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(CNN) —  

The Southern Resident killer whale population has fallen to only 73 because three adult killer whales reported missing in July are now presumed to be dead.

The Washington state-based Center for Whale Research said the killer whales, also known as orcas, were from an extremely endangered population that frequented the Salish Sea in southwestern Canada and northwestern Washington state daily in the summer months. But a lack of Chinook salmon means they are rarely seen in those waters anymore.

“They aren’t getting their prey, and that’s the problem,” said Michael Weiss, a field biologist with the Center for Whale Research.

Weiss said this has been going on since the ‘90s. “That’s when the population was close to 100 whales, and since then, there’s been a steady decline.”

The South Fork Nooksack River in northwest Washington state is home to nine species of Pacific salmon, including Chinook, according to NOAA’s National Center for Environmental Information.

The area used to have an abundance of fish, but the numbers have drastically declined because of climate change. Weiss said overfishing also plays a role.

The National Oceanic and Atmospheric Administration’s National Marine Fisheries Service estimates that there are about 50,000 killer whales globally, with 2,500 living in the eastern North Pacific Ocean.

The Southern Resident orca population is a large extended family made up of three social groups, called pods and named J, K and L. The three missing whales are J17, K25 and L84.

J17 was not in good condition last winter, researchers say.
Dave Ellifrit/Center for Whale Research
J17 was not in good condition last winter, researchers say.

J17 is a 42-two-year-old matriarch and the mother of Tahlequah or J35, a killer whale remembered for carrying her dead calf across the Pacific for more than two weeks last year. The Center for Whale Research said J17 wasn’t in good condition last winter, and researchers assumed it was because of stress. J17 also leaves behind daughter J53 and a son, J44.

K25 leaves behind two sisters and a brother.
Dave Ellifrit/Center for Whale Research
K25 leaves behind two sisters and a brother.

K25, a 28-year-old adult male, was also not in good health last winter, according to the center. He has two sisters, K20 and K27, as well as a brother, K34.

L84 was the last of a female lineage of 11 whales, 10 of whom have died.
Dave Ellifrit/Center for Whale Research
L84 was the last of a female lineage of 11 whales, 10 of whom have died.

The 29-year-old male, L84, was the last of a female lineage of 11 whales, 10 of whom have died. The center said none of the whales from L pod has entered the Salish Sea all summer.

Weiss said the center is going to continue its research efforts and advocating for funding for habitat repair in salmon spawning areas.

CNN’s Gabrielle Sorto contributed to this report.