Lou,
Don't give up! I was wondering if you could, every winter,
temporarily put landscape mesh over the pond. (metal)
I remember yours is raised, and doesn't it have a level, even edge where this would work?
You could make a wood frame for the wire mesh to be easily lowered over the pond, custom fit.
(I know you are handy when it comes to these projects!)
Just thinking .... there HAS to be a solution to this problem!