I did something like this to prevent fish from going through a 15" drain tube from one of our ponds. Dave is correct about debri potentially blocking the spillway if you put the fence right through it. I built a 12-ft square fixed dock over the pipe and placed the fencing on the dock posts. This provided me with 36 ft of fence that is 2.5-ft tall which provides much more surface area with lower flow rates through the fence. You could put posts in your pond so that the fence is extended into the pond and beyond the edges of the spillway. The trick is making something that looks ok. I added my fence when I kept finding lots of bass, cats and bluegill in a pool below the pipe after heavy rains. Like Dave said, I did not think they would dive into the pipe, but in my case, they did. I do not know how many washed away into the swollen creek below the pipe, but there were plenty left in the pool to rescue when the water went down.