Patagonia as we know wasn't always a fly fisherman's paradise. Just like New Zealand, Patagonia had no native trout or salmon so they are all imported by man. The idea of pristine rivers with no trout to be found for millions of years is hard to grasp.
It all started early 20th century when the first trout and salmon eggs were brought to the area. Read more about the history of trout in Patagonia here.
Here are two podcasts with great info about fishing in Patagonia I really enjoyed: part 1 and part 2