Dead planet, living planet

RESTORATION OF A DEPLETED CRAYFISH FISHERY IN EUROPE – LESSONS LEARNT

was considered resistant to the crayfish plague and to occupy the same ecological niche and therefore presumably able to restore the recreational and commercial important crayfish fishery in the plague affected areas. The fact that it was also a natural host of the oomycyte Aphanomyces astaci (the causative agent of crayfish plague) was probably unknown at the time (Unestam 1972). Trials were successful and in 1967–69 a large number of signal crayfish were imported from USA and intro- duced into Swedish and Finnish waters (Svärdson 1995, West- man 1995). These were followed by secondary introductions, mostly from Sweden, to many European countries (Lowery and Holdich 1988, Holdich et al . 2009). This led to an increase in the number of crayfish plague outbreaks, and 65 % of the reg- istered incidences of crayfish plague in Sweden from 1907 – 2004 have occurred after the increased introduction of signal crayfish from 1969 (Bohman et al . 2006) To alleviate the problem of reduced stocks of indigenous spe- cies, the plague carrying North American species Orconectes limosus and Procambarus clarkii is also widespread in Europe, and are now established in 21 and 15 countries/territories re- spectively (Holdich et al . 2009) In Europe, three of the five recognized indigenous species are on the IUCN red-list (Baillie & Groombridge 1996), and some countries are establishing “ark-sites” (secure sites) in an at- tempt to protect indigenous crayfish species (Sibley et al . 2007, Horton 2009). Restoration of many European crayfish fisher- ies with transplants of North American crayfish species may be regarded as a success in terms of establishing new plague resistant stocks for harvest (Ackefors 1999), but has been detri- mental for the indigenous crayfish species of Europe (Holdich et al . (2009). Extreme caution and careful planning must be conducted when restoring ecosystems to ensure the use of in- digenous species and that no infestations or invasive species are introduced unintentionally.

The world´s biodiversity of freshwater crayfish are threatened by numerous factors, including loss and degradation of habitat, overharvesting, climate change and introduction of alien species of crayfish and associated diseases (Taylor 2002, Holdich et al . 2009), and more than one-third of the world´s crayfish species are likely threatened with population decline or extinction (Taylor 2002). Ironically, introduction of alien crayfish species, also in restoration projects aiming to compensate for lost native stocks, is regarded among the most serious threats to indigenous cray- fish populations (Taugbøl & Skurdal 1999, Lodge et al . 2000). The commercially important crayfish fisheries of Europe were heavily depleted during the last century, especially of the noble crayfish ( Astacus astacus ), which was reduced to near 10 % of “historical” catches (Westman et al . 1990). One of the main reasons for this decline was the spreading of crayfish plague ( Apahnomyces astaci ) throughout Europe after its introduction in Italy in 1859 (Söderhäll & Cerenius 1999), most likely as a result of importation of infected North American crayfish (Al- derman & Polglase 1988).

In 1959 a small batch of signal crayfish was introduced to Swe- den from California, USA (Svärdson 1995). The signal crayfish

 Figure 18: The distribution and main spreading route of the Signal crayfish in Europe.

90