So now I know (per the article below) that the fish guy at Whole Foods Market didn't just give me the sample of Paiche (prounounced pie-chay) to try because he liked me--the chain is trying to create a market for this farmed version of an endangered Amazonian river fish.
I have to hand it to them: this is GOOD. The comparison to halibut is apt only because it's mild. The comparison to Chilean sea bass is the best and spot on--it's sweet and full-flavored but not fishy, very much like Chilean sea bass but even richer. It even browned and crisped on the bottom of the roasting pan like Chilean sea bass would have (where the halibut it was next to did not). I'll definitely buy it again.
http://www.bloomberg.com/news/articles/2014-08-28/whole-foods-is-pushing-paiche-in-order-to-support-fish-farms