(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.

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