Michael Milstein, a spokesman with NOAA Fisheries, says researchers on Wednesday spotted the 20-year-old whale known as J35 carrying her dead young off the tip of Washington's Olympic Peninsula.
The population of Southern Resident killer whales is down to 75 individuals.
The young female orca has sparked an global rescue effort by Canadian and American scientists who have developed a novel plan to feed her salmon medicated with antibiotics.
"So when we undertake interventions, we will quite likely disturb not just J50, but other animals around J50", said Sheila Thornton, a scientist with Fisheries and Oceans Canada.
Scientists in both the United States and Canada are hoping they can feed medicated fish to a sick killer whale off Vancouver Island, a process that has approval in the USA but not yet in Canada.
J50 and J35 have not been spotted since J-pod swam out of the Strait of Juan de Fuca to the west coast of Vancouver Island late Thursday and Friday. As a young female, experts say, she has reproductive potential and could play a vital role in the southern residents' recovery.
The recent images of the mother orca pushing her dead calf and trying to keep it afloat in Northwest waters has given greater urgency to the fate of the whales, said Les Purce, co-chair of the task force. The only sighting Tuesday was between 3:52 and 4:31 p.m., when Canadian researchers saw J50, staying close to her mother, moving steadily along, and resting.
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"We don't know exactly what is wrong with her", Rowles said in a teleconference.
"It's very possible that she has succumbed at this point and we may never see her again".
Barre said operations testing for using live Chinook for nutritional support therapy and administering oral medication would be done with the Lummi tribe and the Whale Sanctuary Project - without whales present.
If J50 is in American waters and needs antibiotics, Rowles said the next step will be to administer an antibiotic that lasts 10 to 14 days. They may drop medicated fish next, if that goes smoothly. The emergency plan is to inject the killer whale with antibiotics through a pole system from a boat.
Springer was two years old when she was found in Puget Sound near Seattle, ailing and separated from her pod.
The whale would initially get just a few fish to see whether she takes it and how she and members of her pod respond before deciding whether to give her salmon dosed with medication. Cottrell says Canadian vessels are out on the waters off southwestern Vancouver Island but heavy fog is making the search hard. She was the first orca to be rescued, rehabilitated and successfully released back into the wild.