If you want the easiest way to break a Mithran Fishing rod. Have a friend who is leveling bonecraft make some fly lures (trust me they will be happy to get rid of them) for you. Step out to East Ronfaure and move down to the river and fish. The target is actually Arrowwood Log which the Rod will snap 75% of the time (100% when I tried it). You might catch a couple Cheval Salmon or Shining Trout but that Arrowwood Log will ruin your rod fast.
Do you know if any other bait can be used or if you need to have reatched any specific fishing skill lvl? only gotten cheval salmon and monsters so far.. but there is something i cant get up before the time runs out