Where rivers lead to the sea, many different animal species live – both on land and water. At the mouth of the Elbe into the North Sea, the fish fauna has changed significantly in the past 40 years, as a study now shows. The number of fish in the mouth of the Elbe is therefore at a historical low. The researchers are particularly responsible for this decline, in which suspended matter are released.
Aestuar – wide water bodies that form where a river flows into the sea – make up only about 0.35 percent of all habitats in the world. But they are of great ecological and economic importance. Ships find a natural harbor and especially birds and fish a unique habitat. Because with ebb and flood, salted sea water flows towards the river in an aesties and thus ensures that brackish water is created in the mouth and the water level fluctuates. These special conditions mean that many different animal species feel comfortable on land and in the water.
91 percent fewer fish
The mouth of the Elbe northwest of Hamburg is one of the largest Aesturs in Europe and the largest in Germany. As the animals living there, a research team around Jesse Theilen from the Leibniz Institute has now examined a research team for analyzing biodiversity change. The researchers wanted to know how the intervention of humans and climate change affect the fish stocks. To do this, they analyzed data from 1984 to 2022 from five measuring stations along the Elbe. The stations measure, among other things, the water temperature, suspended matter, oxygen content and the content of the nitrite, which is poisonous for fish. In addition, they evaluated data on the number and the composition of the fish that have been caught in this aestuary in the past 40 years.
The results are gratifying at first: by 2010, water quality improved in the mouth of the Elbe and the number of fishing increased. For example, only about 12,000 stints lived in the Aestuar in the 1980s, but their stock grew up to 168,000 copies by 2010. After 2010, however, the situation changed drastically: the entire fish population in the mouth of the Elbe decreased by 91 percent by 2022. The stint population alone shrank to around 13,000 copies up to this point. The stocks of Finte, Flounder, Kaulbindsch and Brachse also decreased, as the analyzes showed. In many types of fish, the number of larvae and young fishing was declining. In contrast, however, the density of the types of sea fish increased herring and Wittling.
Elbe deepening as a driving force
“The long-term changes in the fish stocks of the Elbe mouth can be closely linked to various environmental factors. While in the 1990s a significant improvement in water quality benefits the recovery of many types, the conditions have been significantly deteriorated in recent years,” explains. The researchers attribute the recently reduced fish density primarily to Schlick, which was said to be in important growth areas of the fish.
This slick accumulated in the course of the so -called “Elbe deepening”. By repeatedly dredging the Elbe, its flow behavior changed: the deeper your river bed is, the more and faster the water flows. In the case of flood, the water flows up streams strongly and brings sediment from the North Sea into the Elbe. At low tide, on the other hand, the water flows more slowly and sinks more – the registered material, i.e. the silt – remains.
With the stronger penetration of sea water, the salinity also increased in formerly less salty areas and disturbed the ecological balance. At the same time, due to less rainfall, less water flowed into the Elbe as a result of climate change. As a result, fewer sediments were washed out. In addition, the floats released during the excavations ensured poorer views in the flow, which means that the fish can look for food worse. “Our data show that fish stocks come under great pressure in the long term if their habitat is burdened by environmental changes,” says Sege.
Source: Jesse Theilen (Leibniz Institute for Analysis of Biodiversity Change) et al.; Estuarine, Coastal and Shelf Science, DOI: 10.1016/J.ecss.2025.109208