This one takes place when the guy next door to you reaches over the fence with a net and scoops out some of your prized Koi from your pond.
"Herons also are know to frequent back yards that have a fish market of tender fresh goodies."
I know exactly what you mean. Some of the Fish Ponds that I install in peoples yards require aquatic scare crows. They have a motion sensor and let out a short burst of water if anything comes within a certain radius of the pond.
They are very effective for the Herons, raccoons or any other potential predator.