Scientists are reporting another challenge to the&nbsp;critically endangered southern resident killer whales in the waters off British Columbia, Washington state and Oregon.

A statement from the Center for Whale Research in Washington says a 47-year-old female identified as L47 has not been seen for nearly seven months and is likely dead.

The centre&nbsp;says its teams have spotted the matriarch's three surviving offspring and their two calves several times since L47 was last seen off B.C.'s Salt Spring Island in February, but the elder killer whale was not with them.

L47's&nbsp;apparent death, along with the confirmed death in July of a 35-year-old male&nbsp;orca&nbsp;from a different pod, would reduce the total number of primarily salmon-eating&nbsp;southern resident orcas to 73.

Older, post-reproductive females hold a key, matriarch-like role in southern resident pods, especially when food is scarce, and the centre&nbsp;says the loss of this female's leadership could have severe consequences.

Chinook salmon supply in winter key to survival of killer whales, study says

Endangered southern resident killer whales would have a much better chance of survival if chinook salmon were in their hunting grounds during winter off the coast of British Columbia, a new study says.

The southern resident killer whales are listed as a species at risk in Canada.

baby killer whale J Pod

It says the risk of death for L47's children and their offspring over the next two years is now three to six times higher, and it could increase if salmon populations continue to dwindle.

Endangered southern residents travel in three separate extended family groups, known as pods: K, J and L.

L47, who was also known to researchers as Marina, was a matriarch of L pod, which now has about 32 members.

The matriarch has had seven calves and they survived long enough to be given alphanumeric designations, the most of any southern resident.

Male southern resident killer whale possibly dead from cancer, says expert

The orca, K-21, was seen in the last week of July and photos showed that it seemed to be wasting away, said Andrew Trites, director of the Marine Mammal Research Unit at the University of British Columbia.

J35 is the matriarch of of J pod, one of three groups of killer whales living off the coast of British Columbia and Washington state. Here she is seen swimming with her new calf, J57. 

J57 baby orca

UBC researcher Andrew Trites says orcas have no predators and usually die of natural causes, with cancer being one of them.

Southern resident orca L41 missing, feared dead

The U.S.-based Centre for Whale Research says L41, an adult male whale and member of the endangered southern resident killer whale population, is missing.

Members of the L pod seen in Haro Strait on January 24, 2020. L41, an adult male, was missing from the group. 

L pod Jan 24, 2020

The centre&nbsp;says its studies show matriarchs support the survival of the pod by acting as "repositories for ecological knowledge,'' guiding their groups to salmon foraging grounds, and that their knowledge is especially important during years when salmon returns are low.

It says older reproductive females can also fill this role, raising the potential that L47's leadership could pass to either of her daughters or any other older female within the pod.

L47 pictured in 2020. The Center for Whale Research in Washington said the matriarch has not been seen in months and has likely died.

Apparent death of 47-year-old orca matriarch could have serious effects on pod, scientists fear

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