Invasive Crayfish in Western Oregon: Distribution and Implications for Native Crayfish

Invasive crayfish can have an array of impacts on aquatic ecosystems. The scope and effects of invasions are poorly known in the western USA, a region where aquatic systems and their biota are priorities for conservation. We sought to determine crayfish distribution and to assess evidence of displacement of native by invasive crayfish in Oregon and southwestern Washington. Native Pacifastacus leniusculus were the most widespread (71 of 283 sites) of the 3 crayfish we encountered. We detected invasive Orconectes neglectus at 11 of 16 sites in the Rogue basin. We also documented their establishment in the Umpqua basin (7 sites), where they were not previously known. We found invasive Procambarus clarkii in 51 sites and in all 3 major basins (Rogue, Umpqua, Willamette/Columbia). Distribution and hydrography suggest these locations represent at least 10 different introductions. We used 2-species occupancy modeling to account for imperfect detection and examine hypotheses regarding habitat and potential interactions between P. clarkii and P. leniusculus. Our analysis showed that P. clarkii occupancy was positively related to silt substrates and constructed/modified habitats. Occupancy of P. leniusculus was positively related to lotic habitat and negatively related to constructed/modified habitats. Our modeling suggests that the two species co-occurred less frequently than would be expected by chance which is consistent with a negative effect of P. clarkii on native Pacifastacus.