I know your pond is probably too big to do the entire bottom this way, but try using some typ of plastic grid in some parts for the eggs to fall into so the parents can't get to them. I can't think of the grid right now, maybe for lighting or something like that. Damn, i saw some a few days ago and thought about it for my friend's pond.
hope this might help if you can't find a way to get the eggs out. if I think of the grid or see it again, I'll let you know.