Occupancy of invasive Northern Crayfish (Faxonius virilis) in northern streams is associated primarily with water temperature.

Invasive species are the 2^nd-greatest threat to global freshwater biodiversity. Crayfish are especially robust invaders due to their omnivorous nature and ability to compete both directly (resource procurement) and indirectly (habitat occupation and modification) with native species. The Northern Crayfish (Faxonius virilis Hagen, 1870) was introduced to the North Saskatchewan River basin (Alberta, western Canada) and has persisted there since the early 1990s. Faxonius virilis ' impacts on native fish assemblages in the North Saskatchewan River have yet to be assessed, even though the watershed is ecologically, economically, and culturally valuable and home to multiple sensitive and at-risk fish species. We aimed to identify the instream environmental characteristics associated with F. virilis occupancy in the North Saskatchewan River basin and to determine which currently unoccupied tributaries are most vulnerable to F. virilis invasion. We used occupancy modeling to meet these objectives. We deployed 24-h baited crayfish traps and measured water temperature, turbidity, flow velocity, and physical complexity at 37 sites along the Alberta portion of the North Saskatchewan River basin. We detected F. virilis at 13/37 sites with no occurrences detected in the upper basin. Occupancy model selection and averaging revealed that water temperature alone was associated with occupancy of F. virilis in the North Saskatchewan River basin. Streams with mean summer water temperatures >∼19°C were ≥50% more likely to be occupied by F. virilis than cooler streams and are at highest risk of invasion. Further, we found that streams with mean summer water temperatures <∼15°C had a <25% chance of becoming occupied by F. virilis than warmer streams. Coldwater streams may thus have some natural protection against F. virilis invasion. The results from this study provide practical guidelines for watershed management of invasive F. virilis populations in western Canadian river basins. Managing F. virilis is particularly important and time sensitive because F. virilis ' range will likely expand when water temperatures in the basin rise because of climate change. [ABSTRACT FROM AUTHOR]