How Japanese anchovy spawn in northern waters: start with surface warming and end with day length shortening

摘要

Concordant with stock increase, Japanese anchovy Engraulis japonicus seems to expand its spawning ground northward from subtropical waters along the Kuroshio Current to subarctic waters affected by the Oyashio Current. However, the description of the northern expansion is still insufficient because quantitative analysis on distribution density of eggs and larvae has not been made and the mechanism of the expansion is still unclear. By analyzing retrospective data, quantitative increase in the distribution density of eggs and larvae in the Pacific waters off northern Japan was confirmed to have occurred with the stock increase in the 1990s. To elucidate how Japanese anchovy spawn in the Pacific waters off northern Japan, generalized additive modeling was performed using sea surface temperature (degrees C), day length (h) and day length difference from the day before (min) as explanatory variables. The northern expansion of the spawning ground was found to have resulted from seasonally warmed surface water (&15 degrees C) that occurs in early July in the subarctic Oyashio Current domain. The northern spawning ceased by early September with the shortening day length (&13.7 h). The early cessation of spawning may contribute to reproductive success in the Pacific waters off northern Japan because larvae hatched by early September would grow larger enough to survive the first winter.