urup,
Do you have an overflow tube on your dam? If you don't, you should, and that will limit fish loss in all but the heaviest flow events (assuming it is properly sized). As RAH says, that would be the place to put some type of screen.
Absolutely do not (repeat do not) put anything across your 6' emergency spillway. With a proper tube in place, the only time you would have water going through the emergency spillway would be a 25-50 year storm. When that amount of rain comes down at once, it will carry a lot of trash into the pond, and that would rapidly block a screen across the spillway.
If you're lucky, the fence wouldn't hold up and would wash over. If you're unlucky, the fence and trash hold, and water overtops the dam. In a matter of hours you could get enough erosion on the backside to loose your dam.
Bill
