Fish and shrimp are once again being suffocated in our waters.

I Sebber Lake and Limfjorden, dead fish were observed at the beginning of July - later in the summer, fish and shrimp were dying in Vejle Harbour.

The oxygen deficiency in our inland waters has increased significantly over the summer, and it is expected to worsen in the coming weeks.

At the end of July, there was oxygen deficiency over nearly 1,200 km² of the Danish inland waters. One month later, the area had grown to over 2,200 km². The oxygen deficiency thus expanded from an area the size of Lolland to an area the size of two-thirds of Funen.

At the same time, the oxygen deficiency has become more severe. In July, there was severe oxygen deficiency over 16% of the affected area. In August, it affected 48%. Oxygen deficiency is defined as water at the bottom containing less than four milligrams of oxygen per liter. Less than two milligrams indicates severe oxygen deficiency.

"A huge area"

The figures are from a new report from the National Center for Environment and Energy (DCE) at Aarhus University. It is based on measurements from Danish, Swedish, and German authorities from July 1 to August 26.

"It is a huge area," says Jens Würgler Hansen, a senior advisor at Aarhus University, to dr.dk.

"In recent years, the marine environment has really been under pressure," he continues.

He explains that the overarching cause of the problem is that there are too many nutrients in the sea - primarily from agriculture, from the air, and neighboring countries. This causes algae to grow, and when they sink to the seabed, the dead plant material is eaten by bacteria, which in the process consume oxygen, leading to oxygen deficiency. Therefore, he also expects that the new fertilization law will have a positive effect on marine life.

Especially hard-hit around Funen

Although the oxygen deficiency increased over the summer, the total area was about one-third smaller than in July and August last year. The area with severe oxygen deficiency in August was at the same level as August in 2020, 2022, 2024, and 2025.

At the end of August, the oxygen deficiency was particularly widespread and intense in the waters around Funen. Among the most affected areas were the northern Belt Sea, the southern Little Belt, Aabenraa Fjord, and Flensburg Fjord. Parts of Limfjorden, Mariager Fjord, Knebel Vig, the western part of the coastal waters in Aarhus Bay, and the Fehmarn Belt were also heavily affected.

In the northern Belt Sea, moderate oxygen deficiency was already measured during the first inspection of the year in early July. This is unusually early, the authors write. By the end of July, the oxygen deficiency had spread throughout the water body, and during August, it became more severe.

Dead fish and toxic hydrogen sulfide

In several places, there was completely or almost completely oxygen-free conditions at the bottom. In Sebber Lake in Limfjorden, Mariager Fjord, Knebel Vig, Nybøl Nor, and Flensburg Fjord, this caused the release of hydrogen sulfide from the seabed. Hydrogen sulfide is a highly toxic compound that can kill bottom-dwelling animals and fish.

At the end of August, dead fish and shrimp were observed in the innermost part of Vejle Fjord, especially in Vejle Harbour. According to the report, this is probably due to oxygen-poor water from outside. Several days of strong west winds had pushed surface water out of the fjord, and instead, oxygen-poor bottom water flowed in from the outer part of the fjord and the northern Belt Sea.

In the worst-affected areas, fish and the most mobile animals seek to escape. Less mobile animals such as worms, snails, and mussels are dying or dead, the authors write. When bottom-dwelling animals die, the fish's food disappears, so after severe and prolonged oxygen deficiency, it can take many years for the seabed to regain a normal animal life.