How Dams Affect California’s Anadromous Fishes and Potential Benefits of Dam Removal

Introduction 

Some of California’s iconic freshwater fishes such as sturgeon, salmon, and trout are anadromous. This means that they are born in freshwater, migrate to the ocean as adults, and return to freshwater to spawn. Several species face extinction or severe decline within the next century [1], an endangerment facilitated by the presence of man-made dams. Dams act as physical barriers that can cause habitat fragmentation and restrict fish from accessing historic habitats [2][3]. They also disrupt the natural flow of water, causing issues with oxygen levels, water temperature, and sediment transport [4][5]. Lastly, dams may facilitate the spread of deadly fish parasites [6][7]. To find effective conservation strategies for California’s anadromous fishes, researchers have also begun to assess the impacts of dam removals which, in general, are predicted to help populations recover [8].

This review will discuss how dams affect anadromous fishes through habitat fragmentation, reduction in water quality, and the spread of disease. By understanding the influence of dams on the environment, we can analyze why they contribute to the endangerment of these species. This review will also discuss the potential benefits of dam removals. By studying completed and in-progress dam removals, we can make more informed decisions in the future on how to conserve California’s anadromous fish. 

Habitat Fragmentation

Some anadromous fishes struggle to swim past dams and thus cannot complete their migration or reach certain habitats. Steel et al. (2018) examined how the Red Bluff Diversion Dam (RBDD) in the northern Sacramento River affected access to spawning grounds by green sturgeon migrating upstream from the San Francisco Bay. The researchers surgically implanted acoustic tags in wild adults and tracked those detected migrating up the river from 2007-2013. Their results found a statistically significant increase of sturgeons passing RBDD when open versus closed, including when accounting for variance in seasonal dam operations. They also found that when the closure of the RBDD gates was delayed, more tagged sturgeon were able to swim upstream past the dam, possibly aligning better with their natural migration timing [2]. This would indicate that the dam and other dams upstream of it may indeed be restricting access to many spawning grounds. Subsequently, dams become barriers that prevent individuals from different populations or subpopulations from interbreeding. The limited gene flow can lead to a decrease in the population's genetic diversity and an increase in the likelihood of inbreeding. 

Another study by Cordoleani et al. (2021) studied migration patterns of Chinook salmon (Oncorhynchus tshawytscha). They analyzed otoliths (earbones) from adult Chinook salmon across various Central Valley watersheds, which can be used to predict habitat use, migration histories, and growth rates of individual salmon through chemical differences in its layers. They also measured historical stream temperatures to compare to predicted stream temperatures in the future. They found that a majority of Chinooks are restricted to the Central Valley floor because their access to high-elevation reaches is blocked by dams [3]. These reaches provide thermal refugia, which are areas of cooler water (compared to low-elevation reaches) needed for a Chinook’s rearing period [3]. Without access to thermal refugia, Chinooks are unable to spawn the next generation in suitable rearing habitats. The results from Steele et al. and Cordoleani et al.’s research both suggest that the presence of dams can decrease habitat connectivity and migration success. 

Reducing Water Quality

Anadromous fishes may also encounter changes in water quality imposed by dams. By restricting the flow of a river, dams hold back upstream water to create stagnant reservoirs (impoundments) with altered biogeochemical properties. Stagnant water is more vulnerable to temperature, oxygen, sediment, and nutrient fluctuations than free-flowing water [4]. Abbott et al. (2022) examined differences in dissolved oxygen (DO) and water temperature upstream and downstream of dams across 15 North American watersheds, and within their impoundment. They also considered dam and river characteristics across these sites such as watershed size, dam height, impoundment volume, etc. They found that DO concentrations in impoundments in 60% of the evaluated sites were lower than upstream reaches, although there were no consistent changes in downstream reaches [5]. They also found a general increase in water temperature in impoundments and downstream reaches compared to upstream reaches [5]. Low DO and high temperatures can create hostile environments for vulnerable fish. Fish experiencing hypoxic stress due to insufficient DO tend to have lower feeding and growth rates, compromised immune systems, and reduced reproductive success [6]. Thus, water quality conditions outside a fish’s ideal range could reduce their fitness and survival.

Dams also affect water quality by interrupting sediment transport and causing changes to the earth’s landscape upstream and downstream river systems. This is dangerous for salmonids as they are broadcast spawners: they dig gravel beds in the sediment, lay their eggs, and then fertilize them. Sediment property is particularly important for successful spawning. If sediments are too large, it is difficult for salmonids to dig suitable beds. Harrison et al. (2018) studied sediment transport along the San Clemente Dam located on the Carmel River in California. They found  that sediment trapped behind the dam could not be transported downstream. This reduced the natural shrinking of sediment grains by erosion, resulting in larger grains [7]. The mean sediment grain size (D_g) was 94 mm, which was considered too large to be a suitable spawning substrate for steelhead trout (Oncorhynchus mykiss irideus) [7]. Therefore, the lack of natural sediment transport puts them at risk of finding no suitable spawning and rearing areas.

