Thermal refuges, patches of habitat that provide physiological refuge from stressful temperatures, form increasingly important habitat in the face of rising ambient temperatures caused by large-scale habitat alteration and climate change. Thermal refuges in river ecosystems are highly heterogeneous, and habitat heterogeneity offers mobile consumers a choice of disparate habitats, allowing them to balance trade-offs between critical resources (e.g., temperature, food availability). Understanding how individuals negotiate fine-scale spatio-temporal heterogeneity in the thermal and prey resource landscape at thermal refuges may help shed light on the potential trade-offs associated with refuge use.
For this dissertation, I developed innovative methods to quantify the probability of thermal refuge use by juvenile salmonids (steelhead, Oncorhynchus mykiss; Chinook salmon, O. tshawytscha) at fine spatial and temporal scales in the Klamath River in northern California (USA), and determine how variability in the thermal and trophic landscapes at refuge sites influenced salmonid diet and thermal habitat use. In Chapter 1, I used temperature-sensitive radio tagging studies to quantify how spatio-temporal variability in abiotic (temperature, flow, time of day) and biotic (body size) variables influenced juvenile steelhead use of thermal refuges. This study represents the most in-depth analysis of spatio-temporal variation in steelhead thermal refuge use to date. I found that mainstem temperature, time of day, and body size were the main drivers of refuge use (>80% of juvenile steelhead moved into refuges when mainstem temperatures reached 22-23°C, and all fish moved in by 25°C). In Chapter 2, I used a combination of radio tagging studies and isotopic diet analyses to quantify how spatial variability in thermal and trophic resources at thermal refuges mediate the foraging behavior and thermal habitat use of juvenile steelhead and Chinook salmon. I found that juvenile salmonids using thermal refuges obtained the majority (~75-95%) of their diet from mainstem prey sources, and that fish thermal habitat use was significantly cooler than diet-inferred fish foraging temperatures, indicating that while fish seek cooler habitat for physiological benefits, they still rely primarily on mainstem prey resources. In Chapter 3, I used lab experiments on internally tagged juvenile steelhead to determine how variation in water temperature and body size affects the amount of time it takes for Lotek temperature-sensitive radio tags to acclimate to a new ambient temperature. I found that tag temperature response time depended on the magnitude and direction (heating vs. cooling) of water temperature change and fish body size (fork length and weight); these experiments determined the sub-sampling interval for data collected in Chapters 1 and 2.
This dissertation demonstrates that fine-scale variability in the temperature and prey landscape mediate how juvenile salmonids use thermal refuges. Moreover, this research highlights the importance of habitat heterogeneity and connectivity for thermal refuges, as refuges may allow fish to more effectively exploit adjacent (and likely more abundant) non-refuge habitat (i.e. mainstem river) by providing temporary thermal respite; this could be a critical and currently under-valued benefit of maintaining refuges.