Damage
While it may seem logical to assume turtles will hurt the fish population of your farm pond, studies have found that most turtle diets contain less than 5-percent fish, according to James T. Davis in his "Turtle Control in Farm Ponds" study for the Alabama Cooperative Extension. In a farm pond, they can compete with the fish for food sources such as crayfish and various aquatic insects, although this shouldn't affect your fish population in severe numbers. If you happen to have a farm pond near a rice field, you may encounter damage to your young rice crops by these turtles feeding on them.
Benefits
Most turtles are not a pest to humans or the ecosystem of your farm pond. Many of the turtles that may be inhabiting your pond have a primarily vegetarian diet. The turtles' many benefits include that they may eat sick or dead fish while also eating dead or decaying organic or animal matters. This alone helps keep your farm pond cleaner, while also improving your fish population by culling the diseased and weak fish.
Types of Farm Pond Turtles
There are plenty of turtle species that will see your farm pond as the ideal environment to make their home. Pond and marsh turtles including sliders and painted turtles will relish your farm pond's slow moving waters and muddy bottoms, particularly if there are large rocks or fallen tree limbs to bask on. Snapping turtles may also inhabit your pond as they, too, enjoy slow-moving waters and a soft, muddy bottom in which to rest.
Control Methods
There really is no way to completely keep turtles out of your farm pond. Trapping is the most effective measure of control, and there are several humane traps on the market. Shooting, while still used some times, is ineffective and illegal in most places. When you do trap the turtles, consult your state's local Department of Natural Resources or similar governmental institution to check on laws for removing turtles from their native habitat and to ask where you can release the trapped turtles.
You can also mow around your pond to make the pond look less attractive to turtle populations. All turtles lay eggs and many see the dense vegetation around a pond as an ideal nest site.