Added to this is climate change, with increasingly hot summers. In order to survive, some species of fish need to find cold-water refuges. But will they succeed? And how important is it for fish to move around, anyway? “We don’t know that”, says Brodersen: “Our aim is therefore to describe and understand how fish migrate within a river system, what factors influence them, and how they are faring.” That is why he and his team launched a project to track the movements of various fish species in the Rhine, Aare, Thur and Reuss rivers using acoustic transmitters and underwater microphones. The project, which is supported by the Federal Office for the Environment (FOEN), forms part of the ‘European Tracking Network’, which facilitates the international exchange of data. For example, the movements of Swiss fish can be tracked all the way to the Netherlands. However, the infrastructure in Switzerland can also be used by the cantons and private environmental consultancies. Some of the tagged fish, and the tags themselves, have a lifespan of several years. Brodersen is therefore hoping that funding for the project will be secured for as long as possible.

Over the past two years, the researchers have installed plastic pipes along the banks of the Rhine, Aare, Thur and Reuss rivers, inside which they have placed underwater microphones, known as hydrophones. The pipes extend up to two metres into the water. The hydrophones themselves are only about the size of a thermos flask. The battery takes up the most space, so that it only needs to be replaced once a year. 140 hydrophones were installed along the riverbanks at intervals of three kilometres on average. Researchers collect the recorded data on site twice a year. In addition to the fish signals, the habitat structure of the respective river sections is also recorded, and the flow rate and water temperature are measured.

An inflatable boat, fitted with an electric fishing device, is being launched into the River Rhine. Each stretch of river between two power stations is considered a section. In the spring and autumn of 2025, the researchers caught fish that were as diverse as possible in every second section. The electric current temporarily paralyses the fish’s muscles, allowing them to be caught with a landing net. However, during the day, large fish tend to swim away from the boat into open water. That is why the researchers mainly fished at night.

The caught fish are anaesthetised in a tank filled with water. A small incision is then made in the abdomen and an acoustic transmitter is implanted. It is shaped like a small cylinder. This allows the location and temperature of the fish to be determined. The wound is sutured and also sealed with a powder. It is also in the researchers’ interest to ensure that the procedure does not harm the fish. After all, they want to study the natural behaviour of the test animals. For this reason, care is taken to ensure that the weight of the transmitter does not exceed five per cent of the fish’s body weight. Last year, the researchers tagged 751 fish of thirteen different species

Initial analyses suggest that seasonal migration patterns may be much less common than previously thought, presumably due to a century of power station activity. However, a definitive assessment will only be possible once observational data covering several years is available. Surprisingly, however, there are significant differences between the species. Carp often cover a wide area, whereas tench tend to stay in the same spot. Eels, too, remain in their holes until they set off on their migration to their spawning grounds in the Sargasso Sea. The researchers have already been able to track two tagged eels as they made their way down the Upper Rhine to Basel.

But individual fish, too, display different behaviours. Some catfish migrate over distances of several kilometres, whilst others were recorded by the same hydrophone every year from March to November. The impact of the catfish on the entire ecosystem is also surprising: This predatory fish is widespread in the major rivers, where there are hardly any medium-sized barbel, dace or other fish that would fit into a catfish’s mouth. It is not known whether they are eaten or whether they manage to escape to catfish-free tributaries in time. In contrast, in the fast-flowing, shallow waters – such as the Reuss near Lucerne – the researchers found no catfish but plenty of barbel of all sizes.

• Eawag
• Federal Office for the Environment (FOEN)