Bob -
DimeBrite has it right.
The Nooksack, Skagit, Stillaguamish and Snohomish all have chum salmon. The fry from the Skagit, Stillaguamish, and Snohomish find their way onto the beaches of Camano and eastern Whidbey in the spring of the year. There is a strong odd/even year difference in abundances with more chum fry typically on odd years. This is balanced with the presence of pink fry on the even year springs however the pink fry migration while over-lapping the chum fry tend to be a bit earlier.
While hitting the beaches with chum/pink fry patterns can produce some good fishing it doesn't seem to be quite what the folks in south Sound enjoy. This due in part to the current depressed levels of chum on those North Sound "S" rivers and differences in the sea-run cutthroat behavior; an extended freshwater stay. Sand lance and surf smelt can be a more consistent forage for much of the season.
The vast majority of the cutthroat on the beaches take place from early March to early October with the summer months being the peak of the fishing. During the same period there can be seasonal peaks of bull trout sub-adult and adult fish on the same beaches adding some diversity to the catch as well a shot at a larger fish. While some years resident coho can be found on Camano and Whidbey beaches they are much less dependable and typically later than down south.
Curt