Yes, they will work on the fish in any sort of impounded water feature such as a small lake or a pond where the fish cannot escape. I learned a little something about this when some buddies and I were working to stock a two acre pond with bass, crappie and catfish. Once we got it stocked pretty good the land owner began to see otters more frequently and saw the same type of evidence that you are that they were working on the fish and even the smaller ducks. Since the landowner wanted them gone... I began to sit on the hillside a hundred and fifty yards or so away with my rifle to watch for them... and it wasn't long before I noticed a catfish (about a three pounder) suspended upside down out of the water and heading for the pond dam... which by the way is quite a strange sight. When I looked through the binoculars... I could see that a good size otter had a death grip on him and (otters) apparently turn these bigger fish upside down to calm the fish until it can get them to the bank to kill them. When he was dragging his prey up onto the pond dam I put a 7MM bullet between his shoulders and the catfish flopped around until it found its way back to the water. This particular otter was a big male and (guessing) weighed about 25 or 26 lbs.
Otters are very aggressive swimmers and can out maneuver and catch most fish even in the deeper water.
"Just Like Iron Sharpens Iron... So it is that One Man Sharpens Another Man." Proverbs 27:17