Rearing in the hatchery will select for fish that survive in those conditions. If they use, for example, water that is the same temperatures as the river will be less damaging than if they use water of a different temperature. Fish are cold-blooded and their enzymes have optimal temperatures to funtion. Incubate and raise them in warmer water and you select for that inheritable trait. In the wild, in cold water, it becomes lethal or sub-optimal.

The greater the difference between the hatchery environment and the wild then the greater the impact when those hatchery fish "go native".