Big dams and salmon evolution: changes in thermal regimes and their potential evolutionary consequences

Abstract

Dams designed for hydropower and other purposes alter the environments of many economically important fishes, including Chinook salmon (Oncorhynchus tshawytscha). We estimated that dams on the Rogue River, the Willamette River, the Cowlitz River, and Fall Creek decreased water temperatures during summer and increased water temperatures during fall and winter. These thermal changes undoubtedly impact the behavior, physiology, and life histories of Chinook salmon. For example, relatively high temperatures during the fall and winter should speed growth and development, leading to early emergence of fry. Evolutionary theory provides tools to predict selective pressures and genetic responses caused by this environmental warming. Here, we illustrate this point by conducting a sensitivity analysis of the fitness consequences of thermal changes caused by dams, mediated by the thermal sensitivity of embryonic development. Based on our model, we predict Chinook salmon likely suffered a decrease in mean fitness after the construction of a dam in the Rogue River. Nevertheless, these demographic impacts might have resulted in strong selection for compensatory strategies, such as delayed spawning by adults or slowed development by embryos. Because the thermal effects of dams vary throughout the year, we predict dams impacted late spawners more than early spawners. Similar analyses could shed light on the evolutionary consequences of other environmental perturbations and their interactions.

Keywords: Chinook salmon; anthropogenic change; dams; development; embryos; emergence; fry; selection gradients; temperature.

Figure 1

A map of Washington and Oregon, USA, depicting the locations of four dams selected for analysis: (A) Mayfield, (B) Fall Creek, (C) Hill Creek, and (D) Lost Creek.

Figure 2

Mean water temperatures before and after dam construction on the Cowlitz River, Fall Creek, the Middle Fork of the Willamette River, and the Rogue River. Mean values were generated by averaging daily mean temperatures among years. Bold lines depicting the mean water temperatures are surrounded by thinner lines depicting the upper and lower bounds of the 95% confidence intervals.

Figure 3

Embryos and fry of Chinook salmon survive well over a range of approximately 10°C. Temperatures >15°C lead to very high mortality. This conclusion was based on models that best fit published data for embryos (Velsen 1987; Beacham and Murray 1989) and fry (Olson et al. 1970). Model selection was performed according to Angilletta (2006); details are provided in Appendix B.

Figure 4

Nine hypothetical relationships between the timing of emergence and the survivorship of juveniles (s₁) used in our sensitivity analysis. These relationships vary in the optimal date of emergence (early, intermediate, and late) and the strength of selection for the optimal date (strong, intermediate, and weak).

Figure 5

Predicted fitnesses (λ) of Chinook salmon before (filled symbols) and after (open symbols) the construction of the Lost Creek Dam on the Rogue River. Each plot corresponds to one of the relationships between emergence date and juvenile survivorship depicted in Fig. 4. From left to right, plots show earlier to later optimal dates of emergence. From top to bottom, plots show decreasing strengths of selection for emergence on the optimal date.

Figure 6

Predicted relationships between spawning date and relative fitness (λ, standardized to a mean of 1) following the construction of the Lost Creek Dam on the Rogue River. The slope of each relationship estimates the strength of directional selection (β). Each plot corresponds to one of the relationships between emergence date and juvenile survivorship depicted in Fig. 4. From left to right, plots show earlier to later optimal dates of emergence. From top to bottom, plots show decreasing strengths of selection for emergence on the optimal date.