I found the same thing a few years ago but they went into the drainage ditch and we were able to get quite a few back to the pond. My overflow is small only 4 ft wide and 4 inches deep (small pond .4 acres excavated) but now I have a screen made from folded hardware cloth and covered with fish netting that I keep in the overflow. The only drawback is that I have to constantly empty it of leaves especially in the fall but since I like to walk around the pond every night it is not much of an inconvience. It has stopped the problem of fish heading out the overflow.

Bob