Mysterious marine mammal injuries hint that some sharks are shifting their range northward into the Arctic

Sleeper sharks are already known to range as far north as the Chukchi Sea, but reports of possible shark attacks on seal and sea lions have increased — and now come from further into the Arctic.

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A Pacific sleeper shark show photographed in the central pacific. The sharks range already extends to the Chukchi Sea, but reports of shark predation on seals and sea lions have grown in recent years, and spread further in the Arctic Ocean. (NOAA Office of Ocean Exploration and Research)

In coastal villages of northwestern and northern Alaska, hunters and residents are seeing more evidence that an unusual predator is feeding on seals and sea lions — sharks.

Over the recent hunting seasons, ice-dwelling seals and sea lions have been showing up, in harvests or dead on the beach, with signs of deadly encounters with the sharp-toothed predators.

“They’re being harvested with flippers amputated or washing up with the head completely gone or a big bite taken out of the middle,” said Brandon Ahmasuk, subsistence director for Kawerak Inc., a Native-owned nonprofit organization based in Nome.

Local Alaska Department of Fish and Game officials were initially skeptical, but a shark expert in Hawaii who reviewed photos of the injured animals came up with a quick verdict: The injuries were perfect matches for shark bites.

Those discoveries of injured animals are among the emerging signs that more sharks are preying on marine mammals in the Bering Strait region and in the Chukchi and Beaufort seas.

Ahmasuk worked on a project with scientists from the University of Alaska’s Sea Grant program and the North Slope Borough that compiled a list of documented shark sightings from 1950 to 2017 in the area stretching from the Bering Strait to the Beaufort Sea. Three of those sightings were in 2017, with sleeper sharks found as far east as Cross Island in the Beaufort. The findings were described in a poster presented at January’s Alaska Marine Science Symposium, held in January in Anchorage.

As ocean temperatures rise, sea ice retreats and fish and marine mammals shift their movements, sharks will get greater access to the seals and sea lions of Alaska’s northern waters, the poster said.

Three species of sharks are found in Alaska waters, according to the National Oceanic and Atmospheric Administration: Pacific sleeper sharks, which range as far north as the Chukchi Sea; salmon sharks, found in the Gulf of Alaska and Bering Sea; and North Pacific spiny dogfish, also found in the Gulf of Alaska and Bering Sea.

There have been some notable sightings in recent years. In one dramatic case in 2011, Ahmasuk said, seal hunters on St. Lawrence Island watched a shark chase a bull sea lion.  Both the sea lion and the shark pursuing it jumped out of the water; after the last jump, the sea lion was struck from below, he said. “After that, it was just a big blood stain in the water,” he said.

The “strangest part” of that incident was that it happened in December — and that the waters around St. Lawrence Island were ice-free then, Ahmasuk said.

It is not yet clear whether there are more sharks in the northern waters or better reporting of their presence, scientists say.

But it would make sense to have more sharks in Alaska’s far north waters, Ahmasuk said. For one thing, prey populations appear to be moving north, he said. When it comes to sea lions at St. Lawrence Island, “there are more and more every year, and they’re staying longer in the fall,” he said.

Markus Horning, science director at the Alaska SeaLife Center in Seward, also said a northward movement of sharks might be underway.

The cold-adapted sharks living off Alaska may be moving to higher latitudes as the marine environment warms, said Horning, who has been studying sleeper sharks in the Gulf of Alaska and Bering Sea.

His work has gathered compelling evidence that sleeper sharks are eating endangered western Alaska Steller sea lions. The western population of Steller sea lions was listed in 1997 as endangered, and the animals in the central and western Aleutians have been faring the worst, according to the NOAA.

“There is probably a fair amount of predation by sleeper sharks on Steller sea lions,” he said. But whether shark predation is responsible for the endangered sea lions’ failure to recover is not yet known, he said. “We can’t really point a finger at a single culprit and say, ‘This is it.’ It’s all inter-connected,” he said.

There is still much to learn about sharks in Alaska waters, Horning said.

Greenland sharks, probably the longest-lived invertebrate on the planet, have not yet been documented in Alaska waters, for example, but sleeper sharks are closely related to that species and may have similar characteristics, he said. Greenland sharks do not reach sexual maturity until they are 150 or 160 years old, “which is mind-boggling, really,” he said. A study published in Science in 2016 estimated that two large Greenland sharks examined by researchers were close to 400 years old.

If sleeper sharks have similar lifespans, Horning said, even small changes in the marine environment or in fishery policies, such as rules for bycatch, could have big implications for the population

Sharks are not the only animals biting marine mammals in the Bering, Chukchi and Beaufort seas.

Killer whale numbers are increasing in around the Arctic, and a study of bowhead whales in the Chukchi and Bering Seas found that injuries from killer whales have been on the rise. The study, published last year in the journal Arctic, evaluated records for 904 bowheads harvested by Native subsistence hunters from 1990 to 2012, analyzing injuries attributable to ship strikes, fishing gear entanglements and killer whale attacks. There was a high prevalence of killer whale-inflicted scars for bowheads longer than 16 meters, presumably, the older animals, and there was a “significantly higher probability of killer whale rake mark scars” for the years from 2001 to 2012 than for the earlier 1990-2002 period, the study said.

That study was by scientists from the North Slope Borough, Alaska Sea Grant, the Alaska Department of Fish and Game and the University of Alaska Fairbanks.