As I am your local poo face, I thougth I might keep my idea to...

As I am your local poo face, I thougth I might keep my idea to myself but ive been called poo face many times so i dont care. Ill share.

Put a motion detector sensor looking across the pond a few metres high. Connect this to a small motor/solenoid that has a long thin rod connected to it, with a bit of string hanging off it and a bit of feather/ball or something bright and light on it.

When the birds fly down when they get a few metres above the water this thing will trigger off and the stick will come flying out with the bright object swinging around all over the place and put them right off their bomb dive and scare them and the fish too.

Seen this done before and it pretty much solved the problem.