Are native fishes and reservoirs compatible?

By Peter B. Moyle and Anna M. Sturrock

Oroville Reservoir during severe drought. The ‘bathtub ring’ of steep barren ground makes poor habitat for native fishes even when the reservoir is full. Photo credit:

The question addressed in this blog comes from a new PPIC report that calls for reforms in management of environmental water stored behind dams in California. The report shows it is possible to manage water in ways that are compatible with maintaining a natural ecosystem in streams below and above dams (Null et al. 2022). An appendix to this report focuses on fishes (Moyle et al. 2022). It provides information on how dams and reservoirs affect native fish populations and supports the need for improved water management to avoid future extinctions.

California has a unique assemblage of fishes native to its rivers and streams. Most of the 129 or so species are found no place else. They are a fascinating mixture of endemic freshwater fishes that cannot live in salt water, and endemic sea-run (anadromous) fishes that migrate long distances through both fresh and salt water environments. There are exceptions, of course, such as delta smelt and splittail, which live in the mixing zone between salt and fresh water. All of these fishes are adapted to a climate that generates extreme floods and droughts, and everything in between, on an irregular basis. This naturally variable climate is also changing, becoming more volatile and making extreme conditions worse and more frequent. Unfortunately for the fish, we humans do not like these extremes nor the unpredictability in water availability; we therefore have built massive infrastructure, centered around dams, to generate a more stable water supply.

In particular, we built dams to store water in reservoirs to get us through extreme droughts and floods and then cement canals to keep a constant flow of water to our farms and cities. There are over 1400 reservoirs in California alone, some of them among the largest in North America. Most rivers in the state support at least one dam-reservoir combination. You would think all this impounded water would provide good habitat for native freshwater fishes. Indeed, some reservoirs in their early years did support high numbers of native fishes such as hardhead, pikeminnow, and hitch. When some of the larger reservoirs filled in the 1950s and 60s, native fishes were so abundant that fisheries agencies talked about solving the ‘hardhead problem’. These native fishes were regarded as trash fish (Rypel et al. 2021) throughout western USA because anglers did not like them and assumed they suppressed populations of game fishes through competition and predation. One solution was the use of fish poisons to kill all the fish in a river before a dam was built. The most egregious example of this was poisoning of the Green River in Utah before the closure of Flaming Gorge Dam. This operation killed millions of fish, native and non-native, including many species (e.g., Colorado pikeminnow, razorback sucker) that are now listed as endangered, even in California. In California, poisoning operations (euphemistically called ‘chemical control operations’) aimed at native fishes in reservoirs were a routine management practice up into the 1980s.

Hardhead, a native California fish that was once abundant in reservoirs but now is uncommon in most of its native range. Photo credit: Ken-ichi Ueda, downloaded from Creative Commons.

The hardhead problem eventually went away on its own after introduced predators, such as largemouth bass, smallmouth bass, striped bass, and channel catfish, became established and devoured and/or out-competed all the native fishes. The non-natives could even thrive in storage reservoirs that were systematically drawn down during summer months, leaving a ‘bath-tub ring’ of exposed dirt along reservoir edges. The raw dirt provides no cover or food for juvenile native fishes that wash in from upstream areas. Most of the non-natives could also complete their entire life cycle in reservoirs, because many of these species are endemic to natural lakes and other warmwater habitats before introduction in California. Only a few native species, such as prickly sculpin, Sacramento sucker, and rainbow trout appear to have adapted to reservoir life and remained abundant in them.

For many native species (24 out of 129), dams and reservoirs have played a predominant role in placing them on an extinction trajectory (Moyle et al. 2022; ‘Dam Impacts’ rated ‘high’ or ‘critical’ in figure below). But, not surprisingly, dams tend to be one of multiple, interacting factors causing their declines, including non-native species and climate change. Over half of California’s native fishes have been rated as headed for extinction, with seven already extinct.

The dominant fishes in most reservoirs today are non-native species, with each reservoir supporting some combination of the 50 non-native species thriving in the state. By and large, reservoir fishes are popular among Californian anglers because they support recreational fisheries for black basses, sunfishes, catfishes, and other familiar fishes in their warm surface waters and rocky bottoms. Such fish are the basis of important sport fisheries elsewhere, especially in the southeastern USA, where game fishes are held in high esteem and managed intensively by fisheries agencies. In California, the reservoir fisheries are pretty much taken for granted, with little concern for harmful effects on native fishes.

