How big is the waterway that your spillway empties into?

If it is generally a trickle, then as the water flows subside, your HSB might work back up to your outlet. Especially if you have a decent stilling pool there.

I have no clue about them escaping the pond, but was hoping that if some did, then you could estimate their numbers and perhaps net some to return to their pond home.

Good luck on your HSB recovery project!