Fair Hooker -
The density dependent interactions with smolt would seem to be a plausible explaination.

However the number of steelhead smolts planted in the region has been relatively constant for the last 20 years or so. In the last 5 years there have been reductions in both hatchery and yearling coho and chinook planted in the North Puget Sound area. For example on the Snohmish the number of hatchery coho has been reduced from 300,000 to 150,000 per year and the number of yearling chinook from 530,000 t0 250,000. Why would those interactions manifest itself now?

In addition survival of summer steelhead, both hatchery and wild have remained constant or even improved in the same time period. The summer and winter steelhead smolts leave the rivers at the same time (peak out migration in early to mid-May). They likely separate at some point on the high seas as they return at different times.

Tight lines
Smalma