Pacific Sleeper Shark: Somniosus pacificus

Family: Somniosidae

Common names

Pacific Sleeper Shark, Mud Shark.

Binomial

Somniosus pacificus.

Synonyms

None.

Identification

A very large, heavy-bodied shark with relatively small fins. Snout length roughly equal to mouth width. Lower teeth form a continuous serrated edge. Small, spiracle behind eye. Eyes often have parasitic copepods attached. Dorsal fins long and low. First dorsal fin origin midway between pectoral fin free rear tip and pelvic fin origin. Second dorsal origin over pelvic fin insertion. Pectoral fins small with rounded apexes and free rear tips. Caudal fin has a very weak pre-caudal notch.
Entire body black or dark reddish-brown.

Size

Maximum verified length 440cm but photographic evidence suggests that Pacific sleeper sharks may reach 7m. If so, the could be the largest predatory fish in the world. Size at birth 40-50cm.

Pacific Sleeper Shark, Somniosus pacificus. A close relative of the Greenland Shark. Prince William Sound, Alaska, North Pacific.

Conservation Status

NEAR THREATENED

The Pacific Sleeper Shark is a bycatch component of trawl and longline fisheries. It is usually discarded, but may be retained for liver oil and (in Taiwan) for its meat. It is sometimes seen in catches in Japan, but has become very rare in Taiwan.
In the Northeast Pacific, the population trend is unclear, but some data indicates a recent decline. Most captured animals are immature, possibly because adults inhabit areas not fished or their large size prohibits capture. Based on evidence in Deep-water videos taken in Monterey Bay and Baja California, there is still a large population of adult Pacific Sleeper Sharks in that region.

Habitat

A boreal / temperate species usually found in deep water of the continental shelf and slope but also enters shallow bays. One individual was found trapped in a tide pool. From the intertidal zone to at least 2000m.

Distribution

Pacific Arctic and North Pacific Ocean. On the west side of the Pacific, it occurs from the Bering Sea southward along the Kamchatka Peninsula to Japan. On the east side of the Pacific it occurs all along the coast of Alaska, southward as far as Baja, Mexico, and possibly further. Generally occurs in deeper water closer to the tropics.
The southern limits of the Pacific sleeper shark’s range is not clearly understood due to confusion with other sleeper sharks from the southern hemisphere.

Reproduction

Presumed aplacental viviparous like other squaloid sharks. Gravid females have never been encountered but a 355cm female examined by Gotshall and Jow in 1965 contained 300 large oocytes (immature eggs).
Jose Castro suggests that the generally large size of adult females, containing large number of similarly sized eggs, and relatively small size at birth (based on one record of free swimming 41cm neonate with a fresh umbilical scar that was trawled from the Challenger Plateau) suggests that litters are probably large, but this is far from conclusive.

Diet

Feeds on flatfishes, Pacific salmon, rockfish, harbour seals, octopi, squid, crabs, tritons, and carrion. It is unclear if the Pacific sleeper shark is capable of catching fast moving prey or if it scavenges for carcasses that have fallen to the bottom. Its long head and oral cavity imply that it has a powerful suction capability but the use of this attribute has not been observed in the field.

Behavior

A slow swimmer that is probably capable of short bursts of speed when hunting fast moving prey such as pinnipeds and squid. The lack of records of large gravid females suggests that Pacific sleeper sharks may segregate by sex or size.

Reaction to divers

Pacific sleeper sharks are rarely (if ever) encountered by divers. However, captured animals that were released in shallow water were approachable until they outpaced observers.

Diving logistics

There are no areas where Pacific sleeper sharks are encountered by divers but they are relatively abundant from southeast Alaska to Washington. Therefore, it may be possible to bring them in by baiting on the substrate close to recreational dive limits.