Have to agree with Todd. We now have some numbers for steelhead smolt. No idea what the mean in an historical context.

And, most of the discussion still seems to be guided by the idea that steelhead are salmon.

The really strong steelhead populations currently in existence (Situk, Kamchatka, Argentina) share one thing in commen; half or more of the returning adults are repeat spawners. Based on that, maybe a smolt-adult survival of 5% can work; if enough fish survive to repeat spawn. Remove them from the equation, and you get extinction.