Influence of the fat content on the auction price of Pacific bluefin tuna Thunnus orientalis at Japanese fish markets

Tunas (genus Thunnus) with a high fat content are considered to be sold at higher market prices. On the other hand, other factors also influence the market price of tunas. In this study, how fat content affects the bid price at auction was investigated by using 150 Pacific bluefin tuna Thunnus orientalis caught around the Yaeyama Islands, southern Japan. Eight factors, year (2015 or 2016), auction site (four categories), date (calendar date), fishing gear (longline or pole-line), condition at catch (alive or dead), sex (female or male), body weight (108-350 kg), and fatness (four independent indicators listed below), were used as explanatory variables, and their effects on the bid price at auction were analyzed using a log-normal generalized linear model. The condition factor (body weight relative to the cube of length), percentage of fat content of the belly, near the first dorsal fin, and near the anal fin estimated by bioelectrical impedance were used as indices of fatness, and the use of fat content near the anal fin area reduced AIC the most. Fish still alive at catch, smaller fish, and fatter fish were sold at higher prices. Of the three areas, fat content near the anal fin had the highest correlation with condition factor, which is an objective indicator to predict fish fatness visually. Fat content near the anal fin is concluded to be a good indicator to predict fatness of the whole body of the fish and therefore the auction prices.