Crayfish occupancy and abundance in lakes of the Pacific Northwest, USA

Crayfish are ecologically important in freshwater ecosystems, but few investigators have evaluated landscape patterns of their prevalence and abundance in temperate lakes. We report results of a crayfish survey for 100 lakes in the Puget Sound lowlands of Washington State (USA) serving as a census of the distributions of the signal crayfish Pacifastacus leniusculus and previously unknown populations of nonnative crayfish, including the red swamp crawfish Procambarus clarkii. We modeled crayfish presence and abundance with classification and regression trees based on multivariate ordinations of lake-, riparianzone-, and watershed-scale predictors. We found 4 nonnative crayfish in 15% and P. leniusculus in 53% of surveyed lakes. The presence of P. leniusculus was best predicted by a lake-scale gradient of greater depth (present) to greater productivity (absent), but forested riparian zones with firm or rocky substrates also supported crayfish presence. No models adequately characterized patterns of P. leniusculus abundance measured as catch-per-unit effort from traps. Exclusion of data on predatory fish communities and lake pH and Ca may have impeded our ability to characterize patterns of crayfish abundance in the study lakes. Prevalence and abundance of P. leniusculus were low relative to what has been reported for native crayfish in other lake districts or for P. leniusculus in its Asian and European invasive ranges. We hypothesize that the erratic distribution of P. leniusculus in Puget Sound lakes may be attributable to challenges of postglacial colonization in this region and potentially to human introductions of this crayfish. Our study expands the geographic and taxonomic scope of inquiry into relationships between crayfish populations and lakes at landscape scales and provides a baseline for monitoring and understanding crayfish in an increasingly urban and invaded region of North America.