The results of the Skagit and Forks Creek studies do make sense to me. Cold blooded animals have temperature specific enzymes that control/assist metabolism. The enzymes are fairly temperature specific and are inherited. That is you will find populations of the same species (take cutthroat, for example) that flourish in alpine cold waters and desert hot waters. Just not the same fish.

The Skagit fish were a product of the time when hatchery steelhead were incubated and reared in warm water; they were selected for the genes that functioned in warm water. it is no wonder that they did poorly in the wild, where they incubated and reared in cold water.

The Forks Creek, and many of the currebnt programs, are incubated and reared on surface waters. the selection is for a fish whose enzymes work in that temperature. Consequently, it should not be surprising that more of them are successful in the wild.

If one wishes to segregate a hatchery stock it is not only necessary to have temporal and spatial separation but also environmental. The less well a fish fits into an stream, the less likely it will be to successfully spawn. All you want them to do is come back and bit a hook.

On a broad-brush view, hatchery salmon are generally more successful in the wild than hatchery steelhead. Hatchery salmon, by and large, are incubated and reared on surface waters and are, therefore, selecting for a temperature regime at least close to the wild. If we switch steelhead programs to surface waters we will find hatchery fish being more successful at spawning in the wild.