Re- Introduce an Apex predator such as wolves &/or cheeters(evolved in the Americas then spread to rest of the world via Bering Strait land bridge at end of ice age.)
Reasons---
-- To inprove the deers genepool by weeding out the sick, injured & most importantly the week & disformed from the avaible breeding stock.
--Hunters mostly want either the best trophy animal &/or the healthiest young animal to eat. Often letting inferiour animals in their sights passby unharmed so as not to scare off the target individuals. Both of these senerios, will greatly reduce the percentage of young healthy breeding animals. Over time the deer will become stunted,runty looking,slow and a drop in fetility or at least waysa major increase in the number of fawns that fail to thrive/ grow and die from infections &/or parasites.