I live on a 15 acre lake and have dealt with the muskrat as you are now. Muskrat traps are not that expensive and very easy to place, in the first 2 years of trapping we got 135 of them. since the numbers have dropped to 10-20 a year. We'll place a trap when we find a den or see one swimming around and follow him back to his area. Again it's not hard to do and all you need are the traps and some waders. After you've eradicated the pests from the hole you want to plug it with rock. Laying old chain link fence down and laying rip-rap over it in problem areas also makes it all but impossible for them to dig a den, they leave and look for easier digging.