Abstract
Nutrient decrease and their effects on fisheries in the Seto Inland Sea, Japan were discussed. It suffered from heavy eutrophication during Japan’s period of high economic growth starting in the 1960s. At that time, red tides often occurred and fish culture was severely affected. Recently, water quality has dramatically improved. Although total nitrogen （TN） and total phosphorus （TP） runoff load from the land were reduced by 40% and 60%, respectively, TN and TP concentrations in seawater have apparently not decreased, despite the apparent nutrient concentration decrease. Nutrient decrease was not due to only nutrient runoff load from the land, and it was thought that nutrient release from the bottom sediment was also important. Despite the water quality improvement, fish catches have gradually decreased. Phytoplankton primary production did not response simply to nutrient decrease, and according to zooplankton, there is no data set to show their biomass variation. The conclusion is that the reason of fish catch decrease is still unknown. Whereas nutrient concentrations decreased, and presumably nutrient decrease will be a contributing factor, land reclamation, decreases in the area of tidal flats and algal/seagrass beds, global warming, and overfishing should be also thought as reasons contributing to fish catch decreases.
https://ror.org/04j7mzp05 
