Signalkrebs i Alling Å: Udvikling 2008-2018

Signal crayfish (Pacifastacus leniusculus) is an introduced or alien species found in nature in Denmark as well as many other European countries. Signal crayfish is considered an invasive species that can drastically affect ecosystems it is introduced to. Both native crayfish as well as other parts of the ecosystem can be negatively affected. For these reasons signal crayfish is included in “The Union List”, a list that contains the most invasive species in EU under the Regulation (Eu) No 1143/2014 of the European Parliament and of the Council of 22nd of October 2014 on the prevention and management of the introduction and spread of invasive alien species.
This report analyzes the results of a monitoring of the distribution of signal crayfish in the AllingÅ conducted during September and October 2018 by fishing with traditional crayfish traps. Themonitoring in 2018 follows a similar monitoring conducted in 2008 (Skov et al. 2009).
During the period 2009-2018 an attempt to eradicate the signal crayfish population in Alling Åwas conducted by a combination of trapping, electrofishing etc. During 2009-2011 a projectfinanced by the four local city councils and the Nature and Forest Agency was conducted(Iversen et al. 2011). From 2012 and onwards the removal of signal crayfish has been done byvolunteers by trapping under the supervision of Randers City Council. In total approximately415.000 signal crayfish were removed during the 10 years, yearly catch varying between 13.744individuals in 2012 to 111.377 individuals in 2018. A part of this variation is due to changes inthe effort between years but the general trend showed an increasing catch per trap during the10 years.
In addition to giving a status for the distribution of signal crayfish in Alling Å the monitoring in2018 could give an answer to whether or not the objectives of the eradication project was met.These were i) to exterminate the population of signal crayfish in Alling Å or if this was notpossible ii) to reduce the density of signal crayfish and stop or delay further spread of signalcrayfish in Alling Å.
 The 2018 monitoring found a considerable increase in the part of Alling Å infested by signalcrayfish from approximately 9 km of the main river in 2008 to ca. 36 km in 2018. In the tributarySkader Å the lower approximately 8 km was infested in 2008. This distribution was unchangedin 2018. In addition the 2018 monitoring found signal crayfish in three other tributaries:Brusgaard Møllebæk (lower 5.8 km), Rosenholm Å (lower ca. 8 km) and Vejle Å (lower 1.8 km).Out of the total approximately 150 km of stream in the river system the fraction infested withsignal crayfish increased from 11 % in 2008 to 38.5 % in 2018.
Mean size of signal crayfish caught during monitoring decreased from 9.7 cm in 2008 to 9.3 cmin 2018 for males and from 9.4 cm in 2008 to 8.4 cm in 2018 for females. The sex ratio in thecatch was ca. 1:1 in 2008 while it was 1 female per 6.5 males in 2018. The reduced mean sizeis most likely caused by fishing with crayfish traps known to be size-biased towards largespecimens during the years 2009-2018. The difference in sex ratio between 2008 and 2018 isexpected to be attributed to the monitoring period. The later period in 2018 (Sept.-Oct.) had theconsequence that mating began before monitoring was completed. In August 2019 a test  showed a sex ration among 327 signal crayfish caught in the main river as close to 1:1 aspossible.
An analysis of signal crayfish habitat preferences in Alling Å regarding both physical/chemicaland biological variables indicated signal crayfish to prefer areas with clay/sand sediment,medium water current, water depth > 50 cm, some maintenance and good shading by low riverbank vegetation.
We conclude that signal crayfish as expected has spread to a much larger part of Alling Åduring the period 2008 to 2018 and that the objectives of the extermination project 2009-2018has not been met.