They are trained quickly with a pellet gun, especially if you hide and they don't know where the sting came from. It makes them scared of the pond area instead of you so they are afraid even if they don't see anyone around.

I have considered a single line of fence 1 foot off the ground. This will deter Geese because they need to walk in and out of the water. I was thinking of putting in an electric fence of this type for Herons.