I caught a runner in the Keys a while back that had a 3/0 hook in it's mouth and had apparently broken the light line the fisherman was using. The hook was still shiny with no rust, but where it was in the fish's mouth, the acid in the fish's system had all but eaten through the metal between the barb and the shank. Mother Nature's way of helpin' 'em out I guess.
By the way ... I love circle hooks ... just had to get used to the idea of not tryin' to cross their eyes with a hookset