Fish are born free, but are everywhere in cages this spring

by Carson Jeffres, Eric Holmes, and Andrew RypelBluff cage

State, federal, and local governments, water users, and the public are all concerned with the survival of salmon.   Over decades, and especially recent years, most salmon runs have severely declined in California.

Part of sustaining salmon populations is improving the survival and fitness of young salmon as they grow for weeks to months before out-migrating to the Ocean.

Growth of these young salmon is particularly low in the river channels confined with levees on each side.  Without adequate food and cold water temperatures, young salmon grow slowly.  Along with slow growing conditions, as spring arrives and water temperatures become warm and clear, predators become more abundant and prey on young salmon.  These two punches of poor growth plus potential predation makes the river channels hostile environments for young salmon.

Historically, young salmon would spread with flows from winter rains and snowmelt and feast over the Central Valley’s vast seasonal floodplains, until levees and upstream dams made these floodplains largely inaccessible.  These floodplains had abundant food for the salmon, particularly insects and zooplankton.  Today, young salmon are confined mostly to river channels, which lack the greater availability of foods in potentially adjacent floodplains (see the video of food abundance on floodplain).

In the early 1900s, the Sacramento Valley’s Yolo and Sutter flood bypasses were built to restore some of the Valley’s natural flood conveyance capacity (a novel flood control idea at the time, and still so today) (Hundley 1989).   Increasingly, these bypasses provide multiple benefits.  They remove dangerous floods from cities (notably Sacramento), are farmed (e.g., for rice and other crops), and provide valuable ecological habitat.

In the 1980s, flood bypasses were modified to also serve migrating waterbirds, restoring some seasonal wetlands along the Pacific Flyway.  Winter riceland flooding was spurred originally by air quality regulations prohibiting burning of post-harvest rice stubble.  This single shift in farming practices effectively doubled available flooded habitat for wintering waterfowl in the Central Valley and the results were very successful (Garone 2011).

In recent years, several groups have been exploring further modifications to Sacramento Valley flood bypasses to restore some of this area’s natural seasonal fish habitat.  This exploration began from observations that salmon migrating through the Yolo Bypass in wet years grew larger than salmon migrating down the leveed Sacramento River channel (Sommer et al. 2001).  Subsequent studies found that young salmon grow much larger on floodplains and flooded bypass lands (Jeffres et al. 2008, Katz et al. 2017).


2019 Salmon Growth Cage Experiment Locations

This year, UC Davis Center for Watershed Sciences and California Trout have four different studies over approximately 100 miles using floating cages with baby salmon inside.  A total of 85 cages were deployed to study fish growth across a variety of habitats and the potential benefits of these habitats to outmigration survival (see map).  These widespread experiments are evaluating differences in water quality, food resources, growth rates and differences in survival between fish grown on floodplains and control fish.

These experiments place young salmon in cages at locations representing different habitat and flow conditions, protecting them from a variety of predators to help clarify effects of habitat, food density, water quality, food density, and location on fish growth.  Raising baby salmon in cages in not always easy.  Watching, tending, and probe instrumentation are especially challenging with so many cage sites in high flows (like this year), requiring special attention to safety.

The insights and information resulting from this year’s field experiments will help prioritize and guide management and restoration throughout the Central Valley.  Better understanding the ties of land and water management for salmon provides opportunities for more effective restoration efforts and the cooperation of land owners and other environmental interests.

This year’s extensive fish growth experiments are a major step forward building on over a decade of collaborative research, observations, and experiments involving a variety of fish and water management agencies and interests, as well as intense and crucial involvement from land owners and the local agriculture.  Such broad collaborations are needed to develop effective solutions and take the broad actions needed reverse recent declines in salmon populations.

little fish

Fish initially caged

successful fish

Fish at end of experiment

Carson Jeffres is a Professional Research Scientist, Eric Holmes is a staff scientist, and Andrew Rypel is an Associate Professor of Wildlife, Fish and Conservation Biology at UC Davis’ Center for Watershed Sciences.

Collaborators in these and earlier studies include: California Trout, California Department of Water Resources, US Bureau of Reclamation, US Fish and Wildlife Service, California Department of Fish and Wildlife, River Garden Farms, Cal Marsh & Farms, Conaway Ranch, California Rice Commission, US Department of Agriculture – Natural Resources Conservation Service, the Delta Science Program, Northern California Water Association, River Garden Farms, Reclamation District 108.

Further reading

Garone, P. (2011). The fall and rise of the wetlands of California’s Great Central Valley: Univ of California Press.

Sommer TR, Nobriga ML, Harrell WC, Batham W, Kimmerer WJ (2001) Floodplain rearing of juvenile Chinook salