The o-tsukuri course in a traditional Japanese meal generally consists of the freshest seasonal fish available, served raw and unfettered. Standard sashimi fish include tai (sea bream), hirame (flounder) and maguro (tuna). There are, however, some fish that are rarely served raw, for one of several reasons.
The first one concerns safety. Certain fish lose quality much faster than others. Mackerel, for example, is not served raw very often. Very fresh local mackerel is delicious served raw, but not always available. It is more common to see it — as well as other silver-skinned, scale-less fish (in Japanese, aomi sakana) — vinegared or salted when served raw.
Some white-fleshed fish, such as amadai (sweet sea bream), have a reputation for not being palatable unless very fresh. The same rule applies to muddy river or pond fish, such as carp. When very fresh these fish present no food-safety hazard, but they are usually shunned in the uncooked form because of taste. Again, when carefully farm-raised and prepared by a professional, occasionally these fish may be enjoyed uncooked.