I've got a small Koi pond that has been doing well for several years. Three Koi about 1' long and 10 were about 6" long. The other day I noticed a Blue Heron take off when I let the dogs out. Checked and ALL of the fish are gone. Now it appears he has taken up residence and obviously I'm not going to restock until he moves on. Is there a simple way to prevent these guys from getting to the fish without screening or fencing around the pond?