The first Autumn eels have been spotted on sonar monitors at a hydroelectric dam in Switzerland.
From September 2026 through January 2027, researchers at the Schaffhausen Hydroelectric Plant will monitor eel activity in the Rhine River.
During that period, many of them will be making their way downstream toward the ocean. One goal of the project is to determine when most eels pass the Schaffhausen Hydroelectric Plant. Initial trials have already detected the first migrants.
Findings from the study could support future measures for migrating eels.
Nobody knows when most eels pass the dam
The Schaffhausen Hydroelectric Dam is one of many hydroelectric facilities in Switzerland.
Like virtually all of them, it is legally required to improve conditions for fish traveling through the river system.
SH Power has plans to upgrade fish passage for upstream migration at the Schaffhausen Hydroelectric Dam. However, the problem is entirely different when it comes to eel migration downstream.
As reported by the project team, developing effective solutions to allow fish to migrate past large river-based hydropower dams remains a significant barrier.
Due to the lack of certainty in developing these types of solutions, Swiss authorities instructed Kraftwerk Schaffhausen AG to conduct further studies.
The timing of the migration remains unclear
European eels are a migratory species recognized for their long-distance journeys between freshwater rivers and the sea.
Learning when these fish pass through the power plant could help answer questions that remain unresolved.
Reduced flow levels within the Rhine River as a result of drought-like conditions this year made the project more challenging to conduct. However, despite these difficulties, the first field trial was able to detect eels near the facility.
The first detections are already appearing on sonar
Following successful preliminary trials, two live-imaging sonar systems were installed at the study site, according to SH Power.
These sonars are designed to monitor eel movement before they reach the power plant.
Unlike conventional observation techniques used prior to this project, sonar offers several advantages. It allows researchers to track the movement of fish underwater in environments with low visibility.
More than movement is being monitored
Once collected, the information will be processed utilizing a specialized artificial intelligence algorithm. Researchers want more than just to count the number of fish passing through.
They also want to determine when most of them begin their journey downstream. Another goal is to determine how weather influences migration timing and how deep the eels travel while approaching the hydroelectric plant.
The monitoring program is scheduled to run from September 2026 through January 2027.
That period was selected because it is believed to coincide with the majority of eels making their downstream migration.
Future protections may depend on the results
Findings from the project could help decisionmakers evaluate future measures for migrating eels. This research does not aim to identify or test a completed solution for protecting eels.
Instead, it focuses on understanding conditions that may influence future actions. Project lead and SH Power Environmental Manager Noëmi Brom stated that “the goal is to get a better understanding of when eels pass the power plant, and we can use this knowledge to support our efforts to protect them during their migration.”
Funding for this research was provided by the Swiss Federal Government.
A final report detailing results from the study should be available sometime during the summer of 2027.
By then, researchers expect to have gained a better understanding of eel behavior near the power plant.
The information gathered could help determine when eel protections should be implemented if they are developed in the future. For now, sonar equipment continues collecting data in the Rhine River as fish move downstream unseen.
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