Crayfish plague continues to devastate Sweden’s native noble crayfish. In just a few decades, populations have plummeted from around 30,000 to fewer than 600 – and the trend continues downward.
The biggest culprit is illegal releases of non-native signal crayfish, which are themselves heavily fished and can also be affected by the disease.
The signal crayfish, which spreads the disease, has increased dramatically and today exists in between 10,000 and 15,000 populations. In Värmland, a province in western Sweden, illegal releases have been documented in as many as 239 bodies of water between 2000 and 2024.
— Noble crayfish populations in Värmland and Dalsland were 430 in the early 2000s; today only 60 remain, says Lennart Edsman, crayfish expert and researcher at the Freshwater Laboratory at the Swedish University of Agricultural Sciences (SLU), to Swedish news agency TT.
When August and crayfish season arrives, the question arises of which species the environmentally conscious consumer should choose for their crayfish party.
— You should eat noble crayfish if you can afford it. That gives them value that makes them worth protecting. And you should eat signal crayfish too, but absolutely not spread them, Edsman believes.
Imports worth hundreds of millions
Swedes eat far more crayfish than the country can produce. Between 70 and 80 percent of the crayfish on tables are imported. In 2023, imports amounted to a value of €40 million, while Swedish crayfish were sold for €27 million.
Previously, the largest portion of imports came from China, but today Spain, Turkey and also Egypt dominate the Swedish market.
Most Swedish-caught crayfish are signal crayfish, with a large share coming from lakes Vättern and Hjälmaren in central Sweden. Recently, however, many consumers have complained that the crayfish have become smaller.
— This is partly because fishing has been too intensive. There is great demand for crayfish in this country. Sweden is quite extreme when it comes to crayfish consumption, Edsman explains.
The signal crayfish originates from western North America, as does crayfish plague – a parasitic algae fungus. Although the species is more resistant than the noble crayfish, it is not immune to the disease.
How the signal crayfish took over
The signal crayfish originally comes from western North America and was introduced to Sweden in the 1960s as a way to replace the noble crayfish, which had been severely affected by crayfish plague at the time. The idea was to preserve crayfish fishing and its economic benefits, since the signal crayfish is more resistant to the disease than the noble crayfish.
The problem is that the signal crayfish carries the very crayfish plague – a parasitic water mold – that is deadly to the noble crayfish. Although the invasive species itself can be affected, it is significantly more resilient, which means it functions as a disease carrier and accelerates the decline of the noble crayfish.
Since its introduction, the signal crayfish has spread rapidly and is now found in between 10,000 and 15,000 populations across Sweden. Many of these have resulted from illegal releases. The consequence is that the noble crayfish has declined from around 30,000 populations to fewer than 600 throughout the country.
