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Sweden has only one indigenous species of crayfish, the noble crayfish (Astacusastacus, Fabricius). There has been a steady decline of noble crayfishpopulations in Sweden since 1907, mainly due to the crayfish plague. To substitute thenoble crayfish fishery lost, the Swedish government launched a large-scale introduction ofthe signal crayfish (Pacifastacus leniusculus Dana).Today, the signal crayfish is regarded as a chronic carrier of the crayfish plague, and anexpansion of the species may seriously threaten the noble crayfish. This paper examinesthe decrease of noble crayfish populations, and the concurrent expansion of signalcrayfish in running waters. Data from the Swedish Electrofishing RegiSter (SERS) was used.We found that in 1980–1984 the noble crayfish occurred in 4.5% of the studied riversections. In 2008–2009 the occurrences had decreased to 1.9%. In contrast, the signalcrayfish had increased in occurrence, from 0.2% (1980–1984) to 11.8% in (2008–2009). Westudied the number of stocking permits for signal crayfish introductions, and theavailable signal crayfish population from the open fishery in Lake Vättern, as possiblecauses of this expansion. A negative correlation between stocking permits and increasedoccurrence in streams, and a positive correlation between the availability of crayfish inLake Vättern and the occurrence in streams was found. This suggests that the expansion ofsignal crayfish may be due to illegal introductions, further endangering the endemic noblecrayfish

Exponential increase of signal crayfish in running waters in Sweden – due to illegal introductions?