The west coast of Sweden

28 February, 2024

On Sweden’s west coast lies the Western Sea with Sweden’s saltiest water and the largest marine biodiversity. The Western Sea includes both Kattegat, Skagerrak, and the Øresund and is home to species such as mackerel, kelp, octopus, sea stars, corals, porpoises, and a total of 17 species of sharks.

Skagerrak, located furthest north in the Western Sea, is home to about 1500 animals and plants, thanks to the high salinity of the seawater, around 35 parts per thousand. Together with the Väderöarna Nature Reserve in the archipelago of Bohuslän, Kosterhavet is one of Sweden’s most species-rich marine areas. Several fjords in Bohuslän are considered so valuable that they are classified as Natura 2000.

However, as you move further south along the west coast, the water becomes fresher due to inflows from rivers, streams, and brackish water from the Baltic Sea. In Kattegat and around Øresund, the salinity is only around 15 parts per thousand. Salinity largely determines which species can thrive in the sea. This results in the number of species in the less salty Kattegat being only about half of that in the saltier Skagerrak.


Since 2009, Kosterhavet has held the title of Sweden’s first marine national park. The varying depths, high salinity, deep hard bottoms, and coral reefs in Kosterhavet are unique habitats for Sweden, contributing to a rich biodiversity of approximately 12,000 species. Moreover, there are around 200 animal species and nine algal species that exist only in Kosterhavet.

The salty and cold deep-sea water that enters from the Atlantic, through Kosterfjorden’s deep trench at 250 meters depth, allows deep-sea animals to live close to the coast. This also provides unique conditions for unusual sponge animals, corals, stingrays, sea pens, sea anemones, and bryozoans to coexist. The national park is also home to the largest population of harbor seals in the Western Sea, studied by researchers since the 1970s.


Gullmarsfjorden is a Natura 2000 area with unique marine life, attracting both divers and researchers. In 1983, Gullmarsfjorden, or Gullmarn as it is sometimes called, became Sweden’s first marine protected area, bordering several nature reserves on land. The fjord is 25 km long, 1–3 km wide, and Sweden’s only threshold fjord, meaning the threshold at the mouth is shallower (around 40 meters) than inside (up to 120 meters). This affects the exchange of water with Skagerrak, creating a distinct thermocline between the salty bottom water from the Western Sea’s depths and the fresher surface water from the Baltic Sea, Øresund, and local runoff.

The environment in Gullmarsfjorden has been well-documented, with scientific studies conducted since the 1830s. Today, several research stations at Gullmarsfjorden allow close observation of small organisms such as krill and coral, as well as a local cod population.


Between Sweden, Norway, and Denmark lies another Natura 2000 area, Bratten, the largest marine protected area in Skagerrak. The over 1,000 square kilometers area is characterized by deep hard and soft bottoms, valleys, and holes believed to have formed through gas leakage from the Earth’s crust. In Bratten, Sweden’s deepest marine area at 560 meters is found. Many rare fish and invertebrates inhabit Bratten due to its unusually deep hard bottom environment. Protected species like deep-sea corals, sponge animals, sharks, stingrays, rabbit fish, and halibut are present. Despite the area being protected, bottom trawling is still allowed in large parts of Bratten, and it is an important area for fishing North Sea shrimp and large fish on the deep mud bottoms.


The Western Sea faces several challenges, with fishing and overfishing being among the most significant. Fish stocks have drastically decreased over the last 100 years. The situation is most severe for cod in Kattegat, but many bottom-dwelling fish have also declined so much that predicting their future is difficult. The decline in cod has led the International Council for the Exploration of the Sea (ICES) to recommend a complete halt to cod fishing in recent years.

There are particularly few old, large cod capable of producing high-quality eggs and larvae in the Western Sea. The disappearance of large fish can affect other parts of the food chain through trophic cascades. Along the coasts of Skagerrak, the lack of large fish is believed to contribute to shallow bays filling with filamentous algae and eelgrass, which serves as a nursery for, among other things, fish fry, disappearing. This can lead to loss of species for the entire ecosystem. In Øresund, on the other hand, trawling has been prohibited since the 1930s, resulting in a more normal distribution of cod sizes in the area.

The situation regarding pollutants is in many ways better on the west coast than in the Baltic Sea. Dioxin levels in herring from the west coast have decreased since the 1990s and are safe to eat. However, in some cases, the development is heading in the wrong direction. Mercury in cod is increasing in Kattegat, and the levels of the perfluorinated substance FOSA and arsenic are higher in herring in the Western Sea than in other marine areas.

Painting boats with bottom paint containing organic tin compounds is a serious problem in the Western Sea, despite being prohibited in both Sweden and the EU since the 1990s. The toxins still exist in sediments from shipping lanes and harbors. Thanks to the ban, these substances have decreased somewhat in recent years, but they seriously harm marine life as they are toxic even at low concentrations. For example, snails have been found to develop reproductive organs of both sexes when exposed to toxins from bottom paint.

Some areas along the west coast show signs of eutrophication. This occurs in places where the nutrient load from the land is high, and water exchange is limited. However, nitrogen levels have decreased since the 1990s, suggesting that measures to reduce nutrient input have been effective. But the disappearance of cod has shown to have as much eutrophication effect as nutrient loading.

Like the Baltic Sea, toxic algal blooms also occur on the west coast, primarily caused by dinoflagellates. In some cases, for example, blue mussels can become inedible when these algal toxins accumulate. Some dinoflagellates can also illuminate the water through a phenomenon called bioluminescence. It occurs in late summer and is not harmful on the west coast, but in the Baltic Sea, it can be toxic.

Due to the changing marine environment and increased transportation, the Western Sea has gained new species in the form of alien and invasive species. Alien species are those that have spread outside their natural range with human assistance. Some of these are considered invasive as they can cause problems. In marine environments, foreign species often find their way to new locations through ballast water taken on board in one port and discharged in another. Some examples of foreign species in the Western Sea are the comb jelly Mnemiopsis leydyi, the Japanese giant oyster, and the Sargasso snare. They can take over large areas and disrupt the biological diversity.

Text: Lina Mattsson
Foto: Göran Ehlmé, Tobias Dahlin, Micke Tilja

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