Somniosus pacificus

A boreal and temperate shark found over continental shelves and slopes. At high latitudes with cold surface waters it ranges into the littoral and even the intertidal (one large individual was found trapped in a tide pool) as well as the surface; however in lower latitudes with temperate water it becomes a deep-water, epibenthic shark, never coming to the surface and ranging down to at least 2,000 m in the extreme southern end of its range (off southern California and Baja California) (Ebert 2003).

Archived depth data from recent tagging studies in the Northeast Pacific showed some sleeper sharks regularly ascended and descended at rates over 200 meters per hour, traveling below the photic zone during the day and approaching the surface at night (Hulbert et al. 2006).

Development is yolk-sac viviparity, but pregnant females have yet to be found, and for some reason are rare, as in the closely related Greenland shark (S. microcephalus). This is possibly due to segregation of pregnant females beyond the usual fisheries gear that capture these sharks or extremely low fecundity, with a small fraction of adult females pregnant at any one time. Although female sharks with ovaries containing over 300 large unfertilized eggs and many small undeveloped ova have been captured in trawls off Moss Landing and Trinidad, California (Gotshall and Jow 1965, Ebert et al. 1987). Size at birth is between 40–65 cm TL suggesting that fecundity may indeed be high for this species given the enormous size of the females. Neonates have been taken in midwater trawls indiciating that they occupy this habitat early in life before taking up a more demersal lifestyle (Ebert 2003).

Mature Pacific Sleeper Sharks have not been reported from the Bering Sea and Aleutian Islands or the Gulf of Alaska, but we know of observers who have seen very large Sleeper Sharks, but they are usually just discarded.

The species attains a maximum length of at least 440 cm with unconfirmed records of up to 700 cm total length (TL) or more (Ebert et al. 1987, Ebert 2003). Adult females are 370–430 cm TL and larger individuals estimated at 700 cm TL or more have been photographed in deep water (Ebert et al. 1987, Yano et al. 2007). Males are mature by at least 397 cm TL (Ebert et al. 1987).

These sharks feed on a wide variety of surface and bottom animals, including flatfish, Pacific salmon, rockfish, harbor seals, octopi, squid, crabs, tritons, and carrion. It is not known if seals and fast-swimming pelagic fish such as salmon are captured alive by these lumbering, sluggish sharks or are picked up as carrion. The small mouths and long heads and oral cavities of these sharks suggest that they are powerful suction feeders, but this has yet to be observed. Pacific sleeper sharks commonly are attracted to traps set at great depths for sablefish (Anoplopoma fimbria), and get trapped themselves or else eat catch and bait-can and escape (Ebert 2003, Ebert et al. 1987).

Red List Category

Red List Criteria

Version

2009

Ebert, D.A., Goldman, K.J. & Orlov, A.M.

Valenti, S.V., Musick, J. & Fowler, S.L. (Shark Red List Authority)

The Pacific Sleeper Shark (Somniosus pacificus) is a deepwater sleeper shark, relatively common in the north Pacific Ocean. Records from the southern hemisphere are most likely Somniosus antarcticus. In the northern part of its distribution it ranges into shallower water, but at lower latitudes it becomes strictly deepwater, extending down to at least 2,000 m depth in the extreme southern end of its range. The species is taken as bycatch by bottom trawl fisheries in the western Bering Sea, and by longline fisheries for sablefish and Pacific halibut in the eastern north Pacific, and is generally discarded. Biomass estimates are increasing in the western Bering Sea and Gulf of Alaska, and have decreased in other areas in the eastern Pacific. Greater depths that are not currently fished may provide some refuge for adult Pacific sleeper sharks. The lack of life history data (e.g. growth rates and fecundity) and robust population assessment information provide justification for a Data Deficient listing.

