With a sharp knife, begin to cut the hard material away from the fish starting at about 1/3 the length of the fish from the tail, cutting towards the pectoral fin. How To Butcher a Whole Tuna: Every Cut of Fish Explained. Make a deep cut in front of the tail on both sides of the fish. Once you get to the fin, cut into the shoulder. Here's another in Bon Appetit magazine's Beatiful Butchery series, this time focusing on how to butcher and break down a whole tuna, including how to fillet tuna.. Our guide for this edition is butcher and fish market owner Yuji Haraguchi.He sets about the tuna using five different knives, breaking down the colossal fish into all the cuts of fish you see at sushi restaurants. Cut the tuna from the head all the way to the tail. Use a strong, sharp filleting knife so you can cut the fish as easily as possible. Do not cut into the fish, rather, shave the scale from the surface of the flesh while pulling on the pectoral fin. Yuji Haraguchi is a butcher and owner of the fish market Osakana. Avoid making a sawing motion with your knife, as that could tear up the meat of the fish. 6.