Effect of tank wall colour and pattern on the survival rate of juvenile Pacific bluefin tuna Thunnus orientalis (Temminck and Schlegel) during ship transportation

Juvenile Pacific bluefin tuna (Thunnus orientalis; PBT) often experience high mortality during ship transportation. This study investigated whether the addition of colours or patterns to the walls of tanks affected survival rate. In the first experiment, three colours and lattice patterns were tested: dark blue single-colour, red single-colour, and red–blue lattice pattern. Fish in all tanks exhibited abnormal behaviours when sunlight entered the tanks between 0800 and 1000 hours, but mortality only increased in the single-coloured tanks as a result of collision with the tank walls. In the second experiment, four colours and patterns were tested: dark blue single-colour, red–blue lattice pattern, red–blue lattice pattern with shade sheet and red–green lattice pattern with shade sheet. Again, we visually observed that fish in all treatment groups exhibited abnormal behaviour when sunlight entered the tanks, but there were no collision deaths in the lattice-patterned tanks and survival in this group was significantly higher than in the single-coloured tanks. Thus, the use of a high-contrast colour pattern can prevent mass death of juvenile PBT during ship transportation.