Maguro Bocho (tuna knife) is a knife specifically made for breaking down whole tuna. They can vary in size anywhere from 400mm to 1500mm. In some cases you can see two fish mongers handling one knife.
When you are dealing with expensive cuts of fish, you require a special tool to get the job done. This knife is designed to make long clean cuts, meant to keep the flesh of the fish as pristine as possible. Maguro bocho sizes can range anywhere from 400mm up to a whopping 1200mm.