It's always amazing how little we know about fish.

Eels, for example, hatch in the Sargasso Sea, migrate into our rivers, grow up there, and then somehow manage to swim 5,000 kilometers back to their place of birth in a year without food, against the Gulf Stream while their bodies remodel.

There they mate, which no human has ever observed.

Salmon, on the other hand, take the opposite route.

They hatch in freshwater streams, grow up in the sea and then, apparently smelling the scent of home, return to their home river to spawn.

Andrea Diener

Correspondent in the Main-Taunus district

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In addition to the usual predators, obstacles such as barrages make life difficult for them.

For example in the Main, which is classified as "severely impaired waters", which is certainly correct from the fish's point of view.

The usual river fish such as roach, barbel or allis shad also need a little freedom of movement when they are looking for food and are reluctant to spend their lives being forced to live between Griesheim and Eddersheim.

Fish ladders do exist now, but most don't work very well.

There's no fish looking for it

Therefore there is a nationwide concept to restore the "ecological continuity" on the federal waterways.

The Eddersheim barrage was given a particularly high priority, as was Kostheim, where the existing facility is to be expanded.

In Eddersheim, however, a completely new type of facility is to be built, as the Federal Institute for Hydraulic Engineering and the Federal Institute for Hydrology have decided.

Because little is known about what fish want and need and why the usual fish ladders are hardly accepted.

The pilot plant is intended to research this.

The Eddersheim barrage is particularly suitable because, like the Griesheim barrage, it belongs to the federal government and not, like all other hydroelectric power plants, to private operators.

A total of seven pilot plants are to be built nationwide, two on the Main, two on the Neckar, two on the Moselle and one on the Weser.

There are two main criteria for fish ladders – findability and passability.

Fish orientate themselves on the main current against which they swim.

The current mainly comes over the power plant levels, only a little over the usual fish ladders, which are usually also on the edge and are very narrow.

No fish will find that, nor will it look there.

Therefore, a lure flow directly next to the turbine flow on the Kelsterbach-Raunheim side should show him the way into the plant.

Through canals it reaches a distribution basin and undertakes the strenuous ascent between the power plant building and the lock in the so-called slot pass.

The Eddersheim pilot system is a double-strand system with two parallel ascent paths in which different construction methods or current strengths can be tested against each other to find out what fish prefer.

For some time now, fish have also been tagged with transmitters and their behavior tested in front of barrages.

This is also to be examined in the new system.

How many entries are optimal?

Where is the best place to place them?

A video tunnel and motion detectors are also installed.

At some point it's everyone's turn

This pilot plant will cost 19 million euros.

It will be even more complex and modern than the first completed facility of this kind at the Moselle barrage in Koblenz, where the Mosellum is also connected to a visitor center that provides information about the life of migratory fish.

However, this will not happen in Eddersheim, only employees have access there.

If you are hoping to see the salmon spawning in the Schwarzbach soon, you still have to be patient.

The next step, explained the Waterways New Construction Office at a public information evening in Eddersheim, is the planning approval process, which will begin at the end of 2023.

The planned construction period of three and a half years is scheduled from 2027 to 2031, from 2031 to 2039 is the investigation period in which research is to be carried out on the facility.

If it is well received, it can serve as a model to make the entire river accessible to fish in the long term.

"There are 34 barrages in the Main, and at some point everyone will have their turn," says Mareike Bodsch, head of the Waterways New Construction Office.

"It's a generational task."