In the case of trout, if some make it through the summer and find suitable habitat, natural reproduction might occur.  Whether or not those young trout make it through the first year of life depends largely on the habitat, particularly summer water temperatures. Many of the streams we manage with stocking of trout are at their very best seasonal trout habitat. They lose flow and experience elevated temperatures during much of the summer and trout don't do well there.

We do not stock hatchery trout to spawn and provide any contribution to the wild trout community. This would be too expensive. We place them out there for anglers to have a chance to fish for them and to catch them. In several places we stock fingerling trout on a put-grow-and-take basis. In those cases the habitat is suitable for year-round survival, but spawning and nursery habitat may be missing. So we add the stocked trout.