Freshness of fish just after catching under various operating conditions by trawlers in the tropical waters was studied, taking opportunity of experimental trawlers by T/V Nagasaki-maru in the Andaman Sea in November 1981 and in the Gulf of Thailand in October 1982. The bottom water temperatures in the fishing grounds were about 28.5°C in both the seas.
The mean K values of fish just after hauling up of the net were rather low; 5.6% for fish by one-hour of towing, 9.6% for fish by two-hours of towing, and 7.2% for fish by three-hours of towing. The mean K value for fish by one-hour of towing was significantly lower than those by two- and three-hours of towing, and this was also true for the same fish species caught in the same fishing ground. The distribution of K values was found to be exponential, the fish with K value of less than 10% accounting for 79.8% in one-hour of towing, 65.0% in two-hours of towing, and 80.0% in three-hours of towing.
The mean K values of the fish caught in the Gulf of Thailand were significantly higher than those in the Andaman Sea. A higher occurrence of trash fish both in catch per hour and in percentage of total in the former area is considered to explain this partially.
A negative correlation was found between the K values just after catching and the body weights of fishes, while between the K values and the increasing rates of K values we found a positive correlation.
