I live in the middle of the swamp...great blue herons, egrets, bitterns wander through the yard all the time. Last year I had losses of my favorite tancho koi and a large number of goldies. Also lost some small fish to snakes. Since then, I keep a net on the pond at all times. They can stab through the net, if they really are determined to get the fish, but it has worked as a deterrent here. Om my above ground pond (where my more vulneralbe fantails are) we keep the net up on a frame, secured with those coffee mug hooks, A heron would have to figure out how to get under the net because it sits up about two feet above the pond, circus tent style.
Also...dogs! My dogs see the pond as "their" turf and they routinely patrol.
There are motion sensor devices that will spray water whenever anything gets near. My step mother uses one of those to keep her dog out of her flower berds, but I don't know how they work on large water birds.
