If its practical, perhaps rebar could be placed over the pond, it could be used to make an a-frame, or flat...
Cover that with plastic and you have both predator, and a cold frame to improve winter protection, at a time when fish are more vulnerable to predators
While your fish are behind bars and under a cover, Mr GBH won't be able to know they or the pond is there, let alone catch anything even if they do...
A bit drastic, but doable
Regards, andy
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