A neighbor once had a hawk raiding her pond;

she added big-leaved water plants to hide/camoflauge the fish & installed a sort of windmill/pinwheel thing that moved intermittantly with the wind.

You could frighten a smaller bird away with an owl or snake decoy, but I don't think a crane would notice-

You might call your county agricultural extension agent & ask what would scare a big bird.

Best luck.

At least temporatily, lay a piece of ply wood or something over a section of the pond to give the fish protection...

Then maybe set up one of those intermittent noise makers?
I love the cranes....but let them eat minnows!
Linda C