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.

中文翻译：

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日本鳗鲡能越过日本九州岛Amikake河水系46米高的天然瀑布

限制进入上游河流栖息地被认为是影响野生日本鳗鱼丰度下降的重要因素。然而，除了堰高之外，阻碍日本鳗上游运动的跨河结构的特征还没有得到很好的记录。为了建立估计运动障碍特征的案例研究，我们对 46 m 高天然瀑布的下游和上游的鳗鱼进行了实地调查。我们的调查结果表明，在46 m高的瀑布上方的河段和其他更上游的障碍物中存在鳗鱼，这表明具有高度积极性的鳗鱼通过攀爬瀑布的岩壁和其他障碍物向上游移动。尽管鳗鱼在瀑布下游最为丰富，但瀑布上游的鳗鱼密度远高于根据先前研究中基于瀑布高度的建模估计的密度。在瀑布上游捕获的鳗鱼的总长度和年龄范围很广（总长度为 217-661 毫米；2+-5+ 年），在瀑布下游捕获的鳗鱼中，体型较小的年轻个体占较高比例。因此，小而幼的鳗鱼似乎经常爬上瀑布，在越过重重障碍后在上游地区生长。除了水流强劲的区域外，瀑布潮湿的表面包括长满苔藓的裂缝。这些特征似乎允许鳗鱼爬上瀑布，并表明除了堰/大坝高度外，还有多种因素，例如结构表面的粗糙度，可能对于表征日本鳗鱼向上游移动的障碍很重要。这表明即使是相对较低高度的结构也可能完全或部分阻止鳗鱼的上游迁移，因为它们的表面条件。未来的研究需要评估考虑多种因素的上游移动障碍的特征。