One exception to the general requirements tor quick freezing of fish requires special mention. Frozen tuna, which will eventually be eaten in its raw state as the Japanese product "Shasimi" seemingly requires to be reduced to a lower temperature than other fish products. Japanese fishing vessels catching fish for this product operate with freezers at -50° to -60°C. Tuna is a large fish and when frozen whole by immersion in sodium chloride brine at a temperature of -12 to -15°C takes up to three days to freeze. Air blast freezing has now replaced brine freezing for this purpose and operation with very low freezer temperatures can result in freezing times of about 24h or less. The exceptionally low temperatures used in these freezers of about -50 to -60°C have given rise to conditions which require special precautions to be taken to avoid low temperature brittle fracture of metal structures in the vessels.