Based on the grading system that I outlined in my previous post, Grade #1, sometimes called sushi grade tuna is as high as it gets. The ideal scenario is quite simple: the fish is caught, drained of blood and treated and chilled immediately. If there are no complications the fish should be in perfect, undamaged condition with no internal sicknesses. Deviations from this ideal scenario are common and fish that was once in pristine condition can be physically damaged in shipping, temperature abused, or can begin to decompose. These sources of degredation need to be minimized in order to save the fish from being down-graded from a #1.
Please consider this point of view:
Freshness is the most important factor.
The longer the fish spends out of the ocean, the more the meat becomes discolored, the odor becomes stronger and the texture of the meat gets softer. Discoloration starts from the skin and belly and is most apparent when the fish is cut into a loin, however you can also see signs when the fish is still whole. Discoloration, odor and softness lower the value of the fish significantly. In addition, any disease that the fish has, such as sashi or cancer, will spread in the time it spends out of the water.
Fishing trips can be anywhere between a day to a month, based on the size of the boat. Day boats are normally smaller and use a rod and reel in what's called the handline fishing method. Although a shorter trip means a fresher fish, the nature of fighting with the fish as it tries to escape causes the core temperature of the fish to rise. This results in what's reffered to in the industry as "burned" meat, a condition that can be minimized if the fish is brought in after only a brief fight.
Longline vessels, on the other hand, can go out for up to a month at a time. For example, a boat leaves port, takes a week to reach a fishing ground, fishes for two weeks, then takes a few days to return. As you can imagine, the first fish that's caught may be two to three weeks old by the time it reaches the port, while the last caught fish may be only a few days old. These vessels use the longline fishing tecnique, which consists of dragging a long line with evenly spaced hooks behind the boat. This method allows the fish to cool down before being hauled out of the water, so although the although the fish caught on these boats is not as fresh, they are generally less "burnt" than fish from smaller boats.
After the fish is unloaded from the boats, it may be shipped out to The United States from its point of origin in a day or two. Next, importers receive the fish and distribute it to the wholesale seafood distributors, who either hold it in inventory or sell it immediately to restaurants. As you can imagine, there is already variation in the freshness and quality of the meat at this point in the supply chain.
- Gutting and chilling
For obvious reasons, fish tend to be distressed when they're brought onboard. At this point, the fish can cause damage to its bones and meat while flopping around on deck. This results in cracked bones and blood clots, causing the meat to “break” as shown below (the breaking is the gap you see between the muscles) THERE IS NO PICTURE CORRESPONDING TO THIS TEXT. Breaking is even more apparent once the meat is cut in to sashimi slices.
Therefore, once onboard, the fish must be drained of blood, immobilized immediately and the nervous system must be killed. Next, the guts must be removed and the fish must be cooled down as soon as possible by covering it in ice or dumping it in ice water. The core tends to be the hottest, so the best way to cool it is to fill the stomach cavity with ice. The sooner this is done, the less likely the meat will come out burnt. From this point on it's a matter of how you can slow down decomposition and control further damage.
- Environmental water temperature
Fish that normally swim in colder climates are less prone to sicknesses, have higher fat, and firmer texture. As you'll remember from my first article on sushi grade tuna, firm and fatty tuna make for better sushi, so fish caught during the colder seasons are generally higher in quality.
Fish that live in warmer climates on the other hand, are more prone to some sicknesses such as sashi, which is not harmful to humans, but is not pleasing to the eye and should not be present in sushi grade tuna.
The fat content and color of the meat can vary based on the fish's diet. Tuna that feed on mackerel and herring tend to have a higher fat content and the discoloration process is slower than in tuna with other diets, making these fish ideal for making high quality fatty tuna sushi.
On the other hand fish that consume leaner fish and squid are more lean themselves and may have lighter red color to the meat. Although fatty tuna is generally preffered when evaluating sushi grade fish, variations in color are a matter of preference. Customers on the East Coast tend to preffer the deep reds of fish caught in Sri Lanka, those on the West Coast preffer the lighter reds of Southeast Asian fish.