River networks have a high amount of thermal habitat heterogeneity which is a critical abiotic factor driving freshwater fish distribution. The fitness repercussions of residing outside a species’ optimal thermal limits, and the resulting behavioural responses to stress, restrict the amount of freshwater habitat available to ectothermic organisms. Changes in ectotherm distribution due to climate-related shifts in thermal habitat availability have been well documented and have been especially pronounced at distributional limits. My objective was to characterize variations in the availability of thermal habitat and quantify its influence on the distribution of a cold-water adapted aquatic ectotherm, Arctic grayling in the Parsnip River watershed in northern British Columbia. The presence of a thermal gradient in the watershed was revealed by a spatial stream network model and its influence on adult Arctic grayling summer distributions was explored with a dynamic site occupancy model using acoustic telemetry data. Results suggest a high probability of occupancy (> 0.75) at temperatures ranging from 8.7-14.2ºC with a peak at 10.9ºC. The distribution of thermal habitat within this range was limiting in only one of the three years during the study period. In 2021 the distribution of thermal habitat with a high probability of use during the study period was reduced to 57% of the accessible watershed length from 89% in 2019 and 87% in 2020. Small streams in high elevation tributaries (e.g., >800 m) are important cold-water sources for Arctic grayling refugia under warm conditions. Increased habitat protections for high elevation streams should be prioritized to ensure a future for cold-water adapted species in the Parsnip River watershed.
