Yes the genetic makeup of the brood changes in one generation.

A group of wild fish are brought into the hatchery, are artificially spawned, and subsequently produce a diversity of genotypes in their offspring. A small minority of those genotypes would flourish in the wild, the rest would perish. A large majority of those genotypes would flourish in a hatchery tank, very few would perish. Among those that perish in the hatchery might be genotypes that produce, for example, fish that prefer to forage near the bottom of the tank rather than at the surface. These could well be fish that would thrive in the wild, but instead die out in a hatchery tank because their surface-feeding brethren snarf up the lion's share of the available food.

It's easy to see how the genotypes of surviving smolts produced in the wild, after enduring the relentless selection pressures of the natural world, would be very different than the overwhelming majority of the genotypes of smolts surviving the cushy-a$$ hatchery environment. Note however that a very small number of the surviving hatchery fish will have genotypes IDENTICAL to the wild fish. If these survive the marine phase of the life cycle, they would be genetically identical to wild adult spawners. It is this tiny fraction of the original hatchery brood that maintains any measurable reproductive potential in the wild. The rest are simply reproductive turds.

So in the end it's not that the hatchery magically transforms "good" wild genes into "bad" hatchery genes in any given fish. What's really happening is that genes that are valuable to survival in the wild during the egg-to-smolt phase are being weeded out of the brood in the hatchery. At the same time, genes that would have been weeded out of the brood in the wild during that same egg-to-smolt life stage are allowed to persist in the VAST majority of hatchery-raised juveniles.

What goes out to sea from each environment is VERY VERY different. And yes, it happens in ONE generation.