I looked it up awhile back. What I found says it's generally offered starting in mid-May. Can't remember how long it was offered. I have a reservation June 7 and am looking for some of that fish.
If I remember correctly, it's so expensive that I will probably just have the fish and wine.
Copper River just came out and here in Seattle is going for $39.99#! It is only available a very short time, if you can get it, good luck, and enjoy it!