But harm to natives by dams is not confined only to reservoirs with non-native fishes. Dams block access to major upstream spawning and rearing areas of salmon, steelhead, and other native fishes. In California, 70% of critical upstream habitat for salmon and steelhead has been blocked. Below the dams, their habitats are often drastically changed by the absence of high flow events that shift and reshape the riverbeds. Such flows create the complex off-channel habitats needed for juvenile rearing and to maintain a diverse fish fauna. The so-called tailwaters below a dam may be cold enough to support salmon and trout, but the embedded substrate limits invertebrate production for food and makes digging nests (redds) for spawning difficult to impossible. As water warms up with distance from the dam, and as flows are further reduced by diversions, non-native species such as carp, catfishes, and basses become dominant in the warm pools of remnant, diked river channels. The habitat, flow and thermal regimes below dams typically bear little resemblance to the historic regimes that supported native fishes and cued important physiological and ecological events. The key ingredient for native fish habitat (cool, high-quality water), is greatly reduced or absent. This water is increasingly stored in reservoirs and not available to native fishes at the right times.

Overall conservation status, vulnerability to climate change, and impacts of dams and reservoirs on native fishes (n =129). See Moyle et al. (2022) for a summary how these ratings of fish species were created and the sources of information. The key findings are that 51% of the fishes are in severe decline overall, that 78% are highly vulnerable to decline due to climate change, and that that dam and reservoir management is endangering 42% of the remaining extant species.

Dams and reservoirs have played a large role in the decline of our native fishes. However, there is a growing need to protect native fishes before even more face extinction and become listed under the Endangered Species Act. It is clearly time to improve management of stored water for native fishes. In our rapidly changing climate, using reservoirs to store designated environmental water could allow such water to be deployed flexibly during droughts and to play a pivotal role in saving endangered fishes from extinction. Nevertheless, major policy changes that revolutionize our ability to store and manage water to benefit native fishes are not likely in the near future. The water is simply too important to California’s economy. However, the restoration of native fishes to lower Putah Creek, a highly managed stream (Yolo and Solano Counties) does provide an example of success with relatively low water costs. Null et al. (2022) provide a framework for creating such successes statewide. The key is making restoration of native fishes a designated function of reservoirs instead of being an afterthought. “Making ecosystem health a primary objective of reservoir operations would enable better overall management of hydrologic uncertainty and ecological risks (p3).” Without such a change, California fishes will likely become just another statistic in the world extinction crisis. It would be better if, instead, California emerged as leader in coping with environmental change through better management of its water. The state’s unique native fish fauna needs all the help it can get!

Peter Moyle is Associate Director of the Center for Watershed Sciences and Distinguished Professor Emeritus at the University of California, Davis, USA. Anna Sturrock is Lecturer in Marine Ecology and UKRI Future Leaders Fellow, University of Essex, Colchester UK.

Further reading:

Moyle, P., A. Sturrock, and J. Mount 2022. California’s Freshwater Fishes: Conservation, Status, Impacts of Dams, and Vulnerability to Climate Change. Storing Water for the Environment, Technical Appendix A. San Francisco: Public Policy Institute of California.

Null, S., J. Mount, B. Gray, K. Dybala, G. Sencan, A. Sturrock, B. Thompson, and H. Zeff. 2022. Storing Water for the Environment: Operating Reservoirs to Improve California’s Freshwater Ecosystems. San Francisco: Public Policy Institute of California.

Rypel, A.L., C.A. Parisek, J. Lund, A. Willis, P.B. Moyle, Yarnell, S., and K. Börk. 2020. What’s the dam problem with deadbeat dams?,

Rypel, A.L., Saffarinia, P., Vaughn, C.C., Nesper, L., O’Reilly, K., Parisek, C.A., Miller, M.L., Moyle, P.B., Fangue, N.A., Bell‐Tilcock, M. and Ayers, D., 2021. Goodbye to “rough fish”: paradigm shift in the conservation of native fishesFisheries46(12): 605-616.

Schreier, A., P.B. Moyle, N.J. Demetras, S. Baird, D. Cocherell, N.A. Fangue, K. Sellheim, J. Walter, M. Johnston, S. Colborne, L.S. Lewis, and A.L. Rypel. 2022. White sturgeon: is an ancient survivor facing extinction in California?

Willis, A., R. Peek, and A.L. Rypel. 2021. Dammed hot: California’s regulated streams fail cold-water ecosystems.

About Andrew Rypel

Andrew L. Rypel is a Professor and the Peter B. Moyle and California Trout Chair of coldwater fish ecology at the University of California, Davis. He is a faculty member in the Department of Wildlife, Fish & Conservation Biology and Director of the Center for Watershed Sciences.
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1 Response to Are native fishes and reservoirs compatible?

  1. Verna Jigour says:

    Perhaps I’ve missed something but at least so far the discussions about functional flows, including the PPIC report (which I’ve only skimmed to date) seem to address solely the water and not the sediments that would be carried by all types of natural flows. Releasing only water from the dams would seem to occasion “hungry water” that will pick up the sediment it requires from existing stream substrates, perhaps scouring where there would have otherwise been deposition – likely not the quality of sediments originally transported by those streams and likely not the functional sedimentation of optimal aquatic habitats for native fish and invertebrate species.

    Will those functional flows achieve their intended habitat-generating functions if they are deprived of the upstream sediments held back by the dams?

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