Three southern resident killer whales are missing and presumed dead as of July 1, according to the U.S.-based Center for Whale Research.

Researchers last spotted the adult whales off the coast of British Columbia last winter in poor, deteriorating health and they have been missing from their family pods this summer.

Usually, the summer location for the whale pods is off the southern end of Vancouver Island and around the U.S. San Juan Islands.

"It's more evidence, on top of all the evidence we already have, that this is an extremely endangered population in decline in need of some serious changes," said Michael Weiss, a field biologist with the Center for Whale Research.

One of the whales — a 42-year-old J pod matriarch called J17 — is the mother of the whale J35, who's known for carrying her dead calf for an unprecedented 17 days last year.

The last photos of J17 show her with peanut head — a misshapen head and neck caused by starvation.

The other two are both males.

These aerial images of J17 show the whale from September 2015 to May 2019. The latest images show she is emaciated and with signs of peanut head because of a drastic loss of fat. (Holly Fearnbach and John Durban/NOAA Fisheries )

The loss of a matriarch is particularly concerning, Weiss said.

"Old females in resident killer whale society are extremely important, they lead group movement when they're foraging," Weiss said.

"Losing J17 is a pretty big blow."

As for the other whales, 28-year-old K25 should have been in the "prime of his life," the Center for Whale Research said in a release.

He was emaciated when last seen in January, though.

J17, the eldest female member of J pod swims alongside a younger resident on Dec. 31, 2018. The matriarch's health has been failing for years. (Center for Whale Research)

The 29-year-old L84 has also been missing all summer. He was the last in a line of 11 whales, 10 of whom died previously.

With these new deaths, the southern resident killer whale population dropped to 73.

The whales are listed as a species at risk in Canada and struggle to find enough food due to the scarcity of suitable chinook salmon prey.

But there are a variety of reasons why a killer whale might lose weight, said Dr. Martin Haulena, head veterinarian for the Vancouver Aquarium Marine Science Centre.

"We have to be very, very careful not to jump to the conclusion that it's simply a starvation [problem]," he said.

Sickness, for example, makes foraging for food more challenging.

The elusive nature of the orca means it's difficult to determine the cause of starvation, he added.

"One of the things we don't get to do is really closely examine these animals," Haulena said.

"Many of them disappear at sea after they die and we never get the opportunity to examine them."

For more on the plight of the Southern Resident Killer Whales, check out Killers: J pod on the brink. You can get it now for free at CBC Podcasts.