There are 2 ponds that are on our land along a small river. Ones about an acre and the other is about 4. Twice otters have cleaned them out. The otters will eat all the fish from the pond, then move on to another pond. This will give you time to think that they are gone, and let you restock it. Once you think everything is going good you will start to notice piles of scales lying back on the ground again. One of my friends sank a trap in his pond with a frozen bass in it and caught two in a week. Hasnt had a problem since.
More than likely the fishing in your pond will have declined dramitically if these are indeed otters that you saw.