ANCHORAGE — Scientists counting marine mammals off Alaska’s Arctic Ocean coast spotted two large groups of killer whales last month, but orca experts are not ready to say the species has increased its numbers in the Beaufort and Chukchi seas.

An aerial survey crew Aug. 20 spotted 13 killer whales 6.2 miles northeast of Barrow, America’s northernmost community. The flight was part of a bowhead whale survey sponsored by the National Marine Fisheries Service, the Bureau of Ocean Energy Management and other federal agencies.

Five days later, crew members aboard the Westward Wind, a vessel in the Chukchi Sea Environmental Studies Program, which is conducting research on behalf of oil companies, spotted 25 to 30 orcas near Hanna Shoal, a shallow-water area northwest of Barrow.

The presence of killer whales in Arctic waters is unusual but not unprecedented. Research biologist Paul Wade of the National Marine Mammal Laboratory said he has no indication from published reports or anecdotal evidence that more orcas are spending time north of the Bering Strait. Russian scientists have reported them on their side of the Chukchi Sea, attacking gray whale calves. That could be why the surveys are seeing them in Alaska, he said.

“If there are more killer whales up in the Arctic, it’s because they’re following prey,” he said.

Biologist Craig George of the North Slope Borough wildlife department, who has authored research papers on killer whales in the Arctic, said Native Alaska seal hunters see orcas every summer but they’re rarely spotted by aerial surveys. The main reason, he said, is that there are hundreds of hunters and few aerial surveys.

There are hints of more sightings but nothing scientifically defensible that he’s aware of, he said.

Killer whales occasionally kill calves of bowhead whales, the same species targeted by Native whalers but competition is not a worry for whalers.

“We are more concerned about line entanglement in commercial fishing gear and ship strikes from commercial fishing,” he said. “However regarding other whales, we are somewhat concerned about competition from gray and humpback whales moving into bowhead whale feeding areas. The former are much more aggressive and efficient feeders in areas with high prey densities.”

Lisanne Aerts, principal researcher for the Chukchi survey, said by email the killer whales were cruising close to the pack ice edge Killer whales have been spotted before, she said but never such a high number or that far north.

Brendan Cummings of the Center for Biological Diversity, which has sued to prevent Arctic offshore drilling, said killer whales in the Arctic are another sign of a reshuffled ecosystem.

“One concern is that ice-adapted species such as beluga and bowhead whales are shielded from killer whale predation by the ice, but in an increasingly ice-free Arctic these species will have a new predator to contend with,” he said.

The documentation this summer of killer whales and an endangered species, the short-tailed albatross, in the Arctic highlights that the region should be protected as a marine sanctuary, not opened for oil development, he said.