Copacabana on Bolivia's side of Lake Titicaca (not the one in Rio!) is the place to eat trout in Bolivia. While not a native fish to the lake (it was introduced in 1939 to provide a source of protein for the locals), it is now a solid part of the diet, with pretty much every restaurant in town offering it as part of their set lunch menu. We definitely ate our fair share, and it was generally pretty good, but we occasionally craved something else.
The hostel where we stayed also operated a nice restaurant with views over the lake and some interesting Andean dishes, which provided a welcome relief from the endless stream of trout we were shoveling into our mouths. This is what we ate there.