The threshold between the Baltic Sea and the Atlantic Ocean
Öresund is the water between Sweden and Denmark that connects the Baltic Sea with the Kattegat, and eventually the North Sea and the Atlantic Ocean beyond. Öresund is shaped like a narrow strait. This strait has been opened and closed many times over the millennia, depending on ice ages, land uplift and the movement of continental plates.
Map of the Öresund area.
The entrance to Öresund, with Helsingborg in Sweden on the left and Helsingör in Denmark on the right. This part of the strait has been closed off many times over the years. As a result, the Baltic Sea has sometimes been a large lake or inland sea, and not part of the ocean.
Photo: Bjoertvedt-CC-BY-SA
Rare species in Öresund
Many species live their natural lives in Öresund. But sometimes other species, from the North Sea or the Atlantic, get lost. Then they find themselves on a short visit to Öresund. Bluefin tuna, the largest species of tuna, swim over huge areas. Occasionally, an individual swims into Öresund. A basking shark, the world’s second largest shark, may also visit from time to time. Other sharks that may visit Öresund are the blackmouth catshark and the starry smooth-hound.
Bluefin tuna, a huge fish that can weigh as much as 250 kg, sometimes visits Öresund.
The basking shark, the world's second-largest fish, sometimes makes unexpected visits to Öresund. The basking shark is completely harmless and eats only plankton.
The blackmouth catshark, a small shark that prefers to live at great depths, is rare in Sweden. But it happens that it visits Öresund.
The common stingray is found along the Norwegian west coast. Sometimes it visits Öresund.
Photo: Chris-Taklis-CC-BY-SA
The Tub gurnard usually stays at the bottom of the Atlantic ocean, but occasionally enters Öresund.
Photo: Frank-Christian-Baum-CC-BY-SA
The Atlantic wolffish, a fish that is listed as endangered in Sweden, sometimes swims into Öresund and a bit north into the Baltic Sea.
Photo: Cephas-CC-BY-SA
The common monkfish has a huge mouth and can grow very large. An adult monkfish can weigh as much as 100 kg. It is rare, but it happens that it visits the waters of the Sound.
Photo: Tiit-Hunt-CC-BY-SA
Species increasing due to climate change
Climate change, which is warming the Earth, has had a huge impact on the oceans. The Baltic Sea is one of those hit hardest – its waters have become warmer and its salinity (salt level) has decreased. The warming of the oceans is leading to excess nutrients (eutrophication) and oxygen depletion. Some species are dying out, while others are declining greatly in numbers.
But there are also aquatic species that are good at adapting to change. They are benefiting from a warmer climate. They can reproduce more, taking over habitats from species that have died out. Most fish species grow and develop faster and have shorter life cycles in warmer waters.
The inactive nuclear power plant Barsebäck, is located by the coast in Lomma bay in Öresund. Locally around the nuclear power plant, the water temperature is higher than the surrounding water, which means that visitors to Öresund who prefer warmer water are happy to linger and even stay around Barsebäck.
Photo: Jorchr-CC-BY-SA
The Viviparous eelpout likes warmer waters.
Photo: Julian-Fahrbach-CC-BY-SA
The whiting is a fish in the cod family, and seems to thrive in the warmer waters around Barsebäck.
Photo: Jesper-Flygare-Malmö-Museer
The lumpfish finds a lot to eat in the warm waters around Barsebäck.
Photo: Tiit-Hunt-CC-BY-SA
In the warmer waters around Barsebäck, unusually large individuals of cod have been found.
Photo: Hans-Petter-Fjeld-CC-BY-SA
Warm water around nuclear power plants
Seawater can also be heated very locally, such as around nuclear power plants, via the discharge of the heated cooling water. The water around the Barsebäck nuclear power plant in Öresund has become warmer than before. As a result, some rare visitors to the Öresund have stayed in Barsebäck for a long time.
Over the past 25 years, several fish species have become more common in the waters around Barsebäck. European eelpout and flounder have both increased. Plaice and pollack have also benefited. Whiting, lumpfish and perch find plenty to eat in the area. In addition, researchers have found unusually large numbers of big eels and cod in the warmer waters around Barsebäck.