(Reuters) - A young killer whale that wandered far up an Alaska river with two adult companions has been found dead near the river's mouth, the National Oceanic and Atmospheric Administration said on Sunday.
Discovery of the dead juvenile means that all three of the whales discovered swimming in fresh water in southwestern Alaska's Nushagak River have died.
The young whale had been swimming with two adult females as many as 30 miles into the Nushagak River, according to NOAA. The whales spent at least three weeks in the river and all had a filmy coating on their skin, a sign of stress from being outside their saltwater habitat, NOAA said.
While killer whales are known to linger near the mouth of that salmon-rich river, which flows into Bristol Bay, there had never before been a documented case in Alaska of killer whales spending prolonged periods in a river's fresh water.
The adults, one of them in the late stages of pregnancy, were found dead in the river last weekend. Initial results from necropsies have failed to pinpoint a specific cause of death.
There had been some hope that the juvenile swam back to saltwater and escaped death, but experts never considered that a likely scenario, said Julie Speegle, a spokeswoman for NOAA Fisheries in Juneau.
"I would say, based on the photos that our biologists saw and the fact that the other two whales died, we felt that it was unlikely that the juvenile would survive," she said.
A local resident reported spotting the dead juvenile whale on Friday, but biologists had needed some time to confirm the sighting and that it was the same whale that had been upriver, Speegle said.
Based on the location, by an island in tidal waters near the town of Dillingham, biologists believe the young whale swam to the site and did not drift there, she said. "It at least had the strength to swim that far," she said.
NOAA has not yet made plans for a necropsy on the juvenile whale