Featured Elasmobranch

May 2005

PACIFIC SLEEPER SHARK

SOMNIOSUS PACIFICUS (BIGELOW AND SCHROEDER, 1944)

FAMILY SOMNIOSIDAE (SLEEPER SHARKS)

Identification: A large, flabby-bodied shark, short, rounded snout, no anal fin, a first dorsal fin that originates about mid-trunk, well behind the pectoral fin tips, and a broad, slightly asymmetrical tail fin with long ventral lobe, and a subcaudal keel. Upper teeth are unicuspid, with a fine, smooth-edged cusp; lower teeth have very high roots with small, smooth-edged, laterally oblique cusps. Color is relatively uniform and varies between blackish brown to gray; no other distinctive color markings.

Size: Maximum verified length is 4.4 m, with unconfirmed reports of individuals reaching 7-8 m.

Distribution: A trans-Pacific species, ranging from Baja California north to the Bering Sea and southwestward to Siberia and Japan. The northern extent of their range is somewhat unclear, as there has been trouble distinguishing this species from the Greenland shark (S. microcephalus). Records of this species in the southern hemisphere are apparently that of Somniosus antarcticus.

Habitat: The depth at which they occur increases toward the equator, probably due to changes in vertical temperature distribution. At high latitudes they have been observed at the surface and as close inshore as the intertidal. In southern and Baja California, the lowest latitudes at which they are found, they are captured relatively frequently on or near the seafloor down to at least 2,000 m. Juveniles have been captured in the mid-water column over extremely deep water.

Biology: Females and males both mature at approximately 3.7 m TL. Reproductive mode is viviparity (live-bearing), without yolk-sac placenta. Litter size is unknown, but may be high, as up to 372 ovarian eggs have been found in one individual. Size at birth between 60-70 cm. Diet includes halibut, rockfish, sablefish, salmon, albacore, cephalopods, crustaceans, pinnipeds and cetaceans. Since sleeper sharks are generally considered to be sluggish swimmers and bottom feeders, it is surprising to find such evasive prey as marine mammals and epipelagic fishes. One hypothesis suggests they swim off the bottom to feed, using their cryptic coloration to closely approach more active species and ambush these faster-moving prey items. Alternatively, these items may be ingested as carrion.

General interest: Pacific sleeper sharks are caught incidentally in longline fisheries, groundfish trawls, and sablefish traps. There is no directed fishery as the flesh is soft and generally undesirable. Considering current conservation issues surrounding pinniped populations (most notably steller sea lions) it is important to determine if Pacific sleeper sharks are feeding on live or dead pinnipeds. The diet and reproductive biology of sleeper sharks are part of an ongoing research project at Moss Landing Marine Laboratories.

By Lewis Barnett
Moss Landing Marine Laboratories
8272 Moss Landing Road, Moss Landing, CA 95039
lbarnett@mlml.calstate.edu