Contrasting responses in larval and juvenile growth to a climate-ocean regime shift between anchovy and sardine

Anchovy and sardine populations have fluctuated alternately with decadal changes in climatic and oceanic states, although the mechanisms remain unclear. In the western North Pacific, anchovy and sardine share nursery grounds in the Kuroshio-Oyashio transitional waters, where the subtropical and subarctic currents converge from the south and north, respectively. We found that northward expansion of the subtropical waters simultaneously changed the local environment in the nursery grounds to be favorable for late larvae and early juveniles of anchovy, but not for those of sardine during 1996-2002. Increased temperature enhanced growth and survival for anchovy, whereas reduced food availability diminished those for sardine. Northward expansion of the subtropical waters have been linked with wind-forced anomaly of sea-surface height in the central North Pacific. After 1988, when anchovy flourished and sardine collapsed in the western North Pacific, previous studies by other researchers documented similar changes in air-sea interaction. Our results suggest that contrasting responses in growth and survival processes to wind-forced oscillation of the current structures caused the alternate population dynamics between anchovy and sardine in the western North Pacific.