Predicting Deer Population

As an ecologist, you want to respond to rumors about overpopulation of deer in the forest. Since you cannot catch all of the deer, you decide to go to random spots in the forest and place a harmless tag on each deer you come by. At the end of the day, you end up tagging 25 deer. What next could you do to with this tagging system in order to predict the entire population of deer in the forest? Explain the necessary procedures and how you would determine this estimation.