At the fishing tourney today Ducky4408 suggested this, and said to suggest this, so all credit to him. There should be a level up to fishing much as there is one to farming, enchanting, etc
Fishing currently contributes to the farming mastery. Until we add more fishing mechanics (which we would like to do at some point) there is little reason to add a fishing mastery. However, when we do, we'll look into adding one.