Correct me if I'm wrong, but I'm pretty sure there were sturgeon in that river before it was impounded. Freshwater sturgeon can grow to over 1500 lbs., and they don't come to the surface often. I don't know if any of them could have survived the damming of the river, but if so, as deep as that lake is, I could see where some might have survived, and grown huge, and rarely been seen.