From 1970s, year round field samplings and observations of Funka Bay have been conducted aboard the R/V Ushio-Maru, Faculty of Fisheries, Hokkaido University. The hydrographic, chemical, and biological data have been used to understand the ecosystem of Funka Bay, especially the relationships among the biological production of low trophic levels, nutrient dynamics, and the behavior of the Oyashio Coastal Water(OCW) from winter through early spring. Tagging experiments conducted by the Hokkaido National Fisheries Research Institute have shown that the Pacific population of walleye pollock occurs from the eastern side Hokkaido Island to northern coastal area of Honshu Island. The distribution of early stage eggs indicates that the spawning grounds are formed from the mouth of Funka Bay to the eastern shelf area outside of the bay. The relationship between the mean days required for 50% hatching(D) and temperature(T) is expressed as follows; D=31.70 exp(-0.12T) by development. Pollock eggs are transported into the bay, where hatchings initiate feeding on copepod nauplii. The feeding success of pollock larvae is higher in Tsugaru Warm Water, which stagnates in winter in the bay, than in OCW, which enters the bay in late January-early February. Early invasion of OCW into the bay is thought to increase the larval pollock mortality due to low temperature and transportation of carnivorous plankton. Strong year classes occurred only in years when late invasions of OCW were observed. With growth, the main food of pollock larvae and juveniles changed to copepodids such as Pseudocalanus newmanii, Neocalanus spp. and Eucalanus bungii. Their biological production might be controlled by the primary production from late winter through early spring, which might be affected by the behavior of OCW. Therefore, the annual fluctuation of the behavior of OCW is a factor affecting complicatedly the year class strength of the walleye pollock Pacific population.