Crayfish

Noble crayfish – a critically endangered species of great value

The noble crayfish (Astacus astacus) inhabits shallow waters along lake shores, in ponds and small watercourses.

It prefers steep banks where it digs deep burrows, or areas rich in roots, stones, or other hiding places. The noble crayfish is nocturnal and almost omnivorous, feeding on insect larvae, mussels, snails, fish eggs, and shoots of various aquatic plants. It can live between 5 and 20 years, with individuals up to 20 cm in length having been caught.

It is estimated that up to 97% of the Noble crayfish population has been wiped out over the past century, making it a critically endangered species. Despite this, the noble crayfish has high cultural, social, and economic value. Conservation efforts are underway across the country, focusing on cultivation and the improvement of noble crayfish fisheries.

More recently, the spread of infection from the invasive signal crayfish has accelerated this decline, along with factors such as acidification, pollution, and water regulation.

Signal crayfish – an invasive species and a disaster for the Noble crayfish

Like the crayfish, the signal crayfish (Pacifastacus leniusculus) lives in shallow lake waters, ponds, and small streams.

It too favours steep banks and hiding places with roots and stones. The signal crayfish is nocturnal and omnivorous, feeding on similar food sources. It also has a lifespan of 5 to 20 years, and can grow up to 20 cm.

Introduced in Sweden in the 1960s, the signal crayfish is a chronic carrier of crayfish plague. It has since established fishable populations in lakes Hjälmaren and Vättern, where commercial catches have increased significantly, mainly due to increased fishing effort.

Signal crayfish has provided good fishing opportunities in southern Sweden. However, its introduction has been catastrophic for the noble crayfish, with native populations declining fourfold since its arrival. As a result, the signal crayfish was officially classified as an invasive species in 2016. It is now illegal to farm or introduce the species into new waters. Introductions were banned as early as 1994, yet illegal releases have continued and remain a major issue, contributing to the continued spread of crayfish plague. Even signal crayfish can die from acute crayfish plague if stressed.

How to tell the difference between Noble and Signal crayfish

Not sure whether you’ve caught a noble crayfish or a signal crayfish? Here are four clear differences:

Noble crayfish

1. A row of small spines can be felt along the border between the head and carapace (A).
2. Dark, uniformly coloured claws with a dark thumb joint, often with a red wart (B).
3. Long, almond-shaped claws with many warts and spiny protrusions.
4. Head and carapace are dark, often blackish, with warts and spiny protrusions.

Signal crayfish

1. No spines along the border between the head and carapace (A).
2. Claws are often lighter underneath, with a white wart in the thumb joint, usually surrounded by a large white or turquoise patch (B).
3. Claws are broad, muscular, and smoother with indented pores.
4. Head and carapace are smooth with indented pores, usually in a clear brown shade.

Crayfish plague

Crayfish plague is a disease that only affects freshwater crayfish. It is a parasitic fungal infection spread via spores between individuals.

Both noble and signal crayfish can be infected, but while signal crayfish often survive and act as carriers, noble crayfish die quickly once infected.

Learn more about crayfish plague

As long as Signal crayfish are present in a watercourse, it is very difficult to restock with freshwater crayfish. However, if all the crayfish disappear from a water body, the fungal spores cannot survive either. A couple of weeks after the last crayfish has gone, the water is free of plague contamination again.

Spread

The spread of crayfish plague has increased dramatically in recent years, due to illegal releases of Signal crayfish. Between 1994 and 2007, nearly a thousand lakes, rivers, and watercourses were affected by crayfish plague.

Unfortunately, crayfish plague spreads quickly, and the number of infected water areas continues to rise. Further information about areas affected by crayfish plague can be obtained from the respective County Administrative Boards.

Spread through illegal releases of Signal crayfish

It is mainly the extensive illegal releases of Signal crayfish that have contributed to the rapid spread of the plague in recent years. The Signal crayfish is always a carrier of the disease, but it is less likely to fall ill. Crayfish plague can also be spread via wet fishing gear, other equipment, bait fish, or spores in water.

Freshwater crayfish more valuable

The areas in Sweden that have completely avoided being affected by crayfish plague are now limited to Norrbotten County. In addition to the loss of biodiversity, significant economic values are lost, as the freshwater crayfish has more than twice the market value of the signal crayfish. The freshwater crayfish is now classified as critically endangered on the Swedish Red List of Threatened Species (2020).

Signal crayfish can also fall ill

Signal crayfish have a naturally higher resistance to crayfish plague than Noble crayfish, but they can also become ill if stressed by other factors, such as pollution, poor food availability, high densities, or unfavourable water temperatures.

It is forbidden to release or move crayfish

It is forbidden to release or move Signal crayfish without permission from the County Administrative Board. Permits are only granted where the species is already legally established. There are also strict regulations on the handling of the species. For example, the entire island of Gotland has been a protected area for freshwater crayfish since 2007.

Dry fishing gear and boats

To avoid spreading crayfish plague, you should not move fishing gear, boats, or other equipment between different water bodies without first disinfecting them. The simplest way to do this is to let the equipment dry completely. This applies to all equipment, including boats, traps, and boots.

Only use bait fish from the same water you are fishing in, or freeze the bait for a day. This will eliminate the crayfish plague.

Education and information materials

Noble crayfish and Crayfish Farming in Sweden: A Handbook Based on Experience

In 2023, SLU launched a handbook on noble crayfish farming. The handbook is in demand among farmers, authorities, and the general public, and it plays an important role in efforts to conserve the increasingly threatened Swedish noble crayfish, as well as in promoting biodiversity. It provides valuable insight into the ecology of the noble crayfish and offers detailed guidance on the conditions needed to successfully start and manage a crayfish farm.

Here you can access and download the handbook (in Swedish).

"Kräftodlingens ABC" – The Movie

In Sweden, we have good conditions for farming noble crayfish. In this video, you'll learn what is required for a successful – and profitable – noble crayfish farming operation.

More information

Read more about the noble crayfish at SLU Swedish Species Information Centre.
Read more about the Signal crayfish at SLU Swedish Species Information Centre.