Effect of onshore-offshore shifts of the Kuroshio axis on the plankton biomass off the Pacific coast of Japan

摘要

This paper examined relationship between onshore-offshore shifts of the Kuroshio axis and interannual changes in the plankton biomass in Tosa Bay and off Boso Peninsula, which are both well-known as a shirasu fishing ground off the Japanese Pacificcoast. It was revealed that plankton biomass during the spring shirasu fishing period in the above two regions was mainly regulated by the frequency of offshore water intrusion responding to onshore-offshore shifts of the Kuroshio axis. This implies thatfrequent intrusion of the Kuroshio water could seriously reduce the food organisms for shirasu in the fishing ground. In addition, particularly in Tosa Bay, interannual change in river water discharge probably related to nutrient input from land could beanother important physical factor affecting the plankton biomass. On the other hand, in the offshore region off Boso Peninsula, meteorological conditions such as wind and solar radiation could be often more critical to the plankton production.