Extension d'une espèce envahissante: Suivi de la population d'écrevisses signal (Pacifastacus leniusculus dana) au léman à l'ouest de Thonon-les-Bains, entre 2001 et 2005 - Impact de la pêche

Abstract

The signal crayfish, which appeared in Lake Geneva at Thonon-les-Bains in 1976, have continuously expanded along the southern shoreline, reaching Séchex in 2001 and Yvoire in 2005 (Fig. 1). The speed of colonization is 500-1,000 m/year. Crayfish populations increase to a maximum and then are likely regulated through limitation of food resources and increasing fishing pressure. Two different traps are used for this study: the professional fishermen's trap, mesh size 20 mm, collects exploitable crayfish (larger than 80 mm total length, TL) in the August 2004 campaign; the "Pirat" trap, mesh size 12 mm, samples even smaller crayfish, from 60 mm TL onward, and is used in the August 2001 and 2005 campaigns. The maximum density is found just behind the front of colonization, i e 82 indiv./trap at Thonon in 2001 and 78 indiv./trap at Excenevex in 2005 (Table I). Due to the combined effects of crayfish expansion, available food and regulation through catches, the mean exploitable crayfish size is globally increasing from the origin zone to the front of colonization, i e from 89.5 mm at Thonon to 108.3 mm at Excenevex, in 2004 (Fig. 2, Tables II and III). The fishing pressure leads to changes on the size of sampled crayfish at Sciez, where professional captures stopped at the end of 2004; the mean exploitable crayfish size increased from 105,1 mm in 2004 to 108,5 mm in 2005, whereas decreasing values were recorded in the neighbour zones of Séchex and Excenevex (Table II); size means between Sciez and Excenevex or Séchex are significantly different with α level = 0.05. The discussion summarizes information on the impact of signal crayfish on the littoral compartment of Lake Geneva, which has been developed in former publications, particularly for macrophyte beds and aquatic snails (Lymnaeids) losses, thus causing an important reduction of swimmer's itch importance in the crayfish colonized zones.