While I don't have the ability read fish minds, I do think that the decision to migrate is a response to low flows, high temperatures, low productivity. In at least some situations, when flows are increased, streams cooled, and productivity bumped up mykiss stays home.

That is why the observation in western WA that setting decent fall flows for spawning benefits salmon but not steelhead because the water used to increase the flows is generally cooler and we still starve streams for nutrients.