Biologists have managed to administer what they believe is a full dose of antibiotics to an emaciated killer whale in the waters off British Columbia.

Michael Millstein with the National Oceanic and Atmospheric Administration in Portland, Ore., says the female, southern resident killer whale was spotted on Monday and researchers were able to deliver a dart filled with the potentially life-saving medication.

“Martin Haulena, the veterinarian from Vancouver Aquarium who administered it, believes that most of the dose was delivered,” Mr. Millstein said in a telephone interview.

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He said team members were pleased with the outcome because the first time a dart was used to deliver medication to the animal in early August, Mr. Haulena suspected it fell out before releasing a full dose.

The nearly four-year-old orca, known as J50, is staying close to her mother and is active as the pod of whales moves toward the mouth of the Fraser River, where they are expected to continue foraging for salmon.

“The teams did remark on just how emaciated J50 is,” Mr. Millstein said after researchers located the young whale.

J50 has often lagged far behind other members of the pod as it travels through coastal waters from B.C. to California and back. Concern that the young whale was dead rose over the long weekend when she wasn’t seen with the rest of her pod in waters between Victoria and Seattle.

“They have never seen a whale this emaciated hang on for this long so she has some fight in her, it seems,” Mr. Millstein said of the researchers who have been watching the whale.

J50 has been in poor condition for months and her death would further devastate the dwindling southern resident population where only 75 orcas remain.

In May, Environment Minister Catherine McKenna said the species faces imminent threats to survival and recovery and the government limited the chinook fishery off the B.C. coast in an effort to increase the main food source for the whales.

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Biologists were not able to administering a deworming medication to J50 on Monday, but Mr. Millstein expected further attempts would be made, depending on available boats, crews, water conditions and the location of the pod.

“There are indications that J50 may have some parasitic worms that are common among marine mammals and typically are not a problem, but in compromised animals they can be, so that is the next step,” he said.

Because the whales appear to be travelling north toward the Fraser River, Mr. Millstein said Fisheries and Oceans Canada would be the lead agency while the pod is in Canadian waters.

A range of options for treatment of J50 has been discussed, including the possibility that she could be captured and held for life-saving treatment before being released to rejoin her pod, but Mr. Millstein said that would be a very last resort.

He said if she was found on a beach and the only alternative was her death, then they would likely intervene.

“There are many advantages to her being with the other whales and we know they share prey so that’s something we certainly want to respect as long as she is still with them and active with the pod.”