i have had hundreds of butchers work for me over the years, very few handled tuna correctly. we always had miles of 3.5mm mono (mainline) laying around in tangles, it works well for separating the vertibre. big eye is by far my favorite fish. fish from water less than 65f contain the color and fat content to bring the good prices in japan. generally north of 36n and south of 36s have the best quality tunas. good big eye fishing is very often close to good swordfishing, the prefered temps are similar. bluefin prefer colder (less than 60f) and yellowfin warmer (more than 70f).
try cutting albacore into chunks, stuff 1 qt. mason jars with fish tighten lid and pressure cook for 1 hour. by far the best tuna salad you will ever taste.can add peppercorn, garlic, etc.