Hunting is available at 436 units (401 national wildlife refuges and 35 wetland management districts) of the National Wildlife Refuge System and almost 20 national fish hatcheries. Hunting is a priority public use at national wildlife refuges. Wildlife hunting is subject to sustainable limits and sometimes used as a management tool to keep wildlife populations in check. Hunters' purchase of Duck Stamps helps buy conservation lands. Hunters must have an appropriate state license.