If the idea that the pollutants are causing Chinook to get younger and smaller can be tested, at least to some extent, by looking at the age and size changes occurring in the New Zealand Chinook. Are they showing the same decreases in both age and size.

I do agree that natural selection will be for what survives to spawn. Life will try to adapt. At some point, though, on the Yukon the fish will be too small to successfully migrate to the spawning grounds and to successfully spawn in big rivers with big substrate.