We had exactly the same - wife came home and found Big Momma koi on the lawn gasping her last. She survived a day but in the end I had to put her out of her misery as she was just lying on her side on the bottom. As very hard thing to do.
I also notice they go for the biggest fish they can :mad: ! A friend of mine found her biggest carp on the lawn dead. Turns out a heron had picked it out but couldn't carry or eat it.
As regards heron scarers - you can buy devices that squirt water jets in the direction of any intruder. Except of course my wife would never remember to avoid them so it's a bit of a non-starter for me. I resorted to a couple of strands of wire right round the pond to a height of about 18 inches and so far it seems to have done the trick.