Signal crayfish – Pacifastacus leniusculus

An alien species legislated as invasive in the EU (List of invasive alien species of Union concern)

Signal crayfish has been laid down as an invasive alien species throughout the EU. The species may not be introduced into the EU nor released into the wild, disposed of, bred or cultivated, transported, transferred, passed on, sold, marketed or handed over to anyone.

However, a management plan has been drawn up for the signal crayfish. On the basis of the plan, the exploitation and trade of the signal crayfish population can be continued, but they must no longer be cultivated or planted in natural water bodies nor should they be allowed in any situation, even short term, to be caught in a keepnet outside the catching area. The management plan and regulations and detailed instructions related to signal crayfish can be found (in Finnish) Kansallinen rapustrategia 2019-2022

The signal crayfish undermines the living conditions of the native noble crayfish through competition and, in particular, by spreading and maintaining the crayfish plague. The signal crayfish is a crayfish plague carrier in most cases. The spread of the signal crayfish therefore reduces the habitats of the noble crayfish. The signal crayfish has been said to affect riverbanks or reservoir structures by burrowing, but in Finland the species is mainly present on the hard-bottomed areas of lakes, seeking shelter among rocks, and no significant damage has been caused by burrowing. The crayfish may also hamper net fishing. No significant adverse effects on fish populations or other aquatic organisms have been so far observed, but as signal crayfish become increasingly common and more widely distributed, they may, at least on a local basis, weaken the populations of slow-moving benthic organisms, the growth of submerged plants and the breeding results of fish, in particular.

Benefits

The signal crayfish is beneficial in economic terms and for recreational purposes. It has mainly been introduced to waters from which the noble crayfish is permanently absent due to crayfish plague, or so low in numbers that it is completely non-productive. When established in former noble crayfish waters, the signal crayfish has restored and maintained the Finnish crayfish catching and crayfish culture. Catches have increased significantly in the 2000s. In economic terms, it is one of the most important beneficial species in the inland fishing economy, replacing imported shellfish. The catch of signal crayfish has varied in Finland from less than one million individuals in the early 2000s to almost 8 million in 2009.

Control of signal crayfish populations and prevention of spreading

Methods for the eradication of signal crayfish have been studied to some extent, but no method has been found that would be specific to the species and which would not place other aquatic organisms at risk. The key way of reducing damage is to limit the spread of signal crayfish into new waters.

In December 2015, the species was included on the list of Union concern under the EU Regulation on Invasive Alien Species, and it is prohibited to introduce the species to new water bodies. However, the EU Commission’s position is that the populations included on the list can be controlled, taking account of the specific conditions of Member States. Therefore, the catching, trade and use of signal crayfish may continue in Finland because of the significant benefits involved, but all introductions, even in waters where the signal crayfish is currently present, are prohibited.

Catching of crayfish

Crayfish pots are the most common method of catching signal crayfish. Crayfishing is subject to the payment of the fisheries management fee and permission being granted to the owner of the local waterway. Crayfishing is prohibited during the off-season from 1 November to 21 July at 12 noon. There is no statutory minimum size for crayfish, but water-area specific limits are usually imposed. In most cases, traded crayfish are over 10 cm in length, as this is the former minimum size.

Crayfishing and other fishing gear used in waters where the signal crayfish is present may not be moved into or used in other waterways due to the risk of crayfish plague. However, if this is necessary for some reason, all fishing gear must be carefully disinfected before being used in another waterway. Bait fish must also be caught in the waterway where crayfishing takes place. If not, bait fish must be frozen before use. Signal crayfish may not even temporarily be stored live in pounds or pools outside the part of the waterway where they were caught, due to the risk of the crayfish escaping or crayfish plague being spread.

Do not move signal crayfish to new water areas, or place them in pounds in waters other than those in which the crayfish was caught.

Do not move any crayfishing or fishing gear from waters infested with crayfish plague before disinfecting them thoroughly.

Empty the bilge water of boats carefully before moving from one waterway to another.

This species is an invasive alien species throughout the European Union. In Finland, the planting of signal crayfish is prohibited in all waterways, including ones in which the species is already present. The farming of signal crayfish is also prohibited.