Habitat also plays a role. Most do not know but the Late Coho in GH & Willapa are from the East Fork Satsop and primarily Bingham Cr. You a number of ES streams go dry in late Spring as the aqua-fur drops and do not water up until nearly Dec thus any Coho that was to spawn in those streams needed to be late. It is simple natural selection driving the adaptability. If you could water up those streams in Sept then the normal timed Coho would utilize it also and probably not to the benefit of the lates. Lates also have a much lower reproductive / survival rate than normal timed Coho here.