Bigeye tuna fishing ground in relation to thermocline in the Western and Central Pacific Ocean using Argo data

The relationship beween thermocline and bigeye tuna ( Thunnus obesus ) fishing grounds in the Western and Central Pacific Ocean was evaluated by Argo data and monthly CPUE (catch per unit effort). The generalized additive model indicated evidence of nonlinear relationships between CPUE and six thermocline characteristics. The results suggested that the fishing grounds distributed where the upper boundary temperature was about 26 °C and the upper boundary depth values between 70 and 100 m. The fishing grounds located between the two high value shapes of the lower boundary depth of thermocline, if the depth was >300 or <150 m, the CPUE tended to be low. The lower boundary temperature of the thermocline in the fishing grounds was lower than 13 °C. Conversely, if the temperature was higher than 17 °C, the hooking rates were very low. The strongest relationship between CPUE with thermocline thickness and thermocline strength was approximately at 60 m and 0.1 °C/m. The optimum ranges for the upper boundary thermocline temperature and depth and the lower boundary thermocline temperature and depth, thermocline thickness and thermocline strength were between 26-29 °C, 70-110 m, 11-13 °C, 200-280 m, and 0.01-0.15 °C/m, 60-80 m, respectively.