007b).
Current abundance and population trends for the Tope Shark in Canadian Pacific waters are unknown. There are no indices of Tope Shark abundance within their northeastern Pacific range. Walker (1999) reported that between 1938 and 1944 approximately 15,600 t of Tope Shark were estimated to have been removed from waters along the west coast of North America (COSEWIC 2007b). This catch can be used as a surrogate for a minimum historic population. Present day population biomass is unknown. Given sixty years of no targeted fishing for Tope Shark, minimal bycatch, and its fecundity rates, it is reasonable to assume a population recovery to at least 10% of the historical level. At this level, at least 1,500 t are present along the west coast of North America.
Figure 6. Distribution of catches of Tope Shark (Galeorhinus galeus) off the west coast of Canada from 1994 to 2007. Positional data of catches retrieved from fisheries and research databases at the Pacific Biological Station (GFCatch; PacHarvTrawl; PacHar3 GFBio).
The Bluntnose Sixgill Shark is considered to be primarily a deepwater benthic species found in waters below 91 m, but is known to occur from the surface to depths of 2,500 m (Ebert 2003). They are often found over the outer continental and insular shelves as well as upper slopes associated with areas of upwelling and high biological productivity (Ebert 2003). Some adults occasionally migrate to shallower waters (Andrews et al. 2007). Two instances of mature females (with pups) stranded in shallow waters of Puget Sound and the Strait of Georgia have been recorded (Williams et al. 2010; King, pers. comm. 2011) suggesting that mature females migrate to shallow waters to give birth. Newborn pups and juveniles 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 (Ebert 2003). In British Columbia, a single tagging study (n=214) was carried out in 1994 in inlets along the west coast of Vancouver Island found primarily juveniles of both sexes with no mature females and a mean length of both sexes of 205 cm (McFarlane pers. comm. 2011). A video surveillance study in the Strait of Georgia conducted in 2001-2002 also observed only immature animals with a mean length of 240 cm (n=35) (Dunbrack and Zielinski 2003). Similar studies in Puget Sound encountered only juvenile fish (Andrews et al. 2007). Using an array of automated acoustic receivers to monitor movement patterns of juveniles in Puget Sound they reported relatively small daily movements of < 3.1 km and a maximum displacement over the entire study period (September 2004 - February 2005) of 23 km. These studies have led to speculation that these inshore areas of Puget Sound, Strait of Georgia and West Coast Vancouver Island inlets may represent important Bluntnose Sixgill Shark nursery grounds.
The Tope Shark is considered a coastal pelagic species, often found well offshore but not oceanic (Compagno 1984). Their habitat is described as temperate continental shelf waters ranging from close inshore, including shallow bays, to offshore waters up to 471 m depth, often near the bottom (Ebert 2003). They have been found in the surfline, as well as in bays and submarine canyons. Offshore, they are generally thought to occur near the bottom but have been captured by pelagic floating longlines over deep waters (Compagno 1984). Pups and juveniles utilize shallow nearshore habitats for one to two years before moving offshore. It is believed the Southern California Bight is the main nursery area for this species (Ebert pers. comm. 2011).
Potential predators of the Bluntnose Sixgill Shark may include Steller sea lion (Eumetopias jubatus), Elephant Seal (Mirounga angustirostris), Killer whale (Orcinus orca), White Shark (Carcharadon carcharias) (COSEWIC 2007a), and possibly other shark species (Ebert, pers. comm., 2011). Bluntnose Sixgill Sharks have been observed to readily attack each other if one becomes distressed (Bigelow and Schroeder 1948; Ebert pers. comm. 2011).
The Bluntnose Sixgill Shark is an opportunistic predator primarily foraging nocturnally on a wide variety of prey items including cephalopods, crustaceans, several species of bony fish (e.g., Pacific hake (Merluccius productus), herring (Clupea harengus), flounders (Pleuronectidae), cod (Gadidae), mackerels (Scombridae, Carangidae), and rockfish (Scoraenidae), sharks and rays (Elasmobranchii) and on the carcasses of marine mammals including porpoises (Phocoenidae), dolphins (Delphinidae), and sea lions (Otariidae) (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 Shark; of these, 48 were empty, seven contained salmon (Oncorhynchus sp), and one contained squid (Order Teuthida) (Benson et al. 2001).
Little is known about predators of the Tope Shark. The few studies available indicate they are predated upon by other elasmobranches, including the White Shark (Carcharodon carcharias) and the Broadnose Sevengill Shark (Notorynchus cepedianus), and possibly marine mammals (Ebert 2003). In New Zealand, the Killer whale (Orcinus orca) has been reported to take Tope Shark off of commercial longlines (Visser 2000).
Worldwide, the diet of the Tope Shark consists mainly of bony fish and cephalopods (Teuthoidea) (Walker 1989). The Tope Shark is an opportunistic predator feeding upon several fish species in both pelagic and demersal environments (Ebert 2003). Juveniles prey less on fish and cephalopods, their diet consisting predominantly of small invertebrates (Walker 1989). Ripley (1946) provides the only documentation of Tope Shark diet in the northeast Pacific. Stomach contents found in his study include fish from a variety of families including herrings (Clupeidae), flatfish, plainfin midshipman (Porichthys notatus), rockfishes, mackerel, and perches (Embiotocidae), as well as cephalopods (Ripley 1946) (Ripley 1946). A recent study in the northeast Atlantic found the diet of adult Tope Shark to exist almost entirely of bony fish (98.8% by weight) (Morato et al. 2003). In Australia, bony fish comprised 47% of the diet by weight followed by cephalopods (37%) (Walker 1999). Diet likely varies considerably by season, location, and size of the shark.
Limiting factors are intrinsic to the biology and ecology of the Bluntnose Sixgill Shark and Tope Shark and, as such, cannot be mitigated or managed. These natural bottom-up, top-down processes are generally mediated by factors such as the availability and quality of prey and by predators, respectively. However, human activities may contribute pressures which alter the balance of these limiting factors, threaten the populations, or influence their conservation potential. In such cases, actions are necessary to ensure that human activities do not place undue stress on limiting factors. Limiting factors for these species are described in the subsequent paragraphs, and include life history features, climate and ocean conditions, and specialized habitat requirements.
Life history features such as longevity (estimated to be 80 years for Bluntnose Sixgill Shark and more than 45 years for Tope Shark), late age at maturity (estimated at 18-35 years for female Bluntnose Sixgill Shark and 13-17 years for female Tope Shark) and low fecundity (47-108 pups for Bluntnose Sixgill Sharks and 6-52 pups for Tope Sharks) characterize them as equilibrium life history strategists (King and McFarlane 2003). As such, they have a low intrinsic rate of increase (Smith et al.1998), and are unable to recover quickly after population reduction.
Climate and ocean conditions are known to impact the abundance and/or distribution and availability of plankton and fish species in the northeast Pacific Ocean (King 2005). The Bluntnose Sixgill Shark and the Tope Shark are opportunistic feeders and changes in prey species are unlikely to limit population growth or stability. However, a long term downward shift in prey availability from natural or anthropogenic causes could influence the behaviour of these species and directly impact their feeding, migration and distribution patterns.
Bluntnose Sixgill Shark juveniles live in shallow nearshore areas, and adapt to live in deep water as adults. Large-scale natural or anthropogenic changes to these environments are likely to have detrimental effects on the species. For example, habitat degradation of nearshore nursery areas could be significant, as demographic models suggest that survival of juvenile sharks nearing maturity is proportionately more important to population maintenance than other age classes (Kinney and Simpfendorfer 2009). This represents one limiting factor that could be mitigated or managed.