They are too slow to catch fish, in most cases, and don’t have a large, negative effect upon your fish population. Large snapping turtles will eat a few fish but typically not enough to cause a problem. The two most detrimental things turtles can do to your lake are: first, steal the fish food away from the fish you want to feed; and secondly, raid the nest of a fish and eat the fish eggs. Think of turtles as “the natural cleanup crew” of your pond.