Digital image processing of fall chum salmon was carried out in order to find an objective criterion to analogize the flesh redness from the nuptial coloration.
The external white distribution rate was significantly correlative to the red and yellow distribution rates. And those rates, especially at the ventral to the lateral line portion, showed a negative correlation with the flesh redness represented as “a” value. On the other hand, the external white distribution rate showed a positive correlation with the “a” value of the flesh especially near the lateral line. This tendency was somewhat more specific in female fish than in male fish. And it was considered to be desirable to discriminate the sex of the fish when the flesh redness is ranked from the external characteristics.