They capture the bison that migrate onto private land with cattle. Bison carry a brucellosis disease that if the livestock catch, they will abort all their calves. They have been doing this for some time now. Some years its a lot and others its a few bison. Once again its all stemming from landowners pressuring that something be done. Its a pretty simple system if you haven't seen it done before. I assume that's why they won't offer more tags (which would be nice).