Where there is one, there will be many. Growing up on a creek and now living on one, I can tell you that they are very destructive animals. They will chew down small trees and girdle larger ones resulting in the tree dying. They will also burrow back in your banks and cause your pond damn to start leaking.
They are easy to manage espicially in a smaller pond, flashlight a .22 or shotgun and buckshot after dark work well. They are very easy to trap as well, using castor glands as lure for bank sets or you can usally find where they are burrowing in the bank by the stained or muddy water in the vincity where they are digging, wade in and find the entrance to the den and either place a snare or conibear type trap over the den entrance, almost always under water, and you'll catch him first night. I have caught them when I was growing up that weighed almost 50 lbs. They pose no threat to you sheperd, he'd kill one quickly, our English sheperd has killed them before and she only weighs approx 45-50 lbs.