Bill,need a little info.Are there den holes ?These should be visible on a sunny day.110 or 160 body grippers can be set in front of these,they will take muskrats entering or leaving the den.
Are there feedbeds?These are bits of cut vegetaion in small mats or piles(though with large populations,these can be large)these can be set with foothold traps,my preference is the #1 stoploss.These can also be set where the rats are coming up on shore.Also can be used at their toilets.
WE will turn you into a A#1 rat man.
Tom