Riverine Habitat Requirements of the White–Clawed Crayfish, <i>Austropotamobius pallipes</i>

Abstract

Ireland remains an important refuge for the freshwater crayfish Austropotamobius pallipes. To determine factors that influence their riverine distribution, forty rivers or tributaries of rivers that flow into Upper and Lower Lough Erne were sampled and surveyed. The nineteen sites where crayfish occurred had more bedrock and moss than those without crayfish, indicating the importance of a stable substrate and an indirect source of food. A subset of these sites, for which hydrochemical data were available, showed greater water hardness where crayfish were present, indicating that a high calcium water content is important to these animals. However, neither biological water quality nor invertebrate community structure varied between sites where crayfish were present and sites where they were absent. This indicates that biological factors have little influence on the presence of crayfish in a river and that crayfish influence biological water quality to a limited degree. M.B. Gallagher, J.T.A. Dick and R.W. Elwood (corresponding author; e-mail: r.elwood@qub.ac.uk), School of Biology and Biochemistry, Queen's University Belfast, Belfast BT9 7BL, Northern Ireland. Received 21 April 2004. Read 2 June 2005. Published 28 February 2006. INTRODUCTION The white-clawed crayfish, Austropotamobius pallipes, was previously widespread in mainland Europe, Great Britain and Ireland, but its distribution and abundance has declined in recent years due to habitat destruction, pollution (Holdich 1991) and the introduction of exotic crayfish (Holdich 1998; Holdich and Gherardi 1999). These exotics tend to be larger, and they compete for resources (Capelli and Munjal 1982; Butler and Stein 1985; S?derback 1991). American exotics are also carriers of the crayfish plague, Aphanomyces astaci, which can eliminate native populations (Capelli and Munjal 1982; Holdich 1998; Holdich and Gherardi 1999). Austropotamobius pallipes has been classified as vulnerable in the IUCN Red List (Groombridge 1993). It is also listed under Appendix III of the Bern Convention (EU/82/ 72/EEC) and Annexes II and V of the EU Habitats Directive (EU/92/43/EEC). In Ireland, however, exotic species have not been introduced and legislation bans their importation. Furthermore, although plague has been noted (Matthews and Reynolds 1990), its effect on A. pallipes was geo graphically restricted and plague is not currently a problem (Matthews and Reynolds 1992). Thus, Ireland retains healthy populations of white-clawed crayfish and has an important role to play in the conservation of this species (Reynolds 1997). Apart from legislation, conservation depends on an understanding of habitat requirements, especially if reintroductions are attempted as part of a conservation/management strategy. Within British rivers, high water quality, hard water and plentiful refugia in terms of tree roots and boulders appear to be required (Foster 1995; Smith et al. 1996; Naura and Robinson 1998). However, river studies pose a problem concerning the unit of analysis. Multiple samples taken from each river are not independent, and thus the assumptions of most statistical tests will be invalid. At the extreme, taking twenty samples from each of two rivers, one with and one without crayfish, and treating the data as independent would not be a valid approach. The present study attempts to overcome this potential problem of pseudo-replication by examining the habitat and crayfish status in the numerous rivers of a large catchment that all flow into a single lake system. Whether or not crayfish are present in specific rivers in this continuous system should depend on the habitat within each river and should be independent of their presence in other rivers. Environmental conditions (water chemistry, phys ical characteristics and invertebrate community structure) were measured at each site in order to evaluate the possibility of using these data to predict sites where crayfish should be present.