It seems to be a seasonal thing. Most of the year they rely on chinook, but there are times that chinook densities are quite low. It seems that they eat a lot of coho in the Salish Sea in September, and then in October and November they take advantage of the chum run and eat a lot of chum, and then they go back to chinook for the rest of the year.
On October 30th, 2018. See this statement in context.