Application of ORP as a rapid indicator for grading tuna's freshness on the ship was studied. The longline trawling process was used for catching the sample tuna in the South Pacific Ocean.
All captured sample tuna were weighed, gender identified and investigated for their morta lity, then measured ORP and K value.
Three species of tuna were caught: blue marlin (Makaira mazara), yellow fin tuna (Thunnus albacares), and swordfish (Xiphia gladius). Most of the fish captured were male and they had been dead after picking onboard.
The measured ORP values of blue marlin varied in the range of 0.295-0.362 Volt, with pH be tween 5.35-5.84. Both ORP and pH of swordfish was similar to that of blue marlin. But for yellow fin tuna, the ORP value was about the same as blue marlin while its pH was significantly higher.
ORP value in all species tended to increase with pH of the fish meat decrease.
It is interesting that ORP value of tuna increased in correlation with K value. These results suggested that ORP and pH change, which are measured in the short time, are the effective indicators for grading tuna's freshness on-board.

Key words: ORP, pH, tuna, grading method, long line trawling, freshness, K value