But why do pacific salmon go through all these changes and die after mating only once, while Atlantic salmon can do the same thing but survive to mate many years in a row?

Yeah but that only takes a couple months even in bad conditions.

Naming the book would be a spoiler because the spoiler was already described. The only way to not spoil the book is to not know what book it is that was spoiled in the description.

This won’t affect us on earth. We will just be able to see it.