The FHM I have observed generally try to head upstream. There will be some loss out the spillway, but usually not enough to hurt the numbers significantly. You might lose a few BG fry. As I stated in another post, I have never seen adult BG or CC go out the spillway downstream. It's more likely that any fish in downstream ponds will try to swim upstream to yours, given sufficient depth. Other than FHM, almost all migrating fish into spillways will be juveniles. Grass carp are the exception. They will try to swim downstream out of the spillway given sufficient water.
A neighbor of mine got gizzard shad in his pond a few years ago from a creek attached to the White River about a half mile away. This upset him initially, but a few months ago his son in law caught a LMB over 11 pounds. It's a 3.3 acre shallow (5 to 7ft) borrow pit.
