Pond electric fence
Herons and cats will fish a pond out as soon as they get the opportunity. You can keep them away from your pond by stretching electric fence. This is effective against robbers and not dangerous for people. Complete sets are available for protecting ponds. These sets contain a sufficient amount of wire, spacers and a transformer. A good set also contains a clear description.
Stretching electric fence
It is a simple and effective method. Stretch the fence round the pond side. The transformer that is supplied along with the set will give low and harmless electric to the wire round the pond.
If touched there will be a shock reaction to the effect that herons and cats will quickly draw away and will never come back to your pond.