05-09-2008, 10:28 AM
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I found one of those motion-sensing water sprayers for sale in the UK: http://www.koicarp.net/koi_accessories1/scarecrow.html
A sturdy netting can also be effective. Here in the States, my grandparents had a koi pond and had a similar problem with Great Blue Herons. They used a fake plastic owl on a tree branch as well as a fake plastic heron. Their pond was also fairly deep and wide at one end, so the fish would hide down there if threatened.