I use deer netting - the black plastic soft stuff, weighted down with rocks. I suspect they don't want to mess with getting tangled up in the stuff. It is inexpensive and even managed to catch a couple of snakes (yuck!). On your small ponds you could tuck under the bottom - also can leave loose in case water lilies and other plants get taller. I used water hyacinths to keep netting from hitting the water otherwise food gets caught and fish have trouble eating it.
