Assessing Japanese eel populations in a river system using eDNA: seasonal variation and modest effectiveness of stocking where natural recruitment is restricted

Japanese eel, Anguilla japonica, is a highly endangered species in the Western Pacific region despite its high commercial value in fisheries. To enhance fisheries resources of this catadromous fish, eel stocking programs are conducted annually in many rivers across Japan. These programs involve capturing naturally recruited glass eels at river mouths, farming them in artificial ponds, and releasing them into rivers. However, the impact of these stocking efforts on eel populations remains unclear. Recently, environmental DNA (eDNA) has emerged as an effective tool for assessing the biomass of target species in aquatic environments. This study aims to evaluate the spatiotemporal changes in Japanese eel population over three years and the effect of stocking using species-specific eDNA marker in the Shigenobu River system in western Japan, where numerous sediment-control dams and weirs are present, and eel stocking is conducted both upstream and downstream of these barriers. Our results show that eDNA concentration was significantly correlated with both the density and biomass of captured eels, confirming the effectiveness of eDNA assays for eel stock assessment. eDNA concentrations, as well as captured density and biomass, were higher downstream than upstream. After stocking, eDNA concentrations increased and remained elevated at upstream stocking sites where natural recruitment was blocked by sediment-control dams. Seasonal variation in eDNA concentrations was most pronounced downstream, where natural recruitment occurs. These findings suggest that stocking enhances eel populations in areas where natural recruitment is restricted, but the effect is modest compared to natural recruitment. Therefore, ecosystem-based habitat management, including habitat restoration and connectivity, should be prioritized for eel conservation.<br>

日本鳗鲡（Anguilla japonica）是西太平洋地区的极度濒危物种，尽管其渔业商业价值颇高。为恢复该降海洄游鱼类的渔业资源，日本诸多河流每年都会开展鳗鲡增殖放流计划。此类计划通常包括在河口捕获天然补充的玻璃鳗幼体，于人工池塘中培育后将其放流至自然河道中。然而，此类增殖放流举措对鳗鲡种群的实际影响仍未明确。
近年来，环境DNA（environmental DNA, eDNA）已成为评估水生生态系统中目标物种生物量的有效技术手段。本研究以日本西部的重信川水系为研究区域——该水系分布有大量拦沙坝与堰坝，且在这些水利屏障的上下游均开展了鳗鲡增殖放流工作——旨在通过物种特异性eDNA标记，解析日本鳗鲡种群三年内的时空动态变化，并评估增殖放流举措的实际效果。
研究结果显示，eDNA浓度与捕获的鳗鲡种群密度、生物量均呈显著正相关，证实了eDNA检测技术在鳗鲡渔业资源评估中的有效性。研究同时发现，eDNA浓度连同捕获的鳗鲡密度与生物量，均在下游区域显著高于上游区域。在增殖放流后，上游受拦沙坝阻断天然鳗苗补充的放流位点的eDNA浓度显著升高并维持在较高水平。eDNA浓度的季节波动在下游存在天然补充的区域最为明显。
上述研究结果表明，增殖放流可在自然补充受限制的区域提升鳗鲡种群规模，但相较于天然补充过程，其提升效果较为有限。因此，鳗鲡保护应优先推进基于生态系统的栖息地管理工作，包括栖息地修复与河道连通性恢复。