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What Happens to Salmon Trapped Behind Dams

Scientists believe that some Chinook salmon “landlocked” behind California’s dams are completing their life cycles without reaching the ocean. Will these fish, stocked by hatcheries, interfere with efforts to return native salmon to the waters above the dams?

Written by K. Martin Perales Published on Read time Approx. 4 minutes

Chinook salmon are a remarkably adaptable species. There is good reason to believe there are multiple populations of landlocked Chinook salmon completing their entire life cycle above Central Valley dams. We recently documented spawning above six of 13 reservoirs that have been stocked with Chinook. In some cases, populations have persisted for several years after stocking of juvenile salmon has stopped, suggesting self-sustaining populations.

The stocked salmon are juveniles that have been stocked by the California Department of Fish and Wildlife (CDFW) to be harvested by recreational anglers. These fish seem to be using the reservoir as a surrogate for the ocean. It is likely that the stocked juvenile salmon feed in the open water and grow into adults in the reservoir. Individuals that avoided being harvested and have matured into adults can go on to reproduce in historic spawning streams and rivers, now inaccessible to anadromous fish because of dams.

What do these populations mean for anadromous salmon? For one, they serve as a reminder that above reservoirs, quality spawning and rearing habitat exists. These are streams where anadromous Chinook salmon have been absent for many years – in some cases, more than 100 years. Some estimate that dams block about 90 percent of spawning habitat in the Central Valley. This habitat loss, along with other changes in the landscape, has been implicated in the decline of salmon. Restoring access to this lost habitat will hopefully address a key limitation in the life cycle of salmon and other migratory fish. The current spawning activity shows that despite being isolated for so long, the habitat is still good for salmon. This validates the idea that we should increase habitat connectivity somehow.

These landlocked Chinook salmon may be a roadblock to increasing habitat connectivity. Most of these planted fish are not native to the rivers below the reservoirs in which they are stocked. Instead, they are “surplus” juveniles from Iron Gate Hatchery, located on the Klamath River, outside of the Central Valley. The presence of these out-of-basin fish spawning above Central Valley reservoirs may complicate our ability to restore native salmon above the dams. Klamath River fish are genetically distinguishable from all fish in the Central Valley, including those that are below the dams where the Klamath River fish are reproducing. Mixing out-of-basin and local salmon will lead to reduced genetic diversity, which makes populations less resilient. Thus, we should avoid mixing these populations to maintain whatever is left of the genetic integrity of these runs.

What now? I am not sure. We have regionally native fish that have been transplanted to streams they are not native to, and are living in an ecosystem that has never existed until recently. Their effect on the restoration of native populations is unknown, but may be detrimental. Should we get rid of these fish to facilitate future restoration efforts? Perhaps a two-way trap and haul program (moving adults above and juveniles below a reservoir), such as is proposed for the McCloud River above Shasta Dam, will be more viable with them gone. It’s possible that we’ve already taken steps to eliminate these unique populations. CDFW recently switched over to planting sterile triploid fish in some of the reservoirs. We shall see whether the landlocked Chinook populations will continue to self-sustain.

However, even if the populations eventually die out, we have few good options to get the desired local native salmon above our huge dams. Many of our dams are too tall for ladders. Fish elevators are inefficient. Some remain skeptical of trap and haul as a long term solution. If we can’t retrofit dams with ladders and can’t move fish easily, are we only left with removing dams as an option, as has been proposed for the Klamath River?

On the other hand, our study raises an interesting question. What if an endangered or threatened run of Chinook salmon, such as winter or spring run, were established above a dam and used the reservoir like an ocean? Can this population act as an acceptable refuge population? Could this population supplement existing runs? There are many examples of “feral” self-sustaining populations of landlocked Chinook salmon around the world; why not rear some threatened varieties from California in our own reservoirs? And sure, it would be a compromise to have landlocked varieties over wild salmon. Everything is a compromise compared to the historic runs California used to sustain. It’s possible that by changing our concept of how we want salmon to persist, we may have discovered a new aid in their recovery.

This story first appeared on California Water Blog, published by U.C. Davis’ Center for Watershed Sciences.

K. Martin Perales worked with Dr. Peter Moyle as an undergraduate investigating relationships among water quality, slough morphology and hydrodynamics and their effect on fish communities of the North Delta. He is currently earning his PhD at the University of Wisconsin–Madison studying human impacts on lake fish communities.

Top image: Joseph DiMarco, 10, prepares to cast his line while salmon fishing on the American River near Nimbus Dam in Rancho Cordova, Calif., Monday, July 16, 2012. Scientists have found that some hatchery salmon are now completing their life cycles above dams. (Rich Pedroncelli, Associated Press)

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