UC Berkeley

Local human impacts and climate change are presenting freshwater species around the world with conditions that are more extreme than those to which they evolved. Human water use and increasing drought severity are causing unprecedented low stream flow and warm water. Consequently, cold-water, migratory fish, such as salmon and steelhead trout (Oncorhynchus spp.), face challenges in completing their life cycles. As Pacific salmon navigate extensive river networks during their migration, individual fish often cannot avoid heavily impaired reaches of streams and rivers, particularly in California, USA. Salmon in the southern extent of their range already experience conditions that approach or exceed their tolerance limits. Yet, we have an incomplete understanding of flow and temperature conditions required to complete their life cycles and sustain healthy populations. Are salmon approaching thresholds in low flows, minimum depths, and maximum temperatures? How do streamflow and temperature influence the critical timing of downstream migration (outmigration) of juveniles? Do minimum thresholds exist in stream water depths that salmon can navigate? How do additional environmental and ecological stressors interact with water temperature to affect physiological thermal limits of salmon?

In this dissertation, I explore how an endangered population of coho salmon (Oncorhynchus kisutch) near the southern extent of the species range in the agricultural Russian River watershed is affected by receding streams and warming waters. In Chapter 1 we tracked outmigration of individually marked juveniles in 7 streams over 13 years, which included an historic drought with extreme dry and warm conditions. We discovered that low-flow years during droughts contracted the outmigration window by 31% (from 11 to 7 weeks), while warm years hastened the outmigration window by approximately three weeks. In Chapter 2 we investigated minimum depth thresholds for migration by applying a novel use of water depth measurements at the riffle crest thalweg, which is the shallowest repeating geomorphic feature that salmon must navigate. We found that steep reductions and a cessation in movement occur at shallow depths, but that movement-depth thresholds varied among streams with different geomorphologies. Finally, in Chapter 3 we develop a framework to assess the degree to which thermal tolerance may be reduced by ecological stressor interactions. Our example assessment demonstrates that temperature-stressor interactions, such as temperature interactions with predation, competition, and disease, may cause uneven distributions of impacts, such as development, growth and carrying capacity, to salmon populations.

Together, the chapters of this dissertation offer insights to better understand, manage, and mitigate the adverse impacts of low flow, shallow water, and warm water in salmon-bearing streams in coastal California. Our findings suggest that maintaining adequate streamflows from March through June is critical to preserving an outmigration window that is sufficiently long in duration to reduce risks of phenological mismatches when juvenile salmon reach the ocean to feed. Policies that set minimum water depth limits are likely needed in many of California’s impaired streams to protect the salmon movement during the outmigration period. This research also underscores the importance of temperature-stressor interactions when considering the potential impacts of warming waters on salmonids. The assessment framework that we introduce can serve in any salmon-bearing system to categorize temperature-stressor interactions and the potential severity of their impacts on a salmon population. Collectively, the site-specific assessment methods and findings of this dissertation can be utilized to inform strategic instream flow policies, ecological stressor reduction, and thermal criteria development. If implemented, these solutions can reduce adverse impacts of human water use and drought on endangered migratory salmon individuals and enhance the resilience of salmon populations in a changing climate.