Optimum Water Temperatures for Bigeye Tuna in the Indian Ocean as Seen from Tuna Longline Catches

Until now, the study of optimum water temperature for bigeye tuna has relied mainly on the use of sea surface water temperature data. However, this method has not proved entirely satisfactory since this species, in particular, occurs at considerable depths. The present authors studied the optimum water temperature for bigeye tuna by determining the in situ temperatures at the hook depths (i.e., the depths at which bigeye tuna were caught) by the use of tuna longline fishing and oceanographic data that had been collected simultaneously in the Indian Ocean. The results indicated that the optimum water temperature for bigeye tuna ranges between 10°C and 16°C in the Indian Ocean, which is virtually identical to the finding of 10°C-15°C for this species in the Pacific Ocean. Moreover, the present results were found to be considerably lower than the previously reported optimum water temperature of 20°C (derived from sea surface temperature data), by approximately 4°C-10°C. The present results also indicated that bigeye tuna have not been caught at depths where the temperatures were lower than 9°C or 10°C, indicating perhaps that they do not occur in waters of such low temperatures.