The fish passage at the new Slussen

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Slussen is now home to one of Sweden’s largest and most advanced fish passages. More than 50 fish species found in the waters around Stockholm can now migrate freely between Lake Mälaren and the Baltic Sea.

Most of Stockholm’s fish species migrate between fresh and brackish water. In waterways, dams, locks and other barriers can make migration difficult, which can have a negative impact on aquatic animals. To help fish move upstream and downstream, we build artificial stream channels known as fish passages or wildlife passages – such as the one now in place at the new Slussen.

More information about the redevelopment of Slussen is available on the Stockholm växer website.

Slussen, Stockholm växer

The fish migrate between the Baltic Sea and Lake Mälaren

Fish and other animals usually migrate for one of two reasons: to reproduce or to find food.

The fish species found in Stockholm have evolved a variety of fascinating behaviours and adaptations.

  • The sea trout migrates upstream into flowing rivers and streams in autumn, where its eggs can develop with less competition.

  • The European eel crosses the Atlantic after hatching and then grows up in Lake Mälaren and its tributaries.

  • Northern pike and European perch migrate into freshwater, seeking shallow, warm bays where their young grow quickly and have plenty of food.

In Stockholm, fish migration has long been restricted by the regulation of Lake Mälaren, locks and strong currents. The new fish passage now allows all fish species to move freely between the Baltic Sea and Lake Mälaren, Sweden’s third-largest lake.

Counting and Measuring Fish

Although the fish passage in Söderström is a highly engineered solution, its design incorporates stones of different sizes, creating a natural-looking stream channel. The stones slow and divert the water flow, creating sheltered areas that allow even weaker swimmers, such as roach, bream and European perch, to migrate successfully.

The fish passage is open year-round and is maintained through gates at either end of the structure, which is just over 100 metres long. At the Lake Mälaren end, a fish counter equipped with a camera records all fish and other aquatic animals migrating to and from the lake.

The camera counts, measures and identifies the fish species using the passage. This data is invaluable for monitoring which species move through the area. It can also be used to detect tagged fish and invasive species that are not wanted in Lake Mälaren.

If you would like to see the fish swimming through the fish passage and learn more about them, you can visit a webpage with live video and statistics.

Fish Counter, Slussen, on the Fiskdata website

Which fish can you see?

These are some of the fish species that use the fish passage at Slussen.

Perch

The perch is a medium-sized predatory fish. As a juvenile, it feeds on small aquatic animals, and once it reaches about 15 cm (6 inches) in length, it mainly eats other fish. It spawns in April–May and is common in most lakes and along the coast. Perch are generally sedentary schooling fish and can live for more than 20 years and weigh over 3 kg (6.6 lb). They are highly popular as both sport fish and food fish.

 

 

Pike

The pike is a voracious predatory fish that begins feeding on other fish just a few months after hatching. It spawns between March and May. The largest pike are generally females, as they grow faster than males. The species plays an important ecological role, particularly along Sweden's coasts, where it helps keep populations of small fish, such as the three-spined stickleback, under control. Pike is highly regarded as a sport fish but is less valued as a food fish. Catch and release is therefore recommended, especially in coastal waters.

 

Bream

The bream is a deep-bodied, laterally compressed member of the carp family with many small bones. It feeds mainly on small aquatic animals. It spawns from May to July in shallow, vegetation-rich waters, during which time the male develops small white breeding tubercles on its head. Bream can weigh up to 9 kg (20 lb). It thrives in warm water and is very common in lakes and along the coast. It is a popular species among coarse anglers.

 

Sea trout

The sea trout is a predatory fish that hatches in flowing freshwater, where it feeds mainly on small aquatic animals. Once it reaches about 20 cm (8 inches) in length, it begins feeding on other fish. It spawns in streams and rivers from October to December. The young remain in the stream for one to three years, passing through three juvenile stages—parr, stirr, and finally smolt—before migrating to coastal waters, where they develop their characteristic silvery scales.

 

Zander

Zander is a nocturnal predatory fish related to the perch. Young fish feed on small aquatic animals before switching to fish when they reach about 10 cm in length. They spawn between May and June.

Zander can grow to weigh up to 14 kg, although most weigh between 3 and 6 kg. During the summer, they usually live in open water in large lakes and sheltered coastal waters. In Sweden, they are common in the larger lakes and the inner archipelago.

Zander is a popular food fish. It is commonly caught in Lake Mälaren by trolling, which requires a valid fishing permit.

Fishing permit

 

Roach

The roach is one of Sweden’s most common species of carp and is found in most lakes and throughout the archipelago. It is easily recognised by its silvery scales and distinctive red eyes. It is usually 15–30 cm long, but can grow up to 50 cm. The roach is an omnivore and feeds on a variety of food sources, including insects, plant material, and zooplankton.

European eel

The European eel is a long, snake-like predatory fish that eats almost anything. It is born in the Sargasso Sea, about 6,500 kilometres from Sweden. The eel larvae drift with the Gulf Stream to Europe. When they arrive, they are called glass eels.

In spring, mainly in April and May, eels move into lakes and rivers. They live there for 5–25 years before swimming back to the Sargasso Sea to spawn. After spawning, they die.

Fishing for European eel is prohibited.

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