Northern shifts in the migration of Japanese glass eels to subarctic Hokkaido Island over the past three decades

Observations of Japanese eel Anguilla japonica recruitment on Hokkaido’s coast in 2020 revealed a poleward shift of the species’ northern limit by several hundred kilometers. Field observations conducted from April to July 2021 in a river in southern Hokkaido, as reported in this study, identified for the first time the potential recruitment period of juvenile glass eels in Hokkaido, suggesting that the recruitment season may have commenced in May and concluded in July. The long-term trend of Japanese eel recruitment to Hokkaido was examined using a three-dimensional particle-tracking model. Virtual larvae were programmed to swim both horizontally and vertically, in addition to being transported by ocean currents, after their release near eastern Taiwan (Scenario 1) and northeastern Japan (Scenario 2). Scenario 1 showed increased recruitment in northern Japan and decreased recruitment in southern Japan during 2014–2023 compared to 1994–2003, which was attributed to the shift in the Kuroshio path. In Scenario 2, focusing on local processes near Hokkaido, the spatial variation in estimated glass eel recruitment exhibited patterns consistent with the natural variation in eel abundance observed across 95 rivers in southern Hokkaido in 2022, with higher recruitment in southeastern Hokkaido and lower recruitment in the Tsugaru Strait. Simulated recruitment trends from 1994 to 2023 indicated an increase in southeastern Hokkaido and a decrease in the Tsugaru Strait. The increased recruitment to southeastern Hokkaido was linked to the northward shifts of the Kuroshio/Kuroshio Extension and Oyashio currents, which weakened the southward currents in the confluence zone of the Kuroshio/Kuroshio Extension and Oyashio. In contrast, reduced recruitment in the Tsugaru Strait was associated with the strengthening of the east-flowing Tsugaru Current. These findings suggest that long-term fluctuations in ocean currents significantly influence the northern limit of anguillid eel habitats, highlighting the impact of changing oceanic conditions on their natural distribution.