Interpopulation Variation in the Thermal Physiology of Chinook Salmon

Background & Motivation

Before embarking on empirical work, it was essential to understand what was already known about thermal variation in salmonid physiology. This review synthesized the existing literature on thermal performance across Pacific salmon species and populations, with a focus on whether and how physiology varies with thermal environment.

Approach

We reviewed studies measuring aerobic scope, critical thermal maxima, gill oxygen uptake, and other performance metrics across multiple salmonid species and populations, synthesizing findings across California river systems and the broader Pacific coast.

Key Results

The review revealed substantial — and systematically underappreciated — variation in thermal physiology among and within salmonid species. Critically, this variation was not random: it tracked historical thermal environments in ways consistent with local adaptation. The review identified major gaps in the literature, particularly around intraspecific (within-species, among-population) variation, and laid the conceptual groundwork for the empirical studies that followed.

This paper has since become a key reference for California salmon management and for researchers designing population-level physiological assessments.

Study Design

Eight hatchery populations of juvenile Chinook salmon were acclimated to a common set of temperatures and measured for aerobic scope — the difference between maximum and resting metabolic rate — across a thermal gradient spanning their ecologically relevant temperature range.

This is one of the largest metabolic performance experiments conducted on any teleost fish, involving hundreds of respirometry trials across populations and temperatures. All fish were treated identically in terms of acclimation, feeding, and measurement protocols, ensuring that observed differences reflect true physiological divergence rather than environmental conditioning.

Results

Populations showed clear and statistically significant differences in their thermal performance curves. Populations from historically warmer river systems tended to have higher optimal temperatures for aerobic performance and maintained aerobic scope at higher temperatures than populations from cooler systems. This pattern is consistent with local thermal adaptation — fish are physiologically tuned to the temperatures they evolved in.

Importantly, no single population performed best across all temperatures, suggesting real trade-offs in thermal specialization.

Study Design

California's Chinook salmon are divided into four seasonal "runs" — winter, spring, fall, and late-fall — that differ in their timing of ocean entry, river migration, and spawning. These runs encounter very different temperature regimes during their freshwater residency, providing a natural experiment in thermal adaptation.

This study compared thermal physiology among populations representing each seasonal run, using the same aerobic scope framework as Study 02 but with a focus on whether run-timing is associated with distinct physiological profiles.

Results

Seasonal runs did show physiological differences consistent with the temperatures they encounter during their freshwater life stages. Winter-run Chinook — which migrate and rear in the coldest conditions and are listed as Endangered — showed the lowest thermal optima and the greatest sensitivity to warming temperatures, highlighting their particular vulnerability to climate change.

These findings provide a physiological basis for understanding why different runs respond differently to thermal stress, and why a one-size-fits-all thermal threshold for water management is insufficient.

Study Design

This study moved beyond aerobic scope to examine acute thermal tolerance — how much heat fish can withstand over a short period before losing equilibrium. Critical thermal maximum (CTmax) was measured across populations using standardized ramping protocols. Critically, the same fish used for aerobic scope measurements were also assessed for CTmax, allowing direct examination of the relationship between these two performance traits.

Results & Trade-offs

As with aerobic scope, populations differed significantly in CTmax, and these differences tracked historical thermal environments. However, the study also revealed an important physiological trade-off: populations with higher CTmax did not necessarily have better aerobic performance at high temperatures, and vice versa. These traits appear to be at least partially independent, reflecting distinct underlying mechanisms.

This has important implications for conservation: a population that can survive a brief heat spike may still suffer physiological costs that impair its long-term growth and reproduction under chronic warming. Management strategies that rely solely on lethal temperature thresholds may underestimate risk.