Duke,
In the future looks like you need to find yourself a reliable trapper to consistently take out your rats every trapping season. It shouldn't cost you a dime, and in fact you may be able to recieve part of the money from the sale of the pelts if you want to. Find the link to your state trappers association and you shouldn't have any trouble linking up to a local trapper.
You may also want to consider gettng rid of their food supply if feasible to discourage them from coming to your pond. Cattails are a prime food for them. Muskrats migrate into ponds in the spring and fall.
I just got a new influx of them this sprng took one out yesterday with a conibear. I was too lazy to get rid of all my cattails last year and lo and behold the muskrats are here.
