Heron Stop from United Aquatics 

A simple and nearly invisible deterrent helping to protect pond fish from herons and cats. Place the stakes around the pond, 10" - 12" from the pond and about 5' apart, with the stakes pointing in towards the pond. 

Thread the nylon string through the holes and fasten the bells to the upper string between the stakes. 

The kit will protect ponds with a circumference up to approx. 30 ft