this depends on alot of things. you need to consider the area your going to be targeting them, whether it be more open water such as lakes or res. or whether it be a small river with lots of structure, or even a large river with more open water and less structure. you also should consider the type of tackle you will be using including the action and length of the rod. there is alot of variables to consider. i use mostly 30 and 40 pound big game, but also spool with 65lb. power pro for certain situations. you can get away with smaller diameter line at times, but i think if you are gonna probe them areas that are full of rock and timber, i would go with a larger mono such as 40lb. big game when you need to lay the meat to them, and larger diameter is obviously better for abrasions...good luck... rollo