They have this one product called a scare crow (no not a man made of straw clothes and hay) it's a motion sensored hose type thing like a sprinkler but if something goes near it it aims and shoot water at them. It might help it might not but hey it might stop them from having you prized koi as they meal. I will try to look for it on the net to show it to you its ment for dogs and cats it should work on a GBH I think.