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Sleeper Sharks: Awake and Hungry
Sleeper sharks Not Culprits in Sea Lion Declines
Pacific sleeper sharks are actually not sleepy at all.
Named for their supposed sluggish nature, sleeper sharks were for many years thought to be quiet bottom dwellers, feeding primarily on arrowtooth flounder and octopus. Recent research by Lee Hulbert, formerly a biologist with the National Marine Fisheries Service and now with the Alaska Department of Fish and Game, revealed that sleeper sharks are not the lethargic bottom dwellers once thought. They spend little time on the bottom. Instead, they move continuously in the water column and are voracious stealth predators of fast moving prey.
However, bringing them aboard the research vessel is an easy feat and suggests how they got their sluggish reputation. They do not thrash around like salmon sharks.
“Salmon sharks will kick your ass,” Hulbert said. “Sleeper sharks are like big tubs of goo. It’s hard to tell if they’re dead or not.”
The sleeper shark’s success as a predator is due to their ability to glide through the water with little body movement, minimizing hydrodynamic noise and eluding acoustic detection by prey. They feed by suction and cutting; their large mouths act as a vacuum to inhale prey, and their lower teeth slice up food too large to swallow whole. “It’s like a chopping blade,” Hulbert said.
His data show that sleeper sharks swim below the photic zone during daylight hours and surface at night—which, because of their dark color also helps them avoid detection by prey. They move in a constant oscillating vertical pattern, averaging about 6 km per day. This vertical swimming pattern is also common to mako sharks, blue sharks, and school sharks.
Hulbert’s research, funded by NOAA and the Steller Sea Lion Coordinated Research Program, tracked the overlapping sleeper sharks and Stellar sea lion habitats and depth ranges to see if the sea lion pups were preyed upon by sleeper sharks.
Sleeper shark predation has been suspected as a cause of the recent sea lion population decline. Since the mid 1970s, the sea lion population has declined about 70 percent; during the same period sleeper shark populations appeared to increase.
According to the NOAA Fisheries domestic sablefish longline surveys, sleeper shark captures increased from 79 to 1,700 between 1988 and 2000, with the largest population increase between 1992 and 1993. ADF&G reported that southeast Alaska fishermen caught very few sharks in the 1980s, but in the 1990s reports increased, and by 1998, ADF&G had numerous reports of sleeper sharks eating halibut off the gear of Cook Inlet longliners.
Hulbert’s study used two types of satellite-transmitting tags from Wildlife Computers, a company in Redmond, Washington: pop-up archival transmitting (PAT) tags and satellite depth recording (SDR) tags. The tags collected data on the vertical movement of sleeper sharks during the periods when sea lion pups are most vulnerable to predation—in July and August when they first enter the water, and again the following April and May when they are weaned. The tags were attached to the base of the first dorsal fin and are later collected at the surface.
The sharks were tagged at the four largest Steller sea lion rookeries in southcentral Alaska: Marmot Island, Sugarloaf Island, Outer Pye Island and Seal Rocks. Between August 1 and 10, 2001, 99 sleeper sharks were collected and 10 were tagged and released. During May 10 and 21, 2002, 99 were collected and 24 were tagged and released.
Sleeper sharks were captured using longline gear. Those that weren’t tagged were killed, weighed, and their stomach contents identified. “We tried to do a lavage-type thing,” Hulbert said, referring to a procedure where stomach contents are forced out with water, “but,” he sighed, “you’ve got to kill them.”
Most of the stomach contents were easily identified, and included intact rockfish, walleye pollock and pink salmon, as well as cleanly sliced larger fish including Pacific cod, chinook salmon, Pacific halibut and chunks of marine mammals such as harbor seals and whales.
“There were occasions where we got mystery meat,” Hulbert said. “It was like – who does that belong to?” A large mess of goo was later determined to be whale. “Every place we found sharks, we found whale in their stomachs. It’s carrion - that’s a lot of dead whale out there.”
The data from the tags were compared to published reports of the foraging and diving movements of immature sea lions. Although sleeper shark and sea lion habitats overlap and the potential for predation exists, no stellar sea lion tissues were found in the stomach contents of the sharks in the study. However, one intriguing find was a fresh chunk of harbor seal flesh shaped like a bicycle seat. It was so fresh that all the hair was intact.
“Usually after an hour the hair will be digested,” Hulbert said. “The question is – do they bite chunks out of live mammals or dead mammals? You just don’t know.”
Hulbert also has numerous photos of marine mammals with bloody bite wounds or scars. “One seal had the whole back part of it gone. I don’t know how it swam. But you just can’t say for sure if it was a sleeper shark that bit them.”
Summarized data from the PAT tags were transmitted to Hulbert’s email via satellite. Much more comprehensive data is extracted from the tags themselves – a reading of depth and temperature every minute. Three tags were found and the data extracted from one was astonishing.
“These sharks never stop moving. One of them logged 12,000 vertical meters in 24 hours. The guys at Wildlife Computers said they had never seen anything like it from any animal,” Hulbert said.
Sleeper sharks are found in the Chukchi Sea, East Siberian and Beaufort seas, the Bering Sea and in the Pacific Ocean south to Baja California and off Japan including the Okhotsk Sea. Reported to reach lengths of over 20 feet, the average length is about 12 feet and the average weight is between 700 and 800 pounds.
You can help: NOAA Fisheries is offering a $200 reward for electronic tags collected from Pacific sleeper sharks. NOAA Fisheries' Auke Bay Laboratory is tagging Pacific sleeper sharks in Lynn Canal/Chatham Strait of southeast Alaska. The 3/4 inch diameter by 2 and 1/4 inch long electronic tags are attached externally near the dorsal fin of each shark. The electronic tags record depth and temperature. Data from the tags will provide information about Pacific sleeper shark behavior in the sea as well as the marine environmental conditions they experience.
Anyone who finds a tag should record the recovery location, retain the tag, and contact Dean Courtney, National Marine Fisheries Service, Auke Bay Laboratory, 11305 Glacier Highway, Juneau 99801; call (907) 789-6006 or 789-6060; or e-mail email@example.com, to aid scientific research and to collect a reward.
For a copy of the reward poster, please visit http://www.fakr.noaa.gov/newsreleases/Sleeper_Reward_Poster.pdf
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