Wow, did you actually do that? I'd love to chat with you about it sometime. Many years back when the obsession was trekking and climbing (and fly fishing just a twinkle in my eye), I spent some time along the Urubamba. It was running heavy chocolate color when I was there and I couldn't imagine a trout being in there.
Up in the Cordillera Blanca I fished a tiny lake near our base camp and couldn't keep the trout off a spinner. I was so proud to bring back a bunch of dinner fish for our group of seven, but get this -- a native had just been by and sold them a bunch of trout!
Hearing about Peru reminds me of good old times