Fish species alternation (FSA) between sardine and anchovy is a drastic phenomenon of ecosystem regime shift responding to climate change. Most FSA are observed in upwelling zones in the eastern part of oceans. The exception is the Kuroshio-Oyashio ecosystem off Japan in the western boundary current system of the North Pacific. In order to understand the mechanism of FSA and to forecast the future FSA in the Kuroshio-Oyashio ecosystem, we started the SUPRFISH (Studies on Prediction and Application of Fish Species Alternation) programme in 2007. We found that physical and ecological shift in the Kuroshio Extension (KEX) region induced the recruitment failure of Japanese sardine in late 1980s. The KEX system shifted at reaching the SSH anomaly which propagated by Rossby wave from the central North Pacific. The positive SSH anomaly shift decreased the wintertime mixed layer depth (MLD) and increased SST. The RPS (recruitment per spawner) index of Japanese sardine is positively correlated with the wintertime MLD and negatively correlated with SST. Ecosystem model study indicated that the shallow mixed layer and high temperature induced the early initiation of spring bloom (February) which mismatch with the arrival of larval fish to the KEX (April-May). Monitoring the SSH anomaly and westward propagation is available for basic technique to forecast the FSA 5-6 years in advance.