Freshwater Crayfsh 17:177–182, 2010 Copyright ©2010 International Association of Astacology ISSN:2076-4324 (Print), 2076-4332 (Online) ISBN: 978-951-27-1322-6 (Print) / 978-951-27-1343-1 (Online PDF) 177 Lotta Bjurström, 1 timo ruokonen, 1,† markku Pursiainen, 2 roger i. jones 1 and Heikki HämäLäinen 1 Impacts of the Invasive Signal Crayfsh on Littoral Macroinvertebrates of Large Boreal Lakes: A Pilot Study in Lake Päijänne, Finland INTRODUCTION Invading species are a major threat to global biodiversity since they affect the distribution and abundance of native species (Vitousek et al. 1996; Sala et al. 2000). Freshwater habitats in particular have suffered from intentional introductions of non- indigenous species, like crayfsh, that interfere with aquatic systems at many levels, and can form a central component of freshwater food webs and ecosystems (Covich et al. 1999; Lodge et al. 2000; McCarthy et al. 2006; Correia and Anastacio 2008). By feeding at several trophic levels, crayfsh can have strong effects on other biota, and hence, on the structure of food webs (Stenroth and Nyström 2003; Dorn and Wojdak 2004). When crayfsh abundance increases, composition of invertebrate assemblages may change towards dominance of less vulnerable prey species. For example, the relative abundance of more mobile invertebrates may increase at the expense of slow moving invertebrates such as snails (Gastropoda), which are preferred prey of crayfsh (e.g., Nyström et al. 1999; Stenroth and Nyström 2003). Predators can also affect the abundance of species to which they have no direct links via trophic cascades (Bernot and Turner 2001). Since snails are important grazers, a reduction in their overall numbers caused by crayfsh predation may indirectly increase the abundance of periphytic algae and mobile insect grazers (Nyström et al. 1999, 2001; Gherardi and Acquistapace 2007). However, the majority of knowledge on the impacts of crayfsh on other invertebrates is based on small scale experimental manipulations. Even though these studies suggest a strong potential of introduced crayfsh to affect native invertebrate communities, there is little documented evidence supporting this potential effect on the natural environment (see Wilson et al. 2004; McCarthy et al. 2006; Rosenthal et al. 2006). Signal crayfsh, Pacifastacus leniusculus (Dana), were introduced into Finland in the late 1960s (Westman 1973). During the frst two decades, introductions to natural waters were very limited, after which the species has been spreading rapidly, mainly by controlled introductions within a strategic area restricted to southern Finland (Kirjavainen and Sipponen 2004). In 1989 – 2004, ca. 1.8 million signal crayfsh were stocked into Finnish lakes and rivers. The species proved to be very productive in large lakes, where it is now an important resource for commercial fshing. In large Finnish lakes, the native noble crayfsh, Astacus astacus (Linnaeus), was only abundant in the southern part of the country, and even there the crayfsh plague, Aphanomyces astaci (Schikora), depleted the noble crayfsh populations by the beginning of the 20 th Abstract.— The introduced signal crayfsh, Pacifastacus leniusculus (Dana), is now a permanent resident in many lakes in Finland and other European countries, but the effects of this large omnivore on lake ecosystems of the region are not well known. Previous studies, mainly experimental, suggest that, among other impacts, crayfsh can directly change the composition of benthic invertebrate communities towards domination by species less vulnerable to crayfsh predation. We conducted a preliminary study on the impacts of the introduced signal crayfsh on the littoral macroinvertebrate communities of large boreal lakes by comparing the invertebrate assemblages of stony shores between an impact area with well established crayfsh populations and control areas without crayfsh in the second largest Finnish lake, Lake Päijänne. The composition of the invertebrate community consistently differed between the impact and control areas, and total abundance, species richness and snail abundance were signifcantly lower in the presence of signal crayfsh. The sites inhabited by crayfsh were strongly dominated by Chironomidae, whilst the control sites were represented by more balanced assemblages of taxa. The results of this pilot study suggest that the introduced signal crayfsh can signifcantly alter littoral communities and food webs in large boreal lakes. [Keywords.— benthic invertebrates; community; impact; invasive species; littoral; Pacifastacus leniusculus]. 1 Department of Biological and Environmental Science, University of Jyväskylä, Finland. Survontie 9, P.O. Box 35, 40014 University of Jyväskylä, Finland † Corresponding Author — timo.j.ruokonen@jyu.fi lotebjur@cc.jyu.fi; roger.i.jones@jyu.fi; heikki.o.hamalainen@jyu.fi 2 Finnish Game and Fisheries Research Institute. Jyväskylä Game and Fisheries Research, Survontie 9, FI-40500 Jyväskylä, Finland markku.pursiainen@rktl.f