Efforts to find a sick young orca from a critically endangered population of killer whales in the Pacific Northwest have come up empty, with a scientist who tracks the animals declaring her dead – though authorities said they would keep looking.

The grim news left just 74 whales remaining in a group that has failed to reproduce successfully in the past three years.

The orcas have struggled with pollution, boat noise and, most severely, a dearth of their preferred prey, chinook salmon, because of dams, habitat loss and overfishing.

“We’re watching a population marching toward extinction,” said Ken Balcomb of the Centre for Whale Research. “Unless we do something about salmon recovery, we’re just not going to have these whales in the future.”

The whales are in such bad shape that experts prepared last-ditch efforts to save the emaciated four-year-old known as J50.

A sharpshooting veterinarian fired an antibiotic-filled dart into her, to no avail, and scientists even mulled capturing her so they could treat her for parasitic worms.

J50 had not been seen since last Friday. As teams scrambled to find her on Thursday, she failed to appear with her pod once again, despite favourable sighting conditions.

Mr Balcomb, who tracks the whales for the US government, declared her dead late on Thursday afternoon.

Michael Milstein, a spokesman for NOAA Fisheries, said the agency gives great weight to Mr Balcomb’s assessment of the whales, given his long experience monitoring them. But, he said, because the US and Canadian governments have so many resources devoted to finding her, they planned to continue searching on Friday.

“We want to make the most of it to make sure that if J50 is there, we haven’t missed her,” Milstein said. “We haven’t given up hope.”

Crews in a US Coast Guard helicopter, 10 vessels, whale watch crews and other resources on both sides of the border were involved in the search. Authorities also alerted a network of people who respond when marine mammals wash ashore.

Whale experts feared the orca was dead earlier this month when J50 lagged behind her family and went missing. But she later turned up and was seen with her family.

The distinctive black-and-white orcas, known as southern resident killer whales, have struggled since they were listed as an endangered species in the US and Canada well over a decade ago.

The orcas’ numbers are now at their lowest in more than three decades.

Another whale in the same pod, known as J35, triggered international sympathy this summer when she kept the body of her dead calf afloat in waters for more than two weeks.

The message, the Cente for Whale Research said in a website post, is that extinction is looming “while the humans convene task forces and conference calls that result in nothing, or worse than nothing, diverting attention and resources from solving the underlying ecological problems”.

Scientist began to be concerned about J50 in July. Teams dropped live salmon from a boat as J50 and her pod swam behind — a test to see whether fish could be used as a means of delivering medication.

Drone images taken earlier this month showed J50 much thinner than she was last year. Her mother, J16, has also declined in condition in the past month.