Cause of death unknown for whales found in Alaska river

ANCHORAGE, Alaska (Reuters) - Veterinarians have not yet determined a cause of death for two killer whales that wandered far up an Alaska river, the National Oceanic and Atmospheric Administration's Fisheries Service said on Thursday.

The two adult killer whales, along with a juvenile killer whale, had spent three weeks in the salmon-rich Nushagak River in southwestern Alaska, seen as far as 30 miles upriver from their saltwater habitat in Bristol Bay.

The adult whales were found dead on Saturday, one floating in the water and one beached on the riverbank. The juvenile whale has not been spotted since Saturday, when it was swimming in the tidal area near the river's mouth, NOAA said.

Although biologists said the whales showed signs of freshwater stress while still alive, including secretion of a filmy coating over their skin, necropsies completed by veterinarians provided no definitive answers, NOAA said.

Although Bristol Bay killer whales often linger near the mouth of the Nushagak, that species of whale had never been reported far up any Alaska river for a prolonged period of time.

Necropsies revealed that both dead whales were female and one was in the late stages of pregnancy, NOAA said. They showed no signs of impact from humans, NOAA said.

Killer whales are known to travel in maternal groups, with juveniles accompanying adults, said Julie Speegle, a spokeswoman for NOAA Fisheries in Juneau.

Officials have made no assumptions about the fate of the third whale, Speegle said.

"I know there's been some concern about its chances for survival," she said. "I guess it's possible that it would make it back to the bay and meet up with a pod, but I don't know if that's likely."

Veterinarians have been taking samples from the dead whales for further analysis, and a more detailed necropsy report is expected in four to six weeks, she said.

(Editing by Dan Whitcomb and Cynthia Johnston)