All our Tuna items are (super) frozen at the ultra low temperature of -60°C. They are directly stored onboard in super-freezer, so the quality does not deteriorate as in fresh or standard frozen condition. As the Japanese sashimi markets have understood for a long time, super frozen tuna is the best quality tuna that money can buy.
Compared to regular frozen processes, super frozen fish is infinitely fresher, as it usually takes days until those fish treated with regular frozen processes are served to your table. So called 'fresh fish' can be 11 to 14 days old and deteriorating all the while before it is ultimately consumed., so super frozen tuna is more value for less catch. Besides that, the most commonly retailed tuna in USA are treated with CO (Carbon Monoxide) processes which prevent oxidation, so that the color keeps remaining fresh.
However, the color remains unchanged even after the fish has gone off. Due to this huge potential risk of food-poisoning, almost all the countries are legally banning CO processed fish, while there is no strict rule in USA for this, which is basically because there is no custom to have raw fishes meaning USA doesn’t have specific benchmark rule of bacterial count.
Please refer to the article for more information on the CO process.