How Dams Affect Salmon Populations
Salmon are an iconic species in the Pacific Northwest both culturally and ecologically. However salmon populations have declined precipitously in recent decades. One of the major culprits? Dams. Dams have fundamentally altered river ecosystems and posed severe challenges to salmon survival. In this article, I’ll explain the major ways dams negatively impact salmon populations.
Blocked Passageways
One of the most straightforward effects of dams is that they physically block salmon migration routes. Salmon are anadromous, meaning they hatch in freshwater, migrate to the ocean to feed and grow, and then return to their natal freshwater streams to spawn. Dams obstruct this natural life cycle by impeding passage between the upper river spawning grounds and the ocean. Without fish ladders or other passage facilities, the dam represents an impenetrable barrier. This permanently eliminates all salmon habitat upstream of the dam. For example, Grand Coulee Dam on the Columbia River completely extirpated all the river’s anadromous fish above the dam site. Even with fish passage facilities, dams cause significant mortality for both juvenile and adult salmon attempting to traverse upstream and downstream.
Altered Flow Regimes
Dams also profoundly alter the natural flow regimes of rivers. They even out seasonal high and low flows by storing water and releasing it when needed for electricity generation, irrigation, or municipal supply. However, this prevents the critical spring flush flows that cue salmon fry emergence and migration. It also reduces crucial peaks during the fall and winter that provide fish access to side channels, wetlands, and other important rearing habitats. The overall reduction in flows caused by dams hampers fish passage. It increases migration times, exposure to warm water temperatures, and vulnerability to predators.
Warmer Water Temperatures
Another major impact is that dams create warmer downstream water temperatures harmful to salmon. Deep reservoir pools stratify, with warm water at the top. This warmer water is what gets released downstream. In free-flowing rivers, flows stay cooler in summer and warmer in winter. But regulated dam releases invert these natural temperature cycles. Salmon life histories are adapted to natural river temperature regimes. Warmer summer and fall temperatures increase disease and mortality particularly in migrating adult and juvenile salmon. Warm winter temperatures can also cause premature egg hatching.
Downstream Habitat Changes
Finally, dams degrade downstream habitat in multiple ways. They trap sediment normally transported downstream, increasing erosion below the dam. This removes spawning gravels and rearing habitat. Reduced peak flows also limit channel migration. Together with reduced sediment, this results in armored channels inhospitable to salmon. Reduced nutrients and changed food webs mean less food available for juvenile salmon. Toxic algae growth in reservoirs produces blooms and die-offs below dams. Overall, dams drastically simplify river habitat complexity essential for salmon survival.
Michael J Angilletta1Department of Ecology & Organismal Biology, Indiana State University, Terre Haute, IN, USAFind articles by
Received 2007 Nov 4; Accepted 2008 Feb 4. © 2008 The Authors. Journal compilation © 2008 Blackwell Publishing Ltd
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: anthropogenic change, Chinook salmon, dams, development, embryos, emergence, fry, selection gradients, temperature
Although multiple factors have contributed to the decline of Pacific salmon (Oncorhynchus spp.) across portions of their natural range, dams have arguably played a major role in many locations (NRC 1996; Lichatowich 1999; Ruckelshaus et al. 2002). Large dams (>15 m tall)–designed to generate hydropower, control floods, or facilitate navigation – have had unintended side effects on the quantity and quality of habitat used by salmon. Specifically, these dams have decreased the area for spawning and rearing, and have altered flows, sediments, and temperatures downstream (PFMC 1979; Ward and Stanford 1979; Ligon et al. 1995).
Since the first large dams in the Pacific Northwest were built in the early 1900s, many generations of salmon have spawned, developed, emigrated, and returned to spawn in the waters downstream. Possibly, these fish have begun to adapt to the new thermal regimes caused by dams (Waples et al. 2008). Although we cannot infer adaptation from the available data, we can estimate the potential evolutionary impacts of dams and discuss the types of data that would be necessary to make such inferences. Under this premise, we present an exploratory analysis of Chinook salmon (Oncorhynchus tshawytscha) in four parts. First, we assess the historical changes in water temperatures downstream of large dams. Second, we review the effects of water temperature on physiological performance at specific life stages. Third, we estimate the degree to which changes in physiological performance would influence the fitness of genotypes. Finally, we assess the possibility of an evolutionary response to selective pressures imposed by dams.
Consequences of dams for the performance of salmon
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.
