Because for sashimi and sushi raw material is used, the tuna must be of very high quality and needs to be ultra-frozen (-60°C) to be well preserved.
The best quality is coming from longliners, but sometimes you see that also high-quality tuna comes from a limited number of purse seiners.
Especially in Asia, they require high-quality tuna, ultra-frozen Bigeye and Yellowfin Tuna. Europe and the USA accept lesser quality tuna from tuna purse seiners frozen @ -40°C.
Sashimi served raw, from either fresh or ultra-frozen tuna. The serving contains net to the sashimi, soya sauce, seaweed, wasabi and a few pieces of (pickled) vegetables.
Sushi is a small shaped rice block, with a topping of a little wasabi, and on top either tuna or other fish, scallops, mussel, eel with a mini amount of vegetables or roe.
The vast majority of the sashimi/sushi quality goes to Japan, where the right product is getting an excellent price. With the introduction of sushi restaurants outside Japan, the Japanese shrinking tuna fishing fleet is not able to supply the new wave of restaurants. Supply is coming from other countries like Taiwan, the Democratic Republic of Korea and China.
In sashimi restaurants, the different cuts of the tuna are obtaining different prices. At the top, you always find the belly of the tuna (Toro), which is the fattiest part and incredibly soft.
Not only Japanese restaurants sell high-quality tuna, but also in many Japanese supermarkets and fish shops, you find the most appealing and naturally treated pieces of tuna.
Japan is by far the top consumer of sashimi quality tuna with a market share of 90%, China, USA and Europa are coming next. The eating habits changed in Japan to more Western Style, but the expectation is the trend will reverse COVID19-virus waning around.