He's been visiting me too Tracey, a couple Ospreys and believe it or not, lots of CROWS.
The crows have been walking around the pond pecking at the small fish sunbathing in the shallows.
They have also been pecking at the bullfrog tadpoles too. (that doesn't bother me)
I am sure the big dead koi has heron or osprey wounds on him, and the one little dead fish I found looked pecked.
They were too stinky and rotten to do a post mortem, but I could see broken skin. :'(
I have a pistol BB gun, CO2 powered. I use it to scare them only.
They seem to know a gun when they see one.
And the popping sound it makes scares them too.