This is how it has been explained to me. Salmon genetics can and do express themselves in a fairly plastic manner. They can adapt quickly, or develop characteristics that are detrimental to survivial quickly. This isnt the same as a human having one offspring at a time. A salmon has a couple thousand offspring. Each one expressing somewhat different characteristics. Those that survive are rewarded with reproductive opportunity. Those that dont survive are not. So a cross with a fish that skipped a step in the survival fitness test (hatchery rearing) have survivors that would not have otherwise made it to reproductive age. These fish are less fit for survival in a natural cycle of reproduction after a single generation. Repeat, repeat, and you have our current dilemma.