"Dolly ropes are used all over the world in various types of bottom trawling. This controversial fishing method involves dragging huge nets across the seabed. To prevent the actual nets from tearing open when they come into contact with rocks or mussels, special polyethylene ropes are attached to the fishing nets - the dolly ropes. Over time, particles or larger pieces of these ropes often break off, drift in the sea and wash up on the beach. In the southern North Sea and the Wadden Sea, they are mainly used in crab and flatfish fishing.
In the breeding colonies of gannets on Heligoland, these plastic threads from fishing can be found in almost every nest. They use the mostly blue or orange-coloured threads to build their nests, often with fatal results. This is because the artificial building material often becomes a trap: many birds strangle themselves with it, drown or starve to death because they are unable to free themselves.
Nets with dolly ropes are currently still permitted in the Lower Saxony Wadden Sea National Park. The foundation Rüm Hart has now launched a petition calling for a ban on the use of synthetic dolly ropes in the Wadden Sea National Parks. If the petition receives at least 5,000 signatures within the six-week period, the Petitions Committee will hear the petitioner in a public committee meeting.
Baltic Sea also affected by plastic pollution
Fishing materials and other plastic waste have an immense impact on the marine environment. A year ago, the organisers of "The Ocean Race" published the results of the Ocean Race Europe. Three teams had taken water samples en route for a scientific study. They took 36 water samples on the way from the Baltic Sea to the western Mediterranean. All samples were found to contain microplastics in subsequent tests by the Geomar Institute in Kiel, with the highest concentration of 430 particles per cubic metre in the Baltic Sea.
Sharp decline in the fish population
In addition, fish stocks in the Baltic Sea are in sharp decline. This is despite drastically reduced catch quotas. In 2017, a fisherman was still allowed to catch 15,000 tonnes of herring; in 2022, it was only around 435 tonnes.
One cause of the fish decline is the so-called dead zones, which now make up a quarter of the Baltic Sea. Dead zones occur where there is a disproportionately high algal bloom in the sea. Studies blame this primarily on agriculture. Fertilisers are washed from the fields into the sea via the watercourses. As soon as microorganisms have used up the nutrients, the plants die and sink to the bottom of the sea. The bacteria that finally decompose them there require a lot of oxygen for the process.
This reduces the oxygen content in the water to such an extent that a large proportion of the resident marine life will die if it does not migrate. The ecosystem is severely disrupted and the consequences are devastating. In addition to the deep areas of the Baltic Sea, the dead zones are increasingly reaching shallower coastal regions. The death of seagrass meadows, for example in the Greifswald Bodden, is evidence of this.
Scientists have recently discovered another reason for the decline in fish stocks. Natural food chains are being destroyed by the warming of the water due to climate change. Because there is simply no food available for them, millions of herring larvae are starving to death.
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Max Gasser
Editor Test & Technology
