Assessing physical habitat and juvenile salmonid populations in small coastal streams in summer using traditional sampling and advanced remote sensing approaches

Freshwater habitat loss is a major threat to the persistence of salmonid (Oncorhynchus spp.) populations in the Pacific Northwest of North America. To better inform where and when to preserve this critical habitat, information on its extent and quality is required over spatial scales fish use for various life stages. Further, current and accurate habitat associations are required to make location-specific conservation recommendations. Given the limitations of fine-scale analysis due to sampling logistics and that land conversion activities and climate change affect stream salmonids on extremely large spatial scales, approaches to extend small-scale fish-habitat assessments accurately and cost-effectively to the watershed level are needed. To address this challenge, I investigated the habitat associations of juvenile rainbow trout (Oncorhynchus mykiss) and Dolly Varden char (Salvelinus malma) in the Nahmint watershed on Vancouver Island, British Columbia, during summer 2019. I also assessed whether 3-D Airborne laser scanning (ALS) derived measurements of small stream characteristics provided similar explanatory power in establishing habitat associations with physical characteristics as conventional field-based surveys. I surveyed 200 m of habitat across varying tributary stream sizes (5.1 m to 10.6 m) and implemented mark-recapture fish sampling using minnow traps and passive integrated transponder tags to establish habitat associations and characterize movement. The density of both species was significantly higher in pool habitats than in glides and riffles. Generally, restricted movement was observed by both species, with some Dolly Varden char moving in a primarily downstream direction. Further, ALS-measured and field-measured variables produced similar habitat associations, with fish presence and density associated with pool habitats classified by the ALS data. Overall, these results highlight the importance of habitat quality (such as abundant pools) and connectivity in small streams, while also illustrating the utility of using ALS -derived measures of small stream habitat to establish fine-scale habitat associations of juvenile salmonids within a watershed. Moreover, ALS has the potential to be used as a predictive tool to assess juvenile salmonid distributions at a broad scale in a timely and cost-effective way.Forestry, Faculty ofGraduat