Maybe our good friend Dr. Dave Willis from SDSU will check in and see this.
Somehow, I don't think that South Dakota box turtles would have much effect on a pond. I'm also wondering if they are truly box turtles. Usually, they aren't much into living in watery homes. Can you post a picture?
In any event, I would not try to eliminate them. Most turtles don't eat a lot of fish, and they eat a heck of a lot of undesirable critters.
Here is a great booklet about South Dakota turtles:
A field guide to South Dakota turtles. I too have to ask -- what kind of rats?
Good luck,
Ken
