Poor cooling in the fish hold leads to the decline of freshness in the set net catch. This phenomenon typically occurs when jellyfish accidentally captured in the set net are kept in the hold with the fish catch, which has been recognized by fisherman for a long time. Such detrimental effects caused by moon jellyfish were investigated experimentally. The temperature of the fish hold only containing the jellyfish remained at around 20°C even cooled with ice. The maximum temperature rise of the fish hold increased in proportion to the total wet weight of the mixed jellyfish, decreasing the quality and freshness of fish in the hold. The temperatures of the fish hold further increased when the mixed jellyfish were naturally vibrated in the fishing boat, reaching 1.5 times as high as that observed when the jellyfish were allowed to stand without vibration. While the heat conductivity of sea water was affected by the mucus secreted by the jellyfish, the quantities and protein concentrations of the secreted mucus increased in proportion to the body weight of the jellyfish.
