[buttercakes]

At least down here in the PNW, hatcheries typically mark the fish they spawn by clipping their adipose fin when they are babies. When they return as adults and return to hatchery fish traps, they are sorted. The unmarked, unclipped native/wild-spawning fish are let back into the river to continue upstream. The hatchery fish are used for spawning the following years run. This creates a diminishing genetic pool and with time produces really measly, smaller and weaker fish with who knows what kind of genetic deficiencies. On some rivers, hatchery fish are trucked in from hatcheries located elsewhere and planted but never sorted on the way back in, assuming the bulk of the returning fish will be harvested by anglers.

Also, there's still the issue of genetic selection on the way out. The millions of fish that are spawned, raised in concrete pools and released back into the river have all been artificially selected to survive the extremely difficult first few months of life, which would naturally be the first genetic screening of the new generation.