Tuna is an extraordinary fish and is one of the most respected. This tuna tail is cut off to be examined by the shop owner, as he checks it quickly (by sight) to access the quality of the fish. Over the years and from experience, an expert can watch the tail section as it has tell tail signs of the fish quality. They are looking to establish the fattiness and general condition after the tuna has been caught and before the auction.
Grading from tail sample gives good information on meat color: any brownish color means poor quality and the meat color may change after being exposed to the air; a change from a dull colour to a more bright red which means good quality.
In fact there are several ways to judge tuna and often fish are cored by some fishermen to sample from the core of the tuna. This is done using a long, thin metal instrument called a sashibo. This probe is plunged into the fish above the pectoral fin, along the lateral line, whereby a tube-like piece of meat is extracted. Then compared to the tail sample, considering its overall texture, including color and visible fat.