Spread of Fish Disease

Fish diseases can be a major cause of mortality for migratory fishes. The most common disease for salmonids is infection by Ceratomyxa shasta, a parasite that releases spores into the water column and causes mass bleeding and necrosis of the intestines. Schakau et al. (2019) studied spore concentrations of C. shasta and the prevalence of infection (POI) in spawning areas of Chinook and Coho salmon along the Iron Gate Dam (IGD) on the Klamath River near the California-Oregon border. They found that opening dams could decrease the spore concentration by 48% and POI by 40% indicative of a potential strategy for disease management [8]. However, Richey et al. (2020), also studied spore concentrations of C. shasta and their exposure to rainbow trout (Oncorhynchus mykiss) via environmental DNA fragments shed into the water and substrates of the environment. They studied dams and reservoirs in the Plumas National Forest in California, studying seven dams along the mainstem Feather River and the Antelope Dam on the Indian Creek. They found no statistical significance between C. shasta concentrations above and below dams. The differing results concerning spores between Schakau et al. and Richey et al. point to the need for further research on more dam and river systems of varying characteristics to clarify the relationship between dams and the parasite’s presence.

Implications for Dam Removal

Removing dams could release some stressors from anadromous fishes. Establishing connectivity between different habitats are possible benefits of dam removals. For example, Cordoleani et al. suggested that improved access to thermal refugia with dam removal would make Chinook salmon more likely to reestablish their populations. Ramos and Ward (2023) studied the effects of four large dam removals on the Klamath River for coho salmon (Oncorhynchus kisutch). They measured summer water temperatures and created habitat models to assess habitat suitability post-dam removal. They found that in pre-dam removal conditions, natural and anthropogenic barriers prevented endangered coho salmon from accessing most of the watershed [10]. In post-dam removal conditions, approximately 80 km of historical habitat and access throughout the watershed were restored to coho salmon [10]. Restoring access to these habitats would therefore give coho salmon a better chance of reestablishing their populations. Ramos and Ward and Cordoleani et al. both acknowledged that restoring access to habitats is crucial for migratory fishes to continue their life cycle. 

Dam removals are attributed to improving water quality in historic habitats. Abbott et al. also studied DO levels and water temperature before and after dam removals. After dam removals, they found DO levels and water temperature were restored to controlled, upstream conditions more suitable for anadromous fishes and with a lower risk of developing hypoxic conditions [5]. Harrison et al. also studied how the removal of the San Clemente Dam affected sediment transport and river characteristics. They found that sediment transport was restored to upstream conditions after the dam was removed, with D_g decreasing to 26 mm in the reaches immediately below the former dam site. Removal also changed the river, with increased bank erosion and coarsening of new river channels that can potentially create new spawning grounds with suitable sediment size [7]. Results from Abbott et al. and Harrison et al. indicate that water quality can return to optimal conditions for migratory fish once dams are removed. 

Sociocultural Implications for Dam Removal

There are also sociocultural implications for dam removal. In particular, it can help restore environmental justice for indigenous peoples who have suffered from some of the highest rates of food insecurity, poverty, and diet-related diseases [12] due to restrictive laws and habitat degradation that severed their access to the anadromous fishes that have served as their main source of protein for generations. The return of salmon after the removal of dams on the Klamath River, for example, restores a foodway of immense importance to the way of life of the Klamath tribes, thereby providing a real chance to reduce food insecurity and revitalize their culture.

However, the needs of others must also be considered. The current dam removal projects on the Klamath River have been controversial for local stakeholders, with conflicts between environmental advocacy groups, commercial and recreational fishermen, farmers and ranchers, and public officials. A survey by Albertson (2019) evaluating the preferred management method for these dams on the Klamath River for each group. Responses ranged from complete approval to complete disapproval of dam removals, often due to differing ideologies and needs between stakeholder groups [11]. While there were those who supported dam removal as a means to restore balance in nature, others opposed it because they either did not believe that salmon declines were caused solely by dams (also blaming downstream tribal and commercial fisheries), or prioritized the availability of irrigation water that would become lost. Although dam removal seems like a solution in the perspective of environmentalists, it also symbolizes a loss of resources for farmers and ranchers that rely for their careers and livelihoods.

Conclusion 

Several studies have demonstrated how California dams have negatively affected anadromous fishes by acting as physical barriers to vital habitats, shifting water temperature and dissolved oxygen (DO) levels, and promoting the spread of fish diseases. In addition, the current progress of the dam removal project on the Klamath River serves as a precedent for the positive effects of dam removals on ecosystem function and the potential recovery of endangered fish species. However, many of these case studies focused on one parameter of dam influence for one species at a time. Future research should focus on how multiple parameters of dam presence affect migratory fish and to what extent they affect each species. Additional research is also required to focus on the long-term effects of dam removal if improved water conditions and habitat connectivity are maintained. Reestablishing certain endangered species susceptible to dams can also bolster food security for coastal and indigenous communities that rely on fish as their main source of protein and symbols of their culture, although river management post dam removal remains a controversial topic in a sociocultural context.

About the Author: Tiffany Huynh

Tiffany Huynh is a fourth-year marine and coastal science major with a focus on marine ecology and organismal biology. She chose this major because of her love for seafood and its significance in cultural cuisine worldwide. She was inspired to write this literature review because of her experience as a research assistant in the UC Davis Fish Conservation Physiology Lab, working closely with sturgeon and salmon. Tiffany wanted readers to be more aware of the role of man-made dams in California and how it has affected fish and people who rely on them. In the future, she hopes to pursue a career in fisheries management or fisheries ecology to preserve culturally and economically important fish species and promote high-quality, affordable, and sustainable seafood.

Author's Note

I wrote this literature review for an assignment in UWP 102B with Amy Goodman-Bide. The lab I’m working in on campus, the Fangue Lab, studies sturgeon, salmon, and smelt, all anadromous fishes that are endangered and affected by man-made dams in one way or another. I wanted to write a review on this topic to learn about how dams are harmful to fish and how we could proceed with future conservation efforts. 

References

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