Biomass estimates are available from the results of bottom trawl surveys in the Bering Sea, off the Aleutians and Gulf of Alaska no deeper than 1,000 m (i.e., within upper part of species' bathymetric range, where mainly juvenile Pacific sleeper sharks occur). In Alaska, sharks of 1.5–2.5 m are most common (e.g., Sigler et al. 2006) and sharks as large as 4.3 m have been caught (Orlov 1999). Data from the Gulf of Alaska and eastern Bering Sea are not robust enough to make statement about population numbers or changes in actual population size. The short-term fluctuations in biomass trend data presented herein are too large to represent trends in overall shark population biomass, but may be a result of sampling vagaries and/or population movements (i.e. changes in local density) rather than actual changes in population size, and could indicate changes in catchability.

Biomass apparently increased in the western Bering Sea from 1,170 mt in 1999 to 87,500 mt in 2002 (Glebov et al. 2003). On the eastern Bering Sea shelf, Pacific sleeper sharks were not well documented in survey catch until the mid-1990s. Biomass in 1999 was estimated at 2,079 mt and increased to 5,602 mt in 2002. However, biomass estimates have decreased since then and were down to 2,944 mt in 2006 (Heifetz et al. 2007).

On the slope of the eastern Bering Sea no Pacific sleeper sharks were recorded in 1979 or 1981. In 1982 biomass was estimated at 12 mt and increased steadily through 1991 when the estimate was 1,235 mt. A new slope survey initiated in 2002 showed a very large biomass of sleeper sharks (25,445 mt in 2002, but dramatically decreased down to 2,260 mt in 2004 (Heifetz et al. 2007).

No Pacific Sleeper Sharks were caught off the Aleutians in 1980. In 1983 biomass constituted about 254 mt. Biomass estimates slowly increased through 1991 when biomass was estimated to be 2,927 mt, but then sharply decreased in 1994 to just over 373 mt. Biomass estimates fluctuated from 1997 through 2004 with the highest biomass estimate being 2,638 mt in 2000 and the lowest estimate being 536 mt in 2002. The 2005 biomass estimate increased to 1,017 mt, and subsequently decreased down to 76 mt in 2006 (Heifetz et al. 2007). However, as previously stated above, large short-term biomass fluctuations may be a result of sampling vagaries and/or population movements (i.e. changes in local density) rather than actual changes in population size.

In the Gulf of Alaska biomass in 1984 was estimated as 163 mt. Since then it demonstrates gradual upward trend and reached 57,022 mt in 2005 (Courtney et al. 2006b).

Unknown

This species is taken as bycatch in several fisheries and usually discarded. It is notably affected by bottom trawl fisheries in the western Bering Sea (Orlov 2005), by longline fisheries for sablefish and Pacific halibut in the eastern North Pacific (Courtney et al. 2006a, b). Incidental catch in US waters in 2006 was 435 mt, in some years it has reached ~ 1,400 mt (Courtney et al. 2006a, b).

From 1997–2001 in the Bering Sea Aleutian Islands area, Pacific sleeper sharks were caught primarily by the Pacific cod longline fishery (30%), walleye pollock pelagic trawl fishery (26%), Greenland turbot longline fishery (17%), flatfish trawl fishery (12%), and sablefish longline fishery (10%). From 1997–2002 in the Bering Sea Aleutian Islands area, Pacific sleeper sharks were caught primarily in two statistical areas, which made up 57% and 20% of the total sleeper shark catch. There has been an increasing trend in catch of Pacific sleeper sharks from two statistical areas in the eastern Bering Sea between 1997-2002, however, this may reflect a change in fishing effort as opposed to any increase in the population size.

Fisheries in the western Bering Sea catch mainly juveniles of this species, present at shallower depths than adults. Greater depths that are not currently fished may provide some refuge for adult Pacific sleeper sharks, however the situation should be monitored.

No current conservation measures are known.

Like many deeper water species more information on biology, ecology and importance in fisheries are required to further assess status and any future conservation needs. Where taken, catches require close monitoring.

The development and implementation of management plans (national and/or regional e.g., under the FAO International Plan of Action for the Conservation and Management of Sharks: IPOA-Sharks) are required to facilitate the conservation and management of all chondrichthyan species in the region.