Recruitment Variability of Small Pelagic Fish Populations in the Kuroshio-Oyashio Transition Region of the Western North Pacific

Climatic and ecosystem changes in the transition region between the subarctic Oyashio and the subtropical Kuroshio Extension fronts may be the key to understanding recruitment variability of small pelagic fish populations in the Pacific waters off Japan. The transition region is characterized by complex oceanographic structure with the meandering fronts, eddies and streamers creating large spatial and temporal variability in the environment. Juveniles of small pelagic fishes such as Japanese sardine (Sardinops melanostictus), Pacific saury (Cololabis saira), and Japanese anchovy (Engraulis japonicus) migrate from the subtropical Kuroshio Current area to the subarctic Oyashio Current area across the transition region. A precipitous decline in number of age 0 recruits of Japanese sardine after the end of 1980s appears to be related to the extremely high mortality rate of young-of-the-year fish in the transition region. In contrast, populations of Pacific saury and Japanese anchovy increased after 1988. These synchronous changes in populations seem to be associated with the weakening of the Aleutian Low Pressure in the North Pacific and weakening of the Oyashio Current in winter, and a sea surface temperature (SST) rise in the Kuroshio-Oyashio transition region in spring. The contrasting responses of the fish populations to the SST rise can be explained by different temperature preference in terms of growth rate in larval and early juvenile stages; cool temperatures are preferred by sardine and warm temperatures are preferred by saury and anchovy.