Graduate Thesis Or Dissertation

How Behavior, Physiology, and Ontogeny of Klamath Redband Trout Shape the Functionality of Habitats in a Landscape of Temperature Extremes

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  • Climate change is a global phenomenon, but natural selection occurs within landscapes. A central tenet of landscape ecology is that mobile species depend on complementary habitats, which are insufficient in isolation, but combine to support animals through the full annual cycle. For coldwater fishes, it is widely assumed that maximum temperatures are limiting, and that summer data alone can predict refugia and population persistence. In a naturally warm basin emulating the future of cooler systems, we demonstrate how such assumptions can overlook portions of the riverscape that are critical for supporting fisheries. In this dissertation, we investigated the complex relationship between redband trout and temperature in the Upper Klamath Basin, from a landscape ecology perspective introduced in Chapter 1. Chapter 2 used parallel data sets from radio telemetry, diet sampling, and fish condition monitoring to explore seasonal movement among complementary thermal habitats. During summer, populations of redband rainbow trout (Oncorhynchus mykiss newberrii) were only found in cool tributaries or springs and distinctly avoided Upper Klamath Lake, which exhibited stressful temperatures >25°C. However, during spring and fall most trout migrated to the lake to forage, achieving high rations and growth that compensated for a net loss of energy in cool summer habitats. In winter, most trout returned to tributaries to spawn, completing a biannual migration totaling 60-300 km. Thus, while perennially cool tributaries supported thermal refuge and spawning, foraging opportunities in the seasonally hot lake ultimately fueled these behaviors. In Chapter 3, we compared coldwater fish from neighboring habitats in a naturally warm, recently fragmented basin to understand how different responses to warming may arise from landscape constraints. Trout in warm, hydrologically connected Upper Klamath Lake fled summer temperatures and sought refuge in cool tributaries, while trout in an equally warm but fragmented reach of the Klamath River endured summer conditions. Respirometry experiments revealed that trout in the river were more physiologically tolerant of high temperatures than trout in the lake across four metrics, including routine metabolism, aerobic scope, recovery from exertion, and loss of equilibrium. Two independent metrics of energetic condition indicated that the behavioral strategy of trout in the lake came at a substantial energetic cost, while the physiological strategy of trout in the river was able to mitigate most energetic consequences of high temperatures. No clear genetic basis for enhanced tolerance was found in trout from the river, which may suggest tolerance was derived from plasticity but cannot rule out genetic adaptation. Chapter 4 examines the mismatch between the extent and resolution of available data for temperature versus for fish habitat use. We surveyed cool springs identified by infrared technology to quantify their functionality as thermal refuges for juvenile and adult salmonids in the warm Sprague River sub-basin. Juvenile trout inhabited approximately half of potential refuges and were most prevalent in ponds and their outflow channels. Larger size classes were present in just 10% of refuges and were more prevalent in plumes. All trout selected cool, oxygenated habitats, but refuges occupied by larger trout were colder and deeper. Regression models for juvenile trout suggested that complex refuges, especially those associated with beaver dams, were more likely to be occupied; further, those containing complementary habitat types also contained more trout. Basin-scale analyses revealed that Sprague River springs provided minimal thermal refuge for the meta-population of large migratory trout in summer. However, these springs were a premier location for winter spawning in the Upper Klamath Basin and offered a moderate degree of warm refuge when temperatures were near freezing. Together, results of the three studies show that applications such as climate change adaptation and water quality standards could benefit from considering temperature as a resource, the use of which is constrained by time, space, and life history. Chapter 5 discusses these applications and concludes that managing for how fish respond to temperature, rather than how temperature controls fish, should be a goal for maintaining resilient ecosystems in a changing climate. Allocation of funds for habitat conservation, as well as policy design for water temperature standards, should consider other aspects of temperature regimes besides summer conditions that can influence how fish survive in warm landscapes.
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