The dependence of yearly recruitment of Japanese sardine Sardinops melanostictus on survival in the Kuroshio-Oyashio Transition region

We have numerically modeled the transport and survival processes of Japanese sardine eggs and larvae Sardinops melanostictus in the Kuroshio-Oyashio region to investigate the dependence of yearly recruitment of the sardine on the mortality rate during the early life stages. The model covers from 130 degrees E to 160 degrees E and simulates the Kuroshio Current and wind-induced current resulting from the winter monsoon in a simple way. As a biological process, natural mortality of the eggs and larvae is parameterized by area and year, considering the oceanographic condition such as the extent of southward intrusion of the Oyashio. The year-to-year variations in the amount of survivors simulated by the model are compared to that of one-year sardine stock based on the field data. The variation in the abundance of one-year old sardine is well correlated to that of the surviving juveniles off the east but not the south of Japan, both of which are calculated by the model. This indicates that the survival condition after the postlarval period off the east coast of Japan, which is strongly affected by the Oyashio, could play an important role in determining the year class strength. A strong year class can be formed by the numerous juveniles transported to the east and/or by the good survival condition in the eastern offshore region of Japan.