Herons prefer feeding at dawn and sometimes again at dusk which is why many people who lose fish are at a loss as to their disappearance.
Maybe in some parts of the world herons are not too 'bright', but here on the Gulf Coast they are quite intelligent and cunning....and extremely patient. They can stand perfectly motionless for a considerable length of time waiting for a pond fish to come out of hiding.
Heron decoys do not work, at least in this area. As stated earlier, strategically placed monofilament and a 'Scarecrow' are your best line of defense, unless perhaps you want to adopt a large dog and get guaranteed results. I wouldn't advise a small dog as Great Blue herons are known to eat small animals also.
"Everybody is ignorant, only on different subjects." - Will Rogers