Actually maybe those two particular ducks. I've had ducks before and never had a problem until last summer just after they showed up. If they show up again I'm going to put the fear of God in them. It could be other factors too such that I am not aware of as in the pond was recently drained and refilled prior to the swimming, no reproduction in the pond other than fathead minnows, and a bad infestation of Sago Pondweed.
I'm gong to eliminate the Sago Pondweed with a herbicide so perhaps that will help this summer. I sure hope so as the wife looks forward to swimming for exercise as she has bad knees.
Cecil, you are scaring me
