A special tunnel for fish has been built under the new lock system under construction in the center of Stockholm, separating the Baltic Sea from the freshwater lake Malaren.

For almost 380 years, fish have not been able to swim freely in the waters, but now a special migration path for fish has been built in the new Slussen lock system.

"We need to allow fish to migrate between the Saltsjon (Baltic Sea bay) and Malaren and thus increase the biological diversity inside Malaren," explained Eva Rosman, Communications Manager, Project Slussen.

The tunnel was built especially for fish who are weaker swimmers such as perch and pike. The path aims to diversify the fish population in Malaren, Sweden's third biggest lake.

When building the new lock, creating a fish migration path was an environmental precondition. Builders used stones from a defensive wall from the 1540s found in archaeological digs, to create a tunnel underground.

Communications manager for the Slussen project, Eva Rosman, said that the tunnel was also being illuminated during the day to mimic the natural conditions as much as possible.

A camera has been installed in one part of the tunnel to monitor the fish swimming through – and sometimes it catches more unexpected creatures, like an otter.

To prevent salt water from entering from the Baltic Sea and to conserve water, Malaren is regulated by opening and closing various dams and sluice gates in Stockholm.

Only one kilometer away, on the other side of the old town, there is Norrstrom, which is one of the shortest rivers in Europe, also connecting Malaren with the Baltic Sea.

There, a great number of fish species can be caught, making it a popular destination for sport fishers.

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