Based on the grading system that I outlined in my previous post, Grade #1 is as high as it gets. The perfect scenario is quite simple. Fish is caught, drained of blood, treated and chilled immediately, with no issue and the fish being in perfect undamaged condition with no internal sicknesses, which is not easy. Everything else just gets a little worse from this pristine condition and is either damaged here and there, temperature abused, decomposing, etc. These are the parts that need to be minimized in order to save the fish from being down-graded from a #1 to lets say a #3.
Please consider this point of view:
1. Freshness
Freshness is the most important factor.
The older (longer since catch) the fish gets, the discoloration grows in the meat, the odor becomes stronger and the texture of the meat gets softer. The discoloration starts from the skin and belly of the fish. It is quite apparent when the fish is cut into a loin however you can see signs of it when the fish is still whole. These 3 points lower the value of the fish significantly. Additionally, if there are any sicknesses in the fish, such as sashi where the meat starts to melt, or cancer, that sickness will start to expand as well.
Based on boat sizes and fishing methods, fishing trips can be anywhere between 1 day to 1 month. The day boats are normally smaller and their catching method is hand-line where they catch the fish by a rod and reel. Although this translates into a fresh fish, due to the nature of fighting with the fish as it tries to escape from the hook, the core temperature of the fish increases and essentially starts cooking the meat of the fish. Therefore there are some negative images of hand-line caught fish being “burned.”
On the other hand, longline vessels can go out for a month at a time. As an example, the boat will leave the port, take a week to go to their fishing ground, fish for two weeks, take a few more days to come back to port. As you can imagine the first fish that is caught may be 2-3 weeks old by the time it reaches the port. The last caught fish may be only a few days old. In terms of the fishing method, the boat will put out a line with equally spaced baited hooks.
After the fish is unloaded off the boats, it may be shipped out to America from its origin in a day or two. Then importers will receive the fish, and distribute it to the wholesalers, who will then hold in inventory or sell immediately to the restaurants, etc. As you can imagine, some fish are not as fresh as you might think they are. Not that the fish is bad but just old. This brings us to the next point
2. Catching method
As mentioned above the hand-line catching method may damage the fish before it even come on board due to the high core temperature it creates when catching. However, if one can minimize this fight and keep it very short that means the fish will be fresh and good quality. On the other hand, the long-line method will give the fish a little time to cool down after getting caught on the hook before it comes on board thus the meat is normally on average not as “burnt” as much as hand-line caught fish. The point is to have the fish fight as little as possible.
[In terms of long-line catching method, sorry I can’t explain it as well as Wikipedia so I think you will get a better idea going over that article there.]
3. Gutting and chilling
Fish tend to get upset when brought onboard as you can imagine. This time, the fish can cause damage to the bones and to the meat while flopping around on board. The result of this is bones cracking and creating blood clots in the meat or the meat to “break” as shown on the picture blow. (The gap that you see between the muscles.) It is even more apparent once it is cut in to sashimi slices since it will essentially be broken into small parts.
Therefore the once onboard, the fish must be drained of blood, kill the nervous system and be immobilized immediately. Then remove the guts, and cool the fish down as soon as possible by covering the fish in ice or dumping the fish in ice water. The core tends to be the hottest so the best way is to replace the stomach where the guts were with ice and just get a solution that is cold enough. The sooner this process is done, the less likely the meat will come out burnt. The fish will only go downhill from this point on. It’s common sense but once the fish is dead, there is no way to improve on the quality, so it is a matter of how you can slow down the process of decomposition and control further damage.
4. Water temperature (the tuna’s living environment)
Fish that normally swim in colder climates are less prone to sicknesses, have a good amount of fat, and a firm texture. Therefore during the colder seasons, the fish are generally higher in quality.
Fish that live in warmer climates are more prone to some sicknesses and parasites. The most common sickness is sashi, which is not harmful to humans if ingested, but is not pleasing to the eye.
5. Feed
Based on what the fish feed on, the fat content as well as the color of the red meat can differ quite a bit. Tuna that feed on mackerel and herring tend to have a higher fat content and the discoloration process is slower than in others.
On the other hand fish that consumer leaner fish and squid are more lean themselves and may have lighter red color to the meat.
(Note: I said “lighter color of red” in the last section. This doesn’t mean that the fish is bad but just light. As you know there are an infinite amount of colors of the “red shade”. There are very dark merlot-esque deep reds to the borderline of where red becomes pink. A good example is that there are customers on the east coast of the US that like the deep red color seen in fish caught from Sri Lanka, but that fish is not preferred as much on the west coast where lighter SE Asian fish are preferred. On the other hand everyone loves the bright red color that is seen in Hawaiian fish.)