Pacific Sleeper Sharks: What We Know

Aptly named for their lethargic and sluggish movements, Pacific sleeper sharks (Somniosus pacificus) are a member of the Somniosidae family of squaliform sharks. Pacific sleeper sharks are gray in color, with a blunted nose and small dorsal fins and a large, cylindrical, torpedo-shaped body (Ebert et al. 1987; Castro 2011). Females of the species are generally larger than males and sexually mature females generally measure in over 3.5 m in length (Ebert et a.l 1987; Yano et al. 2007). S. pacificus may potentially grow to 6 or 7 m in total length, but reports of sharks at these sizes are  unconfirmed. Very little is currently known about these large, cryptic animals, but what we do know paints a picture of an elusive, opportunistic predator that likely plays an important role in arctic food webs.  

What do Sleeper Sharks eat?

Stomach content analysis reveals that S. pacificus feeds upon red squid, giant grenadier, walleye pollock, popeye grenadier, chum salmon, and Kamchatka flounder (Orlov 1999), though does appear to be a relationship between prey selection and shark size. Sharks over 3 m in body length exhibit a shift from invertebrates to a diet more heavy in teleost fishes and marine mammals (Sigler et al. 2006; Yano et al. 2007).

a photo of Alaska pollock

Alaska (or walleye) pollock (Gadus chalcogrammus) are an important prey item for Pacific sleeper sharks.
Credit: NOAA Fishwatch

 

Marine mammal remains found in the stomachs of sampled S. pacificus appear to be both scavenged (in the case of cetacean flesh) and fresh (seals) (Sigler et al. 2006; Yano et al. 2007). And while there’s no evidence of Pacific sleeper sharks preying on Steller sea lions near rookeries or during pupping season, there is evidence that the sharks are preying upon sub-adult juveniles (Horning & Mellish 2014).

Life History

When do Pacific sleeper sharks reach sexual maturity? Where do they give birth? How many pups are in a litter? Do juvenile Pacific sleeper sharks utilize space differently than sexually mature adults?

These are all questions that remain unanswered. What we do know: it’s likely that female S. pacificus do not reach sexual maturity until the are at least 3.5 meters long (Yano et al., 2007). Utilizing available data on the related Greenland shark, this means that Pacific sleeper sharks may not reach sexual maturity until they’re over 150 years old (Nielsen et al., 2016).

Where do they live?

Pacific Sleeper sharks are predominately found on continental slopes and shelves in the North Pacific, the Bering, Chukchi, Okhotsk, and Beaufort Seas, the Gulf of Alaska, down through British Columbia, Washington, Oregon, and through Baja California (Ebert et al. 1987; Castro 2011) . While there are reports of S. pacificus off the coast of Chile and other regions in the south Pacific, it is more likely that these are sightings or catches of the southern sleeper shark (S. antarcticus), a closely related species (Yano et al. 2004). The two species share many morphological similarities, and are easily mistaken for one another where their ranges overlap (Benz et al. 2007; Castro 2011).

Range and distribution of Pacific sleeper shark Somniosus pacifcus. Credit: IUCN 2012. IUCN Red List of Threatened Species

Recorded depths range from as shallow as 100 m up to depths of 2,000 m (Compagno, 1984; Hulbert and Sigler 2006). Several researchers have noticed the tendency for larger sharks to be found at deeper depths than smaller sharks (Yano et al. 2007). There also appears to be a relationship between temperature and depth, where sharks in more temperate or subtropical waters are found near the seafloor (Compagno 1984; Orlov 1999).

Fisheries

Pacific sleeper sharks are not currently targeted as an active fishery, though they are frequently caught as bycatch in pollock and other fisheries in Alaska. Fishery impacts on this species are not currently known, but the total number of Pacific sleeper sharks caught on sablefish longline surveys increased from 1979 to 2003 (Courtney & Sigler 2007).