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.
This knife in particular is forged by Masaharu Shimizu, in Echizen city, Fukui prefecture. He is one of only a few blacksmiths in Japan who forge this type of knife, and in 2013 he was recognized by the Emperor for his contributions to blacksmithing.