Tokyo Health Centers stipulate that refrigerated cases where sushi toppings are stored should be kept at 5℃ or lower. This is to maintain the temperature of the toppings at below 10°C at which point bacterial growth is slowed.
However, sushi chefs will remove the topping from the refrigerator and leave it standing out for a while (in the case of tuna, the fat will melt at around 23℃). They do this because if the topping is cold, it becomes difficult to taste the essential nature of the fish. The temperature of the shari is best at human skin temperature (around 36℃) to maximize the taste and sweetness of the rice. However, the ideal temperature differs very slightly depending on the topping.
For example, conger eel which is often lightly grilled or prepared in another, similar way, should have a slightly higher temperature (around 42℃) than the shari, and kuruma prawn, which are boiled, should be the same temperature as the shari.
Overseas, there are laws that state that sushi must be served at 10℃ or lower. This ignores that sushi is best enjoyed at skin temperature. Serving it straight out of the refrigerator makes it no better than purchasing takeout sushi from the supermarket.
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