"What is clear is that with lower numbers of steelhead, the process of hybridizing increases. I have watched this happen for more than a decade on the Nitinat with the result that cuttbows now predominate over pure searuns. Hybridization also results from river degradation due to logging."
I had to read it twice to find the information: this is all there is about cross-breeding and decline of steelhead. Sounds a bit like impression, not science.