Not advocating it, but even when you decapitate some fish, they will continue to twitch or open and close their mouths. Eels are particularly bad for this - I've filletted eel, and the individual portions (2" x 5") would twitch even after 30 minutes after the fish was killed.
I once watched a carp head do that for well over 3 hours when I worked at a fish market. Just the head, cut away from the body.
All that being said, sushi is best the fresher it is. Although, the animal should be dead (killed quickly) before preparing as the flesh declines in quality if the fish struggles or suffers before dying.
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