A letter to INFORUM about the new management plan says the herd should be large enough to have a healthy breeding population but reproduction should be limited with PZP, not GonaCon.
If the mares can’t bear fruit, why are you worried about herd size?
If you poison them with the pesticide, as the advocates are doing on the Virginia Range, the population will need to be larger than 150.
As for other national parks where you can find wild horses, don’t forget Cumberland Island and the Ozark National Scenic Riverways.
Sadly, the herd on the Maryland side of Assateague Island, mentioned in the letter, has been ruined by a safe, proven and reversible darting program.