Old hydroelectric power stations often provide unsafe migration routes to fish, apart from passing through mobile gates during high flow. The installation of retrofitted bypass is considered to be a potential solution to improve fish passage at such old structures, but their performances are often insufficient. In the Amblève River (Belgium), a mobile 3.3 m high dam feeds two principal Francis turbines (12 + 14 m³ s⁻¹) and is equipped with three mobile gates, a modern vertical slot upstream fish pass and a retrofitted downstream bypass functioning at a very low flow (1% of maximum turbined flow). A bypassed river section (length 8.4 km) downstream of the dam was set at 3 m³ s⁻¹ and supplies a Francis microturbine. From 2015 to 2016, we used 1150 hatchery Atlantic salmon smolts (Salmo salar), and we placed antennae with automatic radio-frequency identification (RFID) stations to analyse the migration routes used (n = 5). We tested the attraction efficiency and the performance of the bypass with and without the placement of a guidance system at the entrance and examined the proportion of smolt passage at the different migration routes under three functioning configurations. The placement of the guidance system markedly improved the attraction efficiency and the overall passage efficiency. The median time spent at the entrance of the bypass was from 7 to 12 min, and the time spent near the entrance of the bypass was less than 1 h for 67.1% (release 1), 88.2% (release 2) and 63.7% (release 3) of the smolts. During the three release events, the smolts arrived near the entrance of the bypass mostly during the dusk and dark periods (87.5%, 96.0% and 95.5%, for releases 1, 2 and 3, respectively). In a configuration without opening a mobile gate, the bypass was the most used migration route, followed by the microturbine and the main turbine. Stopping the microturbine and opening a mobile gate has consequences of making it the first choice of passage followed by the main turbine and the bypass. The re-establishment of safe and quick downstream migration has to be considered with a holistic view of the functioning of the hydroelectric powerstation.

老水力发电站通常会为鱼类提供不安全的迁移路线，除了在高流量时会穿过活动闸门。在这样的旧结构上，安装改型旁路被认为是改善鱼通过的潜在解决方案，但其性能通常不足。在比利时的昂布勒夫河，一座高3.3 m的移动大坝为两台主要的弗朗西斯水轮机供电（12 + 14 m ³  s ^-1），并配备了三个移动闸门，一条现代化的垂直槽上游鱼道以及经过改造的下游旁路流量非常低（最大涡轮流量的1％）。大坝下游的旁路河段（长度8.4 km）设定为3 m ³ s ^-1并提供弗朗西斯微型涡轮机。从2015年到2016年，我们使用了1150个孵化场大西洋鲑鲑（Salmo salar），并在天线上安装了自动射频识别（RFID）站，以分析所使用的迁徙路线（n = 5）。我们测试了在入口处有无引导系统的情况下的吸引效率和旁路的性能，并研究了三种功能配置下不同迁移路线上的香气通过比例。引导系统的布置显着提高了吸引效率和总体通道效率。在旁路入口处花费的中位时间为7至12分钟，旁路入口附近所花费的时间少于1小时，分别为67.1％（释放1），88.2％（释放2）和63.7％（释放3）的mol。在这三个释放事件中，主要在黄昏和黑暗时段（分别针对释放1、2和3分别为87.5％，96.0％和95.5％），到达旁路的入口附近。在没有打开移动门的配置中，旁路是最常用的迁移途径，其次是微型涡轮机和主涡轮机。停止微型涡轮机并打开活动门会导致其成为主要涡轮机和旁路之后的首选通道。必须从水力发电站功能的整体角度考虑重新建立安全而又迅速的下游迁移。