Beståndsutveckling för signalkräfta i de stora sjöarna 2023: resultat inom projektet datainsamling sötvattenskräftor fram till och med 2023

The signal crayfish was introduced into Lake Vättern, Lake Hjälmaren and Lake Mälaren starting in 1969, after the native Noble crayfish had gone extinct because of the crayfish plague in all of Sweden’s large lakes. Today there are productive stocks of signal crayfish in Lake Hjälmaren and Lake Vättern. In Lake Vänern, the stocks have a positive trend, and catches are increasing, so far mainly in the South-eastern parts. The stock of signal crayfish in Lake Mälaren, which initially developed favorably, declined sharply after the year 2000. Today, there is no commercial fishing for crayfish in Lake Mälaren due to low catch rates, but recent years indicate a recovery. This report presents the results of standardized monitoring as well as sampling of size structure in fisheries catches of signal crayfish in Lake Hjälmaren, Lake Vättern and Lake Vänern during 2023. A comparison is also made to previous years’ data (2009–2023). Several sites were sampled per lake and the results differ between and within the sites. The six sites in Lake Vättern generally have a high abundance of crayfish compared to the other lakes. Monitoring data from the sites in the northern parts show that the average size of the crayfish is decreasing and that the number of crayfish per trap night is increasing in all but one site. The three sites in Lake Hjälmaren, also have a high abundance of crayfish. One of the sites in Lake Hjälmaren has an increasing number of crayfish per trap night, decreasing average size and decreasing number of crayfish above the legal minimum size during the monitoring period. For the other sites in Lake Hjälmaren, no clear changes over time are observed in the average sizes and the number of crayfish caught during the monitored period. The catches in the two sites in Lake Vänern are still modest in comparison to Lake Vättern and Lake Hjälmaren. There is, however, a slow general increase in the average size and number of crayfish per trap night. In one of the monitoring sites, total abundance and proportion of legal sized crayfish is increasing. There was no clear significant trend in the second site. Practically no commercial fishing has taken place in Lake Mälaren since 2000 and the catches in recreational and subsistence fishing have previously been very modest. The monitoring in Lake Mälaren indicates a slight recovery of the crayfish population and an increase in the proportion of crayfish above the legal minimum size, even though the over all abundance is still rather low. The potential recovery in Lake Mälaren will be investigated further by SLU Aqua together with the County Administrative Boards around the lake.