When I read the book as a kid, I was certain that the "fish" is a narwhal. I'm rereading it now, many years later, and a quick Google search told me I was wrong all these years: the fish is a marlin. However, in the same book, the old man recalls seeing a big marlin in the past. So he knows what marlin is and what it looks like. Why does he keep referring to the fish as a "fish" and never mentions it to be a marlin?