In connection with a planning of fishing ground development in the exclusive economic zone around Hokkaido, the authors observed currents, water quality, plankton and the primary productivity in the area surrounding the Musashi Bank in August in 2009. As a result, the primary production rate of summer of this sea area was estimated to be about 36.6 (mg-C/m2/day). It is quite lower than that of other bay area in Japan. The density stratification was observed and it suggests that Tsushima Warm Current along the Hokkaido coast influences the height of the layer. Depth in which it meets the requirement of the photosynthesis is 25-55m. It accounts for about 66% of the primary production of the entire water column here. It was confirmed that the lack of the nutrient salt on the surface limited the primary production of this sea area in summer.
