We evaluated the isothermal distribution of subsurface temperatures in the bigeye tuna () fishing grounds in the tropical Indian Ocean. We plotted the isothermal depths at 10, 12, 13on a spatial overlay map using data collected on monthly basis from Argo buoys and monthly CPUE (catch per unit effort) from bigeye tuna long-lines. In addition, the differences in depth between the 12 and 13isolines at the lower boundary of the thermocline were computed to determine the relationship between bigeye tuna vertical distribution and thermocline depth. Our analysis suggested that the vertical range of optimal temperatures (10 and 16) in the area of high CPUE was between 150 and 400 m, and was beneath the lower boundary depth of the thermocline. The overlay maps suggest that CPUE is highest in areas where the 12 isothermal depth is shallower than 350 m (mode: 225−350 m). Conversely, if the depth is >300 m, the CPUE tends to be low. Similarly, the highest CPUEs were observed in areas where the 13 isothermal depth was shallower than 300 m (mode: 190−275 m), whereas CPUE was low when the depth exceeded 400 m. The vertical distribution of high hook rates was concentrated in the area to the north of 15°S. We used a frequency analysis and the empirical cumulative distribution function to compute the optimum range of subsurface factors. The optimum ranges for the12isothermal depths were between 250−340 m and 190−270 m, respectively. The optimum depth difference ranges were 30–130 m for the 12 isothermal depth. We documented the distribution interval and vertical depth range for the central fishing ground of bigeye tuna. Our results provide a reference for improving the efficiency of thelong-line fleet and aiding in resource management.