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.