Ecological characteristics of Japanese eels in Hokkaido, the northern edge of their distribution

Abstract The northern limit of the geographic distribution of the Japanese eel Anguilla japonica is Hokkaido, located north of the Tsugaru Strait, in northern Japan. No commercial eel fishing has been conducted in Hokkaido for several decades, and no comprehensive scientific research has been conducted in the region. To investigate the ecological characteristics of this species at the northern limit of its distribution, we collected a total of 80 individuals (158–675 mm in total length) from seven rivers facing the Pacific Ocean in southern Hokkaido during the summers of 2021 and 2022. Large females, including those in their sexually maturing stage (late oil droplet stage), and males, showing external morphological characteristics of mature silver eels (early spermatogenesis stage), were observed. The age of 62 individuals, estimated using otoliths, ranged from 1 to 13 years old, and the growth rates were lower than those of their conspecifics in lower latitudes in Japan, suggesting that the lower water temperatures in Hokkaido inhibited their growth rate. Stomach content analysis revealed that eels in Hokkaido primarily fed on insects and crustaceans. Additionally, otolith microchemistry analysis indicated that 56 out of 74 individuals (75.7%) were river residents that remained within freshwater habitats until capture, whereas the others (24.3%) migrated between rivers and estuarine-coastal habitats. A greater dependency on estuarine and coastal areas was associated with higher growth rates. These findings demonstrate that Japanese eels can inhabit the rivers of southern Hokkaido, underscoring the importance of riverine conservation in this region.