That is unreal. Too bad you are a ways from me I know places that are completely overrun with them, take all you want for free. And they are so congested the plants fight for sunlight and get almost 2 feet tall. I would not worry about caging them up, you should not have to.
I would be way more worried about what killed them off. You must have some nasties in that water to do that. To be honest, I have never seen water bad enough to kill them. Never. You either are going to have to get that water back to neutral or get the Washington Monument or the Taj Mahal behind the ponds.
Cold water temp will knock them back, but not kill them. What's your surface temp and what is in your pond water as far as treatment chemicals?
