Captain Kevin Beach of The Mexican Gulf Fishing Company takes you through the steps of filleting a blackfin tuna, which is a smaller species of tuna. Beginning with a really sharp knife, start cutting under the peck fin. Begin at the bottom of the fish and cut around and underneath the peck fin all the way towards the eyeball of the blackfin. Then make a cut on the tail and turn the knife to cut along the spine of the blackfin back towards the head. Then cut along the lateral line of the blackfin, angling your knife at about 45 degrees to slide along the bone. Next, do the same on the underside of the fish, making sure to cut on top of the ribs to avoid the guts. You want to cut down to the anal fins. Pull off each chunk of meat on top and under the fish. You should get four, clean-cut loins from the blackfin.