Chances are, fish predation by a heron or a cat is still occurring, even though a “deterrent device” has been installed.

Herons are smart animals and unfortunately do not always let an artificial heron deter them. As decoration, however, you can leave it in place. Much more effective is to use, for example, the Pond Protector Pond Protector, a harmless scare wire that the heron runs into with his paws. When touched, it triggers a startle reaction, causing herons but also cats to prefer to avoid the pond in the future.

Velda also has the Laser Guard in the range. With a.o. laser beams, it is a very effective product against herons and other threats to fish. See here for the entire range of heron scarecrows.