The salmon is a truly amazing creature, whose marvelous adaptation is most visible as it jumps 10 feet over a waterfall, but whose complex and almost entirely hidden lifestyle has evolved over many millions of years. It is finely tuned to home river chemistry and food chains, changing food preferences and protective coloration multiple times. Its entire physiology shifts during migration from fresh river waters to salty northern seas. Even its parasites can't make the change and are replaced by new ones. Young salmon must somehow determine when to leave the nursery, find feeding grounds thousands of miles away, grow to maturity and return home with enormous precision.
American salmon swim east and European salmon west to Greenland.
Salmon time their lives to match local water flow and time of ice-out. Mixing genes from other rivers obviously leads to destructive errors in navigation and distorts many other finely tuned behaviors. Further genetic divergence is then inevitable. There still are a few salmon successfully returning to Down East rivers. The genes and the DNA making these a ''distinct population segment'' as defined by the ESA are still there and salvagable.