Hokkaido University study finds northern Japanese eel distribution range determined by summer water temperatures

Master's Student Kanta Muramatsu at the Graduate School of Environmental Sciences, Hokkaido University (at the time of the research; currently a teacher at Nagano Nishi High School), and Professor Osamu Kishida from the Field Science Center for Northern Biosphere at the same university, in collaboration with the University of Tokyo and the Japan Agency for Marine-Earth Science and Technology, have clarified what types of rivers the Japanese eel (Anguilla japonica) inhabits in its northern limit region. Their findings were published in PNAS Nexus.

The research group investigated the habitat conditions of yellow eels in 105 rivers in southern Hokkaido; they confirmed a total of 222 specimens in 52 rivers. The survey revealed that while some rivers had many eels, others had none at all. Furthermore, these habitat conditions appeared in regional clusters.

When the factors creating these differences in habitat conditions were analyzed, it was shown that not only the quantity of juveniles (glass eels) approaching the river mouths in spring, but also the summer river water temperature during their active feeding period were major factors in determining eel abundance. Specifically, rivers with higher summer water temperatures tended to have larger eel populations.

Differences in summer water temperatures were related to watershed land use (the proportions of farmland and urban areas versus forest areas) and geology (the proportions of sedimentary versus volcanic geology). In particular, rivers with a high proportion of farmland and urban areas in the watershed and a lower proportion of forest or a higher proportion of sedimentary geology and a low proportion of volcanic geology, tended to have higher summer water temperatures.

In other words, rivers with these watershed characteristics exhibit higher summer water temperatures, creating environments suitable for eel habitation. Additionally, given neighboring rivers with similar geology and land use also have similar water temperatures, it was inferred that regional clustering occurred in the number of eels observed.

Journal Information
Publication: PNAS Nexus
Title: Thermal constraints on the distribution of Japanese eel (Anguilla japonica) at its northern limit: Links to land use and geology
DOI: 10.1093/pnasnexus/pgaf384