The link does not say who made this estimate or the methodology used, so it is difficult to respond to these estimates. Further in the note the author suggests potential production of a bunch of different species based on the historical amount of spawning gravel available and says that "I do not have a present estimate we will eventually have for historic Yakima steelhead based on spawning gravel, but I can guarantee it will be well above 100,000."

Really only one thing to say....no one I know in the field thinks that the amount of available spawning gravel controls steelhead production.