Japanese eels, Anguilla japonica, can surmount a 46-m-high natural waterfall of the Amikake River system of Kyushu Island, Japan

Restricted access to upstream riverine habitats is considered to be an important factor affecting abundance declines of wild Japanese eels. However, the characteristics of cross-river structures obstructing upstream movements of Japanese eels have not been well documented, except for weir height. Aiming to establish a case study for estimating the characteristics of barriers to movements, we conducted field surveys for eels downstream and upstream of a 46 m-high natural waterfall. The results of our surveys showed that eels were present in river reaches above the 46 m-high waterfall and other more-upstream obstacles, which indicated that highly motivated eels moved upstream by climbing the rock wall of the waterfall and other obstacles. The eel densities upstream of the waterfall were much higher than the density estimated from modeling based on waterfall height in a previous study, although eels were most abundant downstream of the waterfall. Total length and age of the eels captured upstream of the waterfall ranged widely (217–661 mm in total length; 2+–5+ years), with small, young individuals comprising a higher proportion of the eels captured downstream of the waterfall. Therefore, small, young eels appeared to frequently climb the waterfall and grow in the upstream area after climbing over multiple obstacles. The moist surfaces of the waterfall included cracks that were covered with moss except in areas with strong water flow. These characteristics appear to allow the eels to climb the waterfall and suggest that multiple factors in addition to weir/dam height, such as roughness of the structure surface, may be important for characterizing the barriers to Japanese eels moving upstream. This indicates that even relatively low-height structures may completely or partly block the upstream migration of eels because of their surface conditions. Future studies estimating the characteristics of barriers to upstream movement that consider multiple factors are necessary.