Are you referring to a spillway,(heavy rain event), or overflow pipe (normal discharge)? They will certainly escape either way, but while there isn't much you can do to stop em' escaping while water is overflowing the spillway, you can install a barrier around the pipe....just be sure and keep it clean.
And just for the record, I have it on good authority that new stocker HBG will escape through a 4" PVC pipe that's only running half full.
