Sea water in fixed tanks, especially at sea installed in the hold, chilled by adding ice, for quickly and uniformly chilling, especially pelagic fish and salmon, by immersion. Portable tank systems have also been developed. In a full tank the proportions of sea water, ice and fish should be about 1:1:4 and the water may be circulated by pump or agitated by compressed air (champagne system) to maintain a uniform temperature and prevent temperature stratification.