I can certainly understand not wanting the birds to eat expensive fish! Of course, the birds have no concept of the value of koi, and are simply doing what they do best: find and eat fish.
To get herons to pursue their livelihood elsewhere, people have tried a variety of techniques. Unfortunately, we at the Cornell Lab of Ornithology have different goals, and actively encourage herons to visit our pond here in Sapsucker Woods. For suggestions, visit the Royal Society for the Protection of Birds web page on deterring herons. The Gray Heron of Great Britain is quite similar to our Great Blue Heron, so ways of dealing with either will be quite similar.
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