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According to a U.S. non-profit conservation group, the Pacific coast’s endangered southern resident killer whale population has dropped from 75 to 74. Tweet This

The Centre for Whale Research, a group that tracks killer whale movement and population numbers in the Salish Sea, declared on Thursday evening that it believed critically-ill orca J50, also known as Scarlet, had died.

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1:51 Officials consider plan to capture sick J50 orca for treatment Officials consider plan to capture sick J50 orca for treatment

“J50 is missing and now presumed dead,” wrote the group on its website.

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READ MORE: Massive search on for sick orca J50 that researcher fears could be dead

“Her last known sighting was Friday, September 7 by our colleagues at [National Oceanic and Atmospheric Association (NOAA)], SeaDoc, and others. The Center for Whale Research has had a vessel on the water looking for J50 for the past three days. We have seen all the other members of her family (i.e., J16s) during these outings.”

The NOAA has not confirmed J50’s death, and spokesperson Jim Milbury said officials were still holding out hope that J50 would be located alive.

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0:42 Scientists administer drugs to ailing J50 orca Scientists administer drugs to ailing J50 orca

“The Centre for Whale Research, traditionally they’re the ones who monitor the numbers of the different pods and they have a criteria for determining whether a whale is missing or not from a pod or gone,” he said.

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“We haven’t made that call yet, we’re going to continue to look, exhaust all our resources until we’re confident she’s not there. Hopefully, she’ll turn up.”

Officials raised new concerns about J50’s condition on Thursday, owing to the fact she hadn’t been seen in several days. Officials with Canada’s Department of Fisheries and Oceans (DFO) and the NOAA said they had launched increased water surveillance in hopes of spotting the ailing orca.

In an email, the DFO said it continued to co-ordinate search efforts with the NOAA, and asked anyone who saw J50 to contact its Marine Mammal Incident Reporting Hotline at 1-800-465-4336.

Scientists have been concerned about J50 for several months. The three-and-a-half-year-old female was lethargic, had not been eating and was extremely emaciated.

Her condition had advanced so severely that her skull was visibly protruding, a condition known as “peanut head.”

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0:35 Condition of ailing orca ‘J50’ not improving: scientists Condition of ailing orca ‘J50’ not improving: scientists

Scientists and veterinarians from both sides of the border have been working feverishly to try and diagnose exactly what J50’s problem was and reverse her condition.

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Teams dosed her with antibiotics, and later attempted to administer anti-worming medication after fecal and breath samples revealed she was suffering from parasitic worms.

Officials had also considered a more extreme plan to capture J50 in order to nurse her back to health, should she become permanently separated from her pod or stranded on a beach.

READ MORE: Scientists considering capturing sick orca J50 to give her medical treatment

J50’s survival was of particular importance to scientists and conservationists because of her young age and sex.

With so few southern resident orcas left, it was believed she could play a key role in helping repopulate the species.

The southern residents are facing threats from many angles, including chemical pollutants that accumulate in their blubber and noisy ship traffic that interferes with their communication.

They’re also facing challenges with declining stocks of their primary food source, chinook salmon.

WATCH: A cross-border effort is underway to save ailing orca

2:17 A cross-border effort is underway to save ailing orca A cross-border effort is underway to save ailing orca

“Watching J50 during the past three months is what extinction looks like when survival is threatened for all by food deprivation and lack of reproduction,” wrote the Centre for Whale Research.

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“Not only are the Southern Resident killer whales dying and unable to reproduce sufficiently, but also their scarce presence in the Salish Sea is an indication that adequate food is no longer available for them here, or along the coast.

J50 was a member of the same pod of orcas as J35, the female that captured international attention after keeping her dead calf afloat for several weeks.

–With files from the Canadian Press