[A-OS13] Marine ecosystems and biogeochemical cycles: theory, observation and modeling

[AOS13-12] Recruitment of temperate Japanese eel larvae to the subarctic Hokkaido Island: has eel recruitment moved northward during the past 30 years?

*Yu-Lin Eda Chang¹, Kentaro Morita², Kanta Muramatsu³, Osamu Kishida³, Mari Kuroki² (1.Application Laboratory, Japan Agency for Marine-Earth Science and Technology, 2.The University of Tokyo, 3.Hokkaido University)

Observations on the recruitment of Japanese eels, Anguilla japonica, conducted in the subarctic coast of Hokkaido in 2020 have shown that this species has extended its natural habitat to the north by more than a hundred kilometers. Field observations conducted from April to July 2021 in a river in southern Hokkaido revealed that the glass eel recruitment season extended in July. The long-term trend of the Japanese eel recruitment to Hokkaido was investigated by employing a three-dimensional (3D) particle-tracking method. In this method, virtual larvae were programed to swim horizontally and vertically, in addition to being transported by ocean currents after being released in northeastern Japan, where the historical northern limit of their habitat is located. The simulation captured the observed phenomenon of more glass eel recruitment to Hokkaido in 2020 than that in 2021. The spatial variation in the estimated glass eel recruitment was in good agreement with the natural variation in eel abundance observed in 95 rivers across southern Hokkaido in 2022, thus confirming the accuracy of our 3D particle-tracking model. The recruitment varied from year to year in the period 1994–2023, exhibiting an increase in southern Hokkaido and a decrease in the Tsugaru Strait, which connects the Sea of Japan and the Pacific Ocean. The increase in eel recruitment to southern Hokkaido was linked to the northward shifted Kuroshio/Kuroshio Extension and the Oyashio, resulting in the weakening of southward currents. Conversely, the decrease in eel recruitment to the Tsugaru Strait was linked to the strengthening of the Tsugaru current. Apart from the major currents that affected eel recruitment to Hokkaido, eddies formed in the confluence of the Kuroshio and Oyashio also affected the Japanese eel recruitment success. These results indicate that long-term fluctuations in ocean currents can affect the northern limit of the anguilliform eel habitat, which has important implications for their natural distribution in response to the climate change.