Young pike are quite small and are eaten by pretty much everything that eats fish, including snakes, otters, a wide variety of birds (e.g., herons, mergansers, loons, ospreys, kingfishers), turtles, dragonfly nymphs, bigger fish, etc., etc. As they grow (if they survive, that is), their predator pool shrinks accordingly. Adult pike, which reach over four feet in length and are fearsome predators themselves, have no significant predators other than humans.
