Killer whales prefer to eat only the biggest, juiciest Chinook salmon they can find. The larger the fish, the more energy a whale can get for its meal.

Each year these top ocean predators consume more than 2.5 million adult Chinook salmon along the West Coast. Except for the endangered southern resident population in Washington, all other fish-eating orca populations that live along the coast, called “residents,” are growing in number. Northern residents along the British Columbia coast number more than 300 whales, for example, while Alaska orcas are close to 2,300 individuals.

But large, old Chinook salmon that orcas crave have mostly disappeared from the West Coast. A new University of Washington and NOAA study points to the recent rise of resident killer whales, and their insatiable appetite for large Chinook salmon, as the main driver behind the decline of the big fish.

The findings were published Dec. 16 in the Proceedings of the National Academy of Sciences.

“We have two protected species, resident killer whales and Chinook salmon, and we are trying to increase abundances of both — yet they are interacting as predator and prey,” said lead author Jan Ohlberger, a research scientist at the UW School of Aquatic and Fishery Sciences. “Killer whales don’t show a lot of interest in Chinook until they reach a certain size, and then they focus intensely on those individuals.”