Effect of environmental fluctuations on mortality of juvenile Pacific bluefin tuna, Thunnus orientalis, in closed life-cycle aquaculture

Abstract

Although full-cycle aquaculture of Pacific bluefin tuna, Thunnus orientalis, has been established by Kinki University, Japan, most juvenile die after transfer to net cages offshore due to the stress associated with fluctuations in environmental conditions. We examined the effect of environmental conditions on mortality of juvenile using a generalized liner model (GLM). Juveniles were reared for more than 300days in a net cage and number of dead fish was examined over time and in relation to water temperature, salinity, moon phase, and water transparency. Mortalities soon after transferring the fish from the indoor rearing tank to the net cage occurred independently of environmental factors and were considered to be due to the handling stress and lack of acclimation. In subsequent periods, fluctuations in water temperature were the most critical factor affecting juvenile survival. Our results showed that optimal water temperatures for juvenile bluefin tuna aquaculture are considered to range between 15 and 25°C. Salinity and lunar irradiance (half moon to full moon) fluctuations had less marked effects on juvenile survival. Juveniles in net cages cannot move to avoid those unfavorable environmental fluctuations and this inability to locate areas of optimal temperature may complicate their body temperature maintenance because the capacity for endothermy in juveniles is undeveloped. We propose that sites selected for culturing juvenile Pacific bluefin tuna have water temperatures ranging from 15 to 25°C with constant temperatures that avoid large fluctuations.