I fight annual battles with Herons on behalf of my pond. Herons breed on the Derwent not far from me. They know my pond is here and they come back time and again. I have lost a few fish, but have largely got on top of them now, hopefully. I have planted a lot of deep water hawthorn and that has grown really well. I have lillies and other floating cover too. Also put in a couple of floating reflective balls on advice of a neighbour. Apparently they dont like them. My dog has chased a couple away but they still come back.