i did a fishery a couple seasons back for a friend of mine they had rats were coming from surrounding fields around the holding areas where thefish were kept there were good runs leading to the location , so i just snared them with the hazel benders , until all the runs went dead , caught quite a good lot , it seemed to do the trick, but all locations are not the same ,
like snareman say's try snaring them with benders, once you master the art you will be more hooked on that than the fishing, you will find all the info at the top of the page in the pinned secton.