Genes are exchanged with each spawning. The fish has a set that it lives or dies with. The genes don't, as far as I know, change within a fish.

The combination, the entire genome, degtermines if that fish lives or dies. Say a fish receives genes for warmer water, and we have a year when water in the stream is warm. The fish lives. If another fish had genes for cold water, and the stream was too warm then it would die.