The current lack of high-precision information on subsurface seawater is a constraint in fishery research. Based on Argo temperature and salinity profiles, this study applied the gradient-dependent optimal interpolation to reconstruct daily subsurface oceanic environmental information according to fishery dates and locations. The relationship between subsurface information and matching yellowfin tuna (YFT) in the western and central Pacific Ocean was examined using catch data from January 1, 2008 to August 31, 2017. The seawater temperature and salinity results showed differences of less than ± 0.5°C and ± 0.01 compared with the truth observations respectively. Statistical analysis revealed that the most suitable temperature for YFT fishery was 28–29°C at the near-surface. The most suitable salinity range for YFT fishery was 34.5–36.0 at depths shallower than 300 m. The suitable upper and lower bounds on the depths of the thermocline were 90–100 m and 300–350 m, respectively. The thermocline characteristics were prominent, with a mean temperature gradient exceeding 0.08°C/m. These results indicate that the profiles constructed by gradient-dependent optimal interpolation were more accurate than those of the nearest profiles adopted.
Figure 1. Location of YFT fishery points and the available Argo observations: (a) show the YFT fishing locations in the WCPO and the locations of fishing locations and Argo profiles during August 2017, whereas (b) is the frequency statistic of observation number surrounding each fishing point.
Figure 2. Temperature and salinity differences between 5 to 500 m at each fishery point: (a1) and (b1) show the vertical difference distribution, whereas (a2) and (b2) are the difference frequency at each depth.
Figure 3. Root mean square errors (RMSEs) of 30 sample points from the analysis results (indicated by dots) and nearest profiles (indicated by circles) at a depth range of 5 to 1500 m: (a) temperature and (b) salinity.
Figure 4. Temperature section at the depth range from 5 to 100 m (a), 5 to 500 m (b), and temperature curves at 5 m and 300 m (c) for different catch. The temperatures at every water layer were interpolated to the catch sequence.
Figure 5. Salinity section at the depth range from 5 to 100 m (a), 5 to 500 m (b), and salinity curves at 5 m and 300 m (c) for different catch. The salinities at every water layer were interpolated to the catch sequence.
Figure 6. The statistic of suitable temperature (°C) and salinity for YFT corresponding different catch group. Temperature at 5 m, 150 m, and 300 m are indicated by a1–a3, respectively. Salinity at 5 m, 150 m and 300 m are indicated by b1–b3, respectively.
Figure 7. Statistics for the thermocline variable matching each fishery point: (a) shows the upper bound depth (indicated by h1) and the deeper bound depth (indicated by h2), whereas (b) is the thermocline thickness (represented by thk) and thermocline strength represented by temperature gradient (indicated by gth).