I have a pond & over a 100 fish & i would be sitting in the Lounge & watch 'Colin' the Heron ( Yep the Mrs gave him a name) pinch my fish sometimes three in a row, also had a King Fisher that did the same.
Bought myself a Heron Scarer which cost about £50 & squirts a jet of water at anything that breaks the beam. My Mrs getting caught when she goes to feed the fish is a positive side to this system , although ive been soaked myself.
Can recommend it cos it works, but does require a mains water supply a great little gadget.
Rod