"A couple points to remember. A certain amount of "straying" is key to the survival of anadromous species. Without that behavior how would rivers become colonized or re-colonized. 12,000 years ago most of our rivers where buried under great thickness of ice. After the glaciers receded our rivers would have been fishless and would have remained so without this "straying" behavior."

I think the above pretty much sums up the issue for 'Mother Nature.' Cross breeding by runs of fish keep the 'gene pool' freash and healthy.