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).
