SLU news

More but smaller crayfish in Lake Vättern and Lake Hjälmaren

Published: 15 May 2024
Small signal crayfish

The number of signal crayfish is increasing in Sweden's largest lakes: Lake Vänern, Lake Vättern, Lake Hjälmaren and Lake Mälaren. But in several of the lakes, the proportion of small crayfish is also increasing. "The fact that we see signs of high densities of small crayfish is worrying, and it could pose significant challenges for commercial fishing in the future," says Patrik Bohman, environmental analyst at the Swedish University of Agricultural Sciences, SLU.

SLU's assessment of crayfish stocks is based on data from both test fishing and commercial catches between 2009 and 2023. Despite relatively low catches in 2023 primarily due to cold and windy weather conditions, the analyses show that signal crayfish are increasing in all four lakes. The highest catches are in Lake Vättern and Lake Hjälmaren. In Lake Vänern, the signal crayfish is still spreading to new places and in Lake Mälaren a slight improvement in numbers is noticeable in several places.

More small crayfish can lead to poorer growth

In several of the lakes, however, a worrying trend can be seen; the proportion of small crayfish is increasing. For years, Lake Vättern has recorded a decline in average crayfish size, a trend now also appearing in parts of Lake Hjälmaren.

- High densities of small crayfish can lead to numerous issues. Increased competition for food can hinder their growth, resulting in lower yields for fishermen. Additionally, large populations of small crayfish can negatively impact other aquatic species and the overall ecosystem." says Patrik Bohman.

Questioning the efficacy of reduced fishing

Intensive fishing, which disproportionately remove larger crayfish and leave smaller ones behind, are believed to be the primary cause of the declining average sizes. SLU has therefore previously proposed that fishing pressure should be reduced in areas with many small crayfish and declining average sizes.

"Unfortunately, we are not sure that a reduction in fishing is now enough to reverse the trend. We have seen this in the area around Tängan in Lake Vättern, where the intensive fishing ceased around 2016. Despite the reduced fishing pressure, the proportion of larger crayfish remains low," says Patrik Bohman.

SLU now wants to, together with the county administrative boards and local fishermen in Lake Vättern, launch a comprehensive study. This project will investigate crayfish growth patterns, their diet, and how various fishing strategies might influence the stock composition.

SLU's latest assessment of crayfish populations has been published in the report (in Swedish) Beståndsutveckling för signalkräfta i de stora sjöarna 2023

In collaboration with the Lake Vättern Society for Water Conservation (Vätternvårdsförbundet), SLU has also published a report on the impact of the signal crayfish on Lake Vättern’s ecosystem (in Swedish).


The signal crayfish is an invasive, alien species that can be fished in Sweden.

Signal crayfish began to be released in Sweden at the end of the 1960s, in waters where the native noble crayfish had become extinct due to crayfish plague. That the release and cultivation of signal crayfish is now prohibited in Sweden.

The signal crayfish has been shown to be able to be infected by crayfish plague - and spread it further - without dying itself. The native noble crayfish, on the other hand, always dies if it is infected. Illegal releases of signal crayfish are today the greatest threat to the critically endangered noble crayfish.

Read more about signal crayfish, noble crayfish and crayfish plague 


Patrik Bohman, Environmental Monitoring and Assessment Analyst
Department of Aquatic Resources, Institute of Freshwater Research, SLU, +46 10 478 42 17