Spatial analysis of vertical active layer of albacore tuna（Thunnus alalunga）in the South Pacific Ocean

In order to understand the vertical distribution characteristics and suitable vertical active layer depth range of albacore tuna in the South Pacific Ocean, we plotted the subsurface isothermal depths at 20 ℃ and 25 ℃ on a spatial overlay map, using Argo data and monthly CPUE ( catch per unit effort) fromlogbook of Chinese albacore tuna long-lines vessels in this area to analyze the vertical distribution characteristics of albacore tuna in the South Pacific Ocean. The analysis shows that the subsurface isothermal depths at 20 ℃ and 25 ℃ had significant seasonal changes; the distribution of central fishing grounds of albacore tuna had significant correlation with the seasonal variation of the isobaths at 220 mof isothermal depths at 20 ℃ and the isobaths at 140 m of isothermal depths at 25 ℃. The central fishing grounds of albacore tuna were distributed to the south of 10S,between160E and 175E, where the isothermal depths at 20 ℃ was mostly deeper than 220 m, or the isothermal depths at 25 ℃ was mostly shallower than 140 m; if the depth at 20 ℃ wasmore than250 mor the depth at 25 ℃ was less than 80 m, the CPUE tended to be low. We used the methods of frequency analysis and the empirical cumulative distribution function (ECDF) to estimate the suitable range of vertical active layer depth of albacore tuna(88～238 m). Thus, the preliminary results on vertical distribution characteristics and the suitable vertical active layer depth range of albacore tuna have been obtained in this study, which can be used as guides for settlement depth of longline hook and provide references for longline production operation.