Myth 1: False
Noble crayfish can be found in about a thousand Swedish lakes and streams.
Noble and signal crayfish distribution in Swedish lakes and streams in 2010.
Noble crayfish, nearly 1,000 sites.
Signal crayfish, over 4,000 sites.
The number of sites has decreased by almost 500 in just 8 years because of crayfish plague outbreaks. The Species’ future in Sweden is critically endangered according to the Species Database Red List.
Source: Institute of Freshwater Research’s crayfish database.
Myth 2: False
Noble crayfish grow just as fast if the conditions, such as access to food, water temperature, and the population density, are the same.
Total length of three year old crayfish (Based on documented studies and cultivation data for crayfish of known age)
Noble crayfish:
Signal crayfish:
Source: SLU Aqua, Institute of Freshwater Research
Advanced reading:
Myth 3: False
They walk forwards. They can back up (by paddling with their tail), if they are frightened, but who can’t do that?
We have never seen a crayfish walk backwards, despite more than 100 hours of videotaped behavioral studies and direct observations of crayfish in aquariums and troughs at the Swedish Agency of Marine and Water Management’s freshwater laboratory. They have always turned 180 degrees and walked forward.
Source: Swedish Agency of Marine and Water Management, UR
Myth 4: False
20% of the restocking in the south of Sweden has failed. Where successful it will take at least 5 years before one can get an average take of 1 crayfish per cray pot and night.
Signal crayfish waters with very poor catches in recent years (from a decline of more than 2/3 of the catches down to almost total elimination of the stock). Total 134 dots.
Blue – streams
Red – lakes
Yellow – ponds
Source: Institute of Freshwater Research’s crayfish database.
Myth 5: False
It is illegal to stock crayfish without a permit.
Legally, crayfish are the same as fish.
Regulation (SFS. 1994: 1716) on fisheries, aquaculture, and the fishing industry
The stocking of fish, etc.
16 §
A permit from the provincial government is required in order to introduce fish or move fish from one body of water to another, or to build and operate a fish farm. Such permit may be subject to conditions.
Swedish Agency of Marine and Water Management’s regulations (FIFS 2001: 3) on cultivation, planting and moving of fish
4 §
Permits may not apply to signal crayfish in bodies of water
- on Öland, Gotland, or north of Dalälven
- within or adjacent to areas with acute crayfish plague
- within or adjacent to areas with noble crayfish populations
- which reach into Norway
- where the species is not present today or where permits have not previously been granted for planting the species.
Permits may not apply to crayfish in a body of water in which the crayfish plague has occurred during the last two years.
Source: SLU Aqua, Institute of Freshwater Research
Myth 6: False
Plague spores starve to death in a couple of weeks after the last crayfish have died.
The crayfish plague is a parasitic fungal disease that needs nourishment from the crayfish. Thus, crayfish plague spores have a short lifespan without the crayfish.
No resting stages, oospores, or intermediate hosts for the disease have been found.
There are examples from Sweden with up to 60-year’s survival of noble crayfish after a plague outbreak and the re-establishment of the noble crayfish.
Today, some 20 bodies of water in Värmland have good stocks of noble crayfish, even though they have a history of crayfish plague.
Source: Institute of Freshwater Research’s crayfish database.
Advanced reading:
Myth 7: True
When the temperature has dropped to about 10 degrees Celsius, the crayfish are willing to mate. The male turns the female on her back and presses his package of sperm between her third pair of legs and her tail.
Mating takes place in October and happens quickly. The male holds the female’s claws, turns her on her back, and applies the sperm capsules. From one day to a week after mating, the female squeezes the roe out of her genital openings, which sit at the base of the third part of walking legs. The eggs are fertilized by the sperm and are stuck to the swimming legs under the tail in a gelatinous substance secreted by special glands. The spawn hatches in June of the next year.
Source: Swedish Agency of Marine and Water Management, UR
Myth 8: False
Signal crayfish are more resistant than noble crayfish to the plague because they can encapsulate the disease in their shell. Therefore, they are always carriers of the infection and may also suffer from the disease if they become stressed.
- Signal crayfish carry the crayfish plague
- If they are stressed they can die in an acute plague
- High mortality occurs in crayfish populations – due to acute crayfish plagues
- Acute crayfish plagues have struck signal crayfish in Hjälmaren 3 times
- Two strains of crayfish plague are known in Sweden (DNA technology)
- The strain that came to Sweden with signal crayfish occurs in all cases of crayfish plague since analyses began in the 1970s.
Source: SLU Aqua, Institute of Freshwater Research
Advanced reading:
Myth 9: False
Restocking noble crayfish is often successful. If it fails, it’s often because people put plague-bearing signal crayfish in the same waters.
Waters in Värmland where noble crayfish have been re-established after the plague.
| Locale | Plague declared |
|---|---|
| Billan | 1970 |
| Bysjön | 1972 |
| Bålgårdstjärnet | 1981 |
| Finnsjön | 1956 |
| Fjällsjön | 1971 |
| Glasälven | 1972 |
| Gravtjärn | 1956 |
| Dalsälven | 1971 |
| Stenbybäcken | 1971 |
| Stora salungen | 1981 |
| Vikareälven (Viksälven) | 1972 |
| Vrångsälven | 1970 |
| Yrsjöälven | 1971 |
| Älgån | 1972 |
| Yngen | 1991 |
| Ullen | 1981 |
| Stömnesjön | 1973 |
| Vargsjön | 1956 |
| Öjesjön | 1971 |
Source: Swedish Agency of Marine and Water Management’s crayfish database
Myth 10: False
The noble crayfish has good prospects for survival if the illegal stocking of signal crayfish ceases.
Number of lakes in Stockholm County that have been affected by crayfish plague during various time periods (County Administrative Board)
Today there are scarcely 100 crayfish populations remaining within the county. Each year 4-5 populations of crayfish are affected. At this rate there will be no noble crayfish left in the Stockholm region in 20 years.