The thermal sensitivities of physiological performances can mediate the direct and indirect effects of dams on the fitness of salmon. Because temperatures in the fall can approach those that stress Chinook salmon (e.g., see Fall Creek in Fig. 2), warming by a dam could directly increase the mortality of embryos. In the laboratory, brief exposure to 17°C during embryonic development caused more than 98% mortality between the fertilization and emergence (Geist et al. 2006). More likely, however, dams indirectly influence fitness by advancing the timing of emergence. Emergence describes a developmental transition in which fry leave the interstitial gravel and enter the water column, thereby initiating a sustained, free-swimming residence in a stream or river. At this point in development, the energy stored as yolk has been virtually exhausted and the juvenile must begin to feed exogenously. Early or late emergence could lead to mortality from excessive flows, abundant predators, or insufficient resources (Jensen and Johnsen 1999; Einum and Fleming 2000). The warming caused by a dam in fall and winter would accelerate the development of embryos, leading to early emergence. Thus, a dam could disrupt a match between the actual and the optimal dates of emergence.
Adaptation of behavior or physiology would ameliorate the thermal effects of a dam. If salmon below the dam spawned later in the year, the temperatures experienced by their offspring would be lower because of seasonal cooling (see Fig. 2). Consequently, development would proceed more slowly and the timing of emergence would correspond to that before the dam’s influence. Alternatively, an evolutionary decrease in developmental rate would enable salmon to delay emergence despite the warm water below the dam. Whether spawning behavior or developmental physiology would experience greater selective pressure depends on the cost of each strategy and the impact on other stages of the life cycle.
Despite the ideas discussed above, current models of thermal adaptation cannot generate deep insights about the responses of salmon to dams. First, these models focus on physiological responses to environmental temperature (reviewed by Angilletta et al. 2002; Kingsolver and Gomulkiewicz 2003), ignoring the potential for behavioral responses that seem relevant to salmon (e.g., timing of spawning, placement of nests). Second, these models assume that temperature influences either survivorship (Lynch and Gabriel 1987) or fecundity (Gilchrist 1995) uniformly throughout the life of the organism. Because both the thermal impacts of dams and the thermal tolerances of salmon vary throughout the life cycle, simple models will not accurately predict the strength of selection. In the next section, we use an age-structured model to explore the impact of physiological performance on the fitness of a genotype. This approach should yield a better understanding of the selective pressures imposed by dams.
Salmon vs Dams
FAQ
How can dams affect salmon?
… block fish from moving along their natural pathways between feeding and spawning grounds, causing interruptions in their life cycles that limit their …Feb 3, 2020
Does dam removal help salmon?
After decades of advocacy by Tribes and river advocates, the recent removal of four Klamath River dams opened over 400 miles of habitat for Chinook salmon, coho salmon, and steelhead. Construction activities wrapped up earlier this month on what is widely regarded as the largest salmon restoration project in history.
How does hydropower affect salmon?
… on the survivorship of juvenile salmon due to mortality in the turbines and spillways, increased water temperatures, predation, and a myriad of other factors
Can salmon get past beaver dams?
Many fish that migrate up streams small enough to be dammed by beavers have evolved significant jumping ability. A good example of this is salmon – which are impressive leapers in order to get around obstacles like waterfalls, and can clear beaver dams as well.
How do dams affect fish?
Thanks to uniquely extensive monitoring over many decades to track the fate of fish migrating throughout the Snake system, we know the impact of the dams is much, much greater: what the most recent science indicates is that each of the dams reduces survival by 20 to 25 percent. How do the dams harm fish?
Do dams block salmon spawning & rearing habitat?
Dams block passage of salmon and steelhead between spawning and rearing habitat and the Pacific Ocean. Where fish passage is not provided the blockage is permanent. More than 40 percent of the spawning and rearing habitat once available to salmon and steelhead in the Columbia River Basin is permanently blocked by dams.
How do dams affect salmon & steelhead?
Dams impact salmon and steelhead in a number of ways, from inundating spawning areas to changing historic river flow patterns and raising water temperatures. Dams block passage of salmon and steelhead between spawning and rearing habitat and the Pacific Ocean. Where fish passage is not provided the blockage is permanent.
How does a dam affect salmon?
The presence of the dam may also change the way predators and prey interact. In many cases the negative effects of these changes are greater than the direct effects of the dam itself. Most salmon are adapted to living in rivers so changing their habitat to a lake often has negative consequences on their life cycle.
Do dams affect Chinook salmon?
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.
How do dams affect fish downstream migration?
Dams affect fish downstream migration, which puts them in many dangers. For example, during low flows in Columbia Basin, the Chinook salmon reaches the estuary almost 40 days later. This exposes them to intensive and overfishing salmon is a result of these kinds of delays.