Improving the habitat for the noble crayfish in the Stockholm archipelago

The noble crayfish is Sweden's only native large freshwater crayfish. The greatest threat to the species is the crayfish plague, which is spread by the illegal release of signal crayfish. Another threat is silting of the seabed. However, dredging of silted lakebeds can improve the crayfish's habitat.

About the project

Since 2020, we at SLU Aqua have been investigating, together with the County Administrative Board of Stockholm County and local water owners, whether the habitat for noble crayfish can be improved by removing sediment through dredging. Crayfish live in shallow waters where they dig burrows in places with plenty of roots, rocks or other hiding places. Dredging exposes rocks and gravel and creates a habitat that can benefit crayfish. The idea is that crayfish fry and adults will find their way back to new habitats after dredging has been carried out.

Crayfish populations before and after dredging – studies in the Stockholm archipelago

The experiment includes two lakes with different conditions for crayfish in the Stockholm archipelago. To detect changes in the lakes, we will study the populations before and after dredging for a few years. We will carry out standardized test fishing for crayfish to estimate the size of the populations. We will also test fish for crayfish fry using specially designed fry catchers, and divers will survey roe-bearing females. Futhermore, we will film the lake bottoms to document the effects of the dredging.

In 2021, we have conducted the following surveys:

• standardized test fishing of crayfish in the lakes,
• test fishing of juvenile and adult crayfish in the dredged areas as well as in undredged reference areas have been carried out,
• diving and mapping of roe-bearing females,
• measurements of water temperature.

Trial fishing of crayfish fry

With data from specially built fry traps, we can estimate changes in the populations and growth of the younger crayfish. Two types of fry traps have been tested so far in the experiment. The traps are checked regularly and the fry are counted, measured and their sex is determinened. We also get a measure of the catching efficiency of the gear.

Results

More information coming.