We examined variability in sea surface temperature (SST) and satellite-observed net primary productivity (NPP) in seven regions around the Japanese Archipelago during 2002–2022: Okhotsk Sea (OKH), Oyashio (OY), Kuroshio-Oyashio Transition Zone (KOTZ), Kuroshio (KR), Northwestern Japan Sea (JSN), Southeastern Japan Sea (JSS), and East China Sea (ECS). SST was relatively stable in all regions and seasons before the 2013/14 regime shift and after that increased. While the spring (April–June) NPP in the subtropical regions (KOTZ, KR, JSS, and ECS) decreased in 2014 and onward, it was stable or increased in the same regions before 2014 and in the subarctic regions (OKH, OY, and JSN) through the research period, suggesting effects of the 2013/14 regime shift and global warming. Positive linear relationships were detected between winter (January–March) SST and spring NPP, in the subarctic regions, whereas relationships were negative in the subtropical regions. We also examined the effects of the major climate/ocean indices (North Pacific Index, North Pacific Gyre Oscillation Index, and Arctic Oscillation Index in winter, and annual Southern Oscillation Index) on winter SST and spring NPP, by using cross correlation analysis with lags from -5 and 0 years and correlation mapping between the climate/ocean indices and either winter SST or spring NPP at a resolution of one-degree latitude and longitude. The results suggested the effects of climate/ocean indices differed by the regions and lagged by 1–3 years.

The Kuroshio–Oyashio transition area has water originating from subtropical and subarctic areas and forms productive ecosystems. Mesoscale eddies play important roles in these ecosystems, such as by transporting heat and organisms. Micronektonic mesopelagic fishes play crucial roles in biogeochemical cycles; however, further research is needed to understand the physical, chemical, and biological effects of mesoscale eddies on these fishes. Observation from a ship was conducted using Underway CTD and a quantitative echo sounder in June 2019 to examine the impact of a mesoscale eddy on fish distribution in this area. The observed anticyclonic eddy originated from the Kuroshio extension and had warm and saline water inside it. Acoustic volume backscattering strength (SV) at 38 kHz showed peaks along the density isolines around 75 dbar and 400 dbar, and the comparison of the SV at 120 kHz and 38 kHz suggested that the peaks of the SV were associated with fishes. The distribution of fishes inside the eddy became 13–19 dbar and 27–37 dbar deeper than outside of the eddy in surface and intermediate depth. Mean SV in intermediate depth was higher inside the eddy compared to that outside the eddy, suggesting the presence of denser fish within the eddy.

The quantitative evaluation of the trend of seawater temperature is of significant importance, and the development of a statistical model that accurately reflects real data and can be readily estimated represents a key objective. In this study, we used a linear Gaussian state-space model to estimate the level components representing the trend, taking into account the seasonality, the influence of the Kuroshio, and changes in the trend, for seawater temperatures observed over a 25-year period in Jogashima, Kanagawa Prefecture. The results showed that the seawater temperature exhibited an increase when the distance from Nozimazaki to the Kuroshio axis is close, and during the fall and winter months when the Kuroshio is A type. The incorporation of Kuroshio data into the model resulted in enhanced precision in seawater temperature predictions. The trend of seawater temperature exhibited a downward trajectory until 2007, after which it exhibited an upward trajectory. When the trend was estimated using a simple linear regression model, the model residuals exhibited significant autocorrelation and did not satisfy the underlying assumptions of the linear regression model. It would be prudent to exercise caution when applying simple linear regression models and interpreting the results of seawater temperature trend analysis.