Killer whale J50 is shown off the coast of Washington State in this August 12, 2018 handout photo.

Image Credit: THE CANADIAN PRESS/HO - NOAA Fisheries, Katy Foster

SEATTLE - A scientist says there is "no chance" a sick killer whale at the centre of an international effort to provide the orca with medical treatment will be found alive.

Ken Balcomb of the Center for Whale Research in Washington state said the southern resident killer whale known as J50 hasn't been seen for several days and he believes she died between last Friday and Monday.

"I know that she is gone for a week and deceased," he said Thursday. "There is no chance that she will still be alive, so we're writing her off."

He said the last sighting was by a biologist on Friday evening and her pod, including her mother, has been seen off the coast of British Columbia and Washington state since then without the three-year-old whale.

"The remains have sunk, no doubt, because she was so skinny that she didn't have anything that would float her," said Balcomb, who tracks killer whales. "No significant blubber layer, her lungs wouldn't hold much air to float the carcass so she probably sank somewhere … in deep enough water that she's not going to buoy up from decomposition."

However, the U.S. National Oceanic and Atmospheric Administration and Fisheries and Oceans Canada haven't given up hope.

NOAA spokesman Michael Milstein said the search for J50 continues. The West Coast Marine Mammal Stranding Network and airlines flying in and out of the San Juan Islands are on the lookout, he said.

He said J50 was last seen Friday afternoon off the west side of the islands and hasn't been seen since.

Fisheries and Oceans said in a new release it was co-ordinating with NOAA to continue the search by water and air. Teams are ready to respond if J50 is located and requires medical attention, it said.

Whale experts feared J50 was dead earlier this month when she lagged behind her family and went missing, but she later appeared with her family. She has been given antibiotics twice, in August and September.

On Wednesday, experts on both sides of the border said they could take extraordinary measures to save the emaciated whale, which is one of 75 remaining members of the endangered southern resident killer whale population.

Officials said they were setting out steps to capture and treat J50 if the whale was found stranded or separated from her family, and that she would then be released back to the wild.

The young whale was presumed dead in the first week of September but was spotted again as she swam with her family.

Balcomb said her mother was "very sprightly chasing fish" on Wednesday.

"The mother just eventually has to give up and the baby will keep trying to catch up and live but the mother has to make a living and keep going."

In August, another killer whale known as J35 was seen carrying the carcass of her dead calf for about two weeks.

Balcomb said the whales are trying to send a message.

"They are starving. And the food is natural salmon so Canada and the U.S. have to emphasize their salmon restoration programs. It isn't noise, it isn't pollution, it's food."

— By Hina Alam in Vancouver, with files from The Associated Press.

News from © The Canadian Press, 2018