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

Abstract

Key-words: Pacifastacus leniusculus, Astacus astacus, illegal introductions, fisheries legislation, electrofishing Sweden has only one indigenous species of crayfish, the noble crayfish (Astacus astacus, Fabricius). There has been a steady decline of no- ble crayfish populations in Sweden since 1907, mainly due to the cray- fish plague. To substitute the noble crayfish fishery lost, the Swedish government launched a large-scale introduction of the signal crayfish (Pacifastacus leniusculus Dana). Today, the signal crayfish is regarded as a chronic carrier of the crayfish plague, and an expansion of the species may seriously threaten the noble crayfish. This paper examines the de- crease of noble crayfish populations, and the concurrent expansion of signal crayfish 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 river sections. In 2008-2009 the occur- rences had decreased to 1.9%. In contrast, the signal crayfish had in- creased in occurrence, from 0.2% (1980-1984) to 11.8% in (2008-2009). We studied the number of stocking permits for signal crayfish introduc- tions, and the available signal crayfish population from the open fishery in Lake Vattern, as possible causes of this expansion. A negative correla- tion between stocking permits and increased occurrence in streams, and a positive correlation between the availability of crayfish in Lake Vattern and the occurrence in streams was found. This suggests that the expan- sion of signal crayfish may be due to illegal introductions, further endan- gering the endemic noble crayfish.