Little Fish Give Way to Big Shifts in Distributions in a Changing Climate: Cryptic Loss in Occupancy Across the Landscape

Many stream fishes are predicted to be substantially influenced by climate change and obstacles to upstream movement are likely limiting their ability to seek refuge upstream from adverse environmental conditions. In western North America, Bull Trout are a common focal species for understanding impacts of climate change in streams; however, they are more mobile and less impacted by obstacles than many native stream fishes. We geo-referenced historical State and Federal presence–absence records from 1993-1995 and expanded the existing dataset in 2011-2013 to increase sites revisited two decades later. We used a Bayesian multi-season occupancy framework to assess the change site extirpation and site colonization and investigate different landscape drivers that maybe associated with any changes of native stream fishes with differing swimming abilities in the Bitterroot River, MT over twenty years. Our results revealed site extirpation probabilities of Bull Trout and sculpin increased with warming habitats, however sculpin, a weak swimming fish, had no site colonization while Bull Trout did. The presence of physical barriers to longitudinal movements of stream fishes may exacerbate climate change effects by impeding range shifts resulting in range retractions and requires understanding both obstacles and movement capabilities of fishes under future climate conditions.