Sea water in fixed tanks chilled by mechanical refrigeration, especially at sea installed in the hold, for quickly and uniformly chilling pelagic fish, in particular, by immersion. In a full tank the proportion of sea water to fish should be between 1:3 and 1:2 and the water is circulated by pump to maintain a uniform temperature and prevent temperature stratification. The sea water in the tank should be pre-cooled before fish are loaded. The sea water can be lowered to about -1°C without freezing the fish to obtain the maximum benefit, and the catch can be cooled more rapidly than by stowage in ice. The refrigeration capacity of an RSW system should be sufficient to cool the fish to 0°C within a few hours. Some species take up unacceptable amounts of water and salt when kept in sea water, the eyes become cloudy and the gills are bleached as the blood is leached out.