4salt -
Interesting observation, I have actually thought about that situation some for the last several years.

Salmo is correct in that there is a fair degree of partition of the rearing habitats between coho and steelhead in the tributary streams. That is not to say that if one species was at low abundance the other would not use some of that habitat. In the case of the Snohomish I doubt that relative abundance of coho and steelhead have had much to do with the decline of steelhead.

On the other side of the coin for decades the biomass being returned to the Snohomish by the natural spawning salmon was typcially between 1 and 2 million pounds. In the last 4 years that has increased to between 3.5 and 7 million pounds of carcasses. If the amount of salmon carcasses is an important factor in over all productivity of the river we should be seeing benefits soon.

The situation with the wild steelhead on the Snohomish likely very little to do with the management of those fish. The last few years the over all fishing rate on the wild steelhead has been about 3.5% (hooking mortaltiy in WSR and tribal commerical catch) while the fishing rate on its chinook have been in the 20 to 30%, and for coho in the 30 to 50% range. The wild steelhead by far are being managed the more conservatively than all other anadromous salmonids yet they are the only ones not doing well.

Tight lines
S malma