Do our freshwater crayfish grow differently? Master’s student Nick Paulus is seeking the answers

Through data analysis, field studies, and laboratory experiments, Nick hopes to contribute to new insights that can support both aquaculture and conservation efforts in his ongoing master’s thesis project at SLU. 

What is your master’s thesis project about?

I’m working with Swedish freshwater crayfish – the native noble crayfish and the invasive signal crayfish. What I want to find out is whether these species adapt to the different environments they live in, and if so, how. I’m particularly interested in their life history, for example why noble crayfish in the north tend to grow more slowly than those in the south, which could be due to food in the north being more seasonal. For the signal crayfish, the focus is more on how management practices might be influencing population traits, like body size, and whether high fishing pressure could be driving changes over time.

How will you study the noble crayfish?

There is already collected data from a previous study, so my task is to analyse these and compare key traits between different populations of noble crayfish. Since the noble crayfish has been in Sweden since the ice age, it seems reasonable to assume that it has had time to adapt locally – but surprisingly, no one has studied this in depth yet.

And the signal crayfish – what makes it interesting for your work?

The signal crayfish was introduced to Sweden in the 1960s and 70s, so in theory it’s too early for any major evolutionary adaptations. Still, some populations seem to be getting smaller, and many believe this is due to competition for food. One problem is that we don’t know whether the different populations of signal crayfish have had time to adapt to their environments – and that’s where my thesis comes in. I will head out into the field to catch signal crayfish and then carry out experiments in the lab at the Institute of Freshwater Research. 

How will the experiments work?

I’ll be following signal crayfish from different populations that hatched this year. We’ll catch crayfish in lake Mälaren, Vättern and Åvaån and then place them in crayfish aquariums. Here, I'll rear each crayfish indiviudally in tanks under identical conditions – same food and same temperature – from September 2025 to January 2026. If they grow at the same rate, it means that the differences in nature are due to the environment. But if they still grow differently despite identical conditions, that would point to genetic differences that may at least partly reflect adaptations to different environmental conditions in the lakes.

Why is it important to understand these patterns?

For the noble crayfish, the results can help us improve conservation efforts, for example when choosing which populations are best suited for different waters. For the signal crayfish, it is more about management. If food scarcity makes them smaller, or if certain populations have adapted in different ways, we may need to rethink how we fish the species. Understanding whether the changes are driven by environment, competition, or genetics can therefore help us improve how we manage this invasive species.

What do you hope to achieve with your work? 

To contribute to a better understanding of Swedish freshwater crayfish. I also hope that the results can be applied in conservation, aquaculture and fisheries management and help us develop smarter ways to handle invasive species in the future.

Finally, what do you appreciate most about your master’s thesis project?

I’m really looking forward to the whole project, but the most exciting part is the fieldwork and setting up and designing the experiments. Fieldwork is something that us students don’t always get the chance to take part in, which makes it especially rewarding and adds an extra dimension to my master’s thesis.

Read more about our Swedish freshwater crayfish

Read more about the program where Nick is studying: Conservation and Management of Fish and Wildlife

See available degree projects at the Department of Aquatic Resources