Swedish researchers tag 2,500 crayfish weekly to probe stunted growth in lakes Hjälmaren and Vättern

Swedish researchers have launched one of Europe’s largest crayfish tagging projects to investigate why a high proportion of the population in lakes Hjälmaren and Vättern remain small, measuring less than 10 centimeters.

The initiative, led by the Swedish University of Agricultural Sciences (SLU), involves marking and releasing 2,500 crayfish per week this summer, according to Patrik Bohman, an environmental analyst at SLU. Bohman noted that it takes a week to tag that number of crayfish. Researchers aim to determine why so few crayfish grow large enough to be commercially viable, even in unfished areas where growth remains stunted.

“It’s very strange,” Bohman said. “We have a very small proportion that become large and commercially viable. Even in areas that aren’t fished, they grow very poorly.”

One hypothesis is increased competition for food due to high crayfish density in the water. “There are a lot of crayfish in the water, so it could be due to increased competition,” Bohman said. “But we don’t know that yet. These are the things we’re going to look at.”

The tagged crayfish will be recaptured, measured, and weighed multiple times, with the first follow-up scheduled for September. SLU is collaborating with American universities to conduct molecular tests, analyzing amino acids to assess the crayfish’s health and diet in both lakes.

Preliminary results may be available as early as next year. Bohman said researchers hope to extend the project for three years and potentially expand it to Vättern, where the problem is more pronounced. “We actually have bigger problems there,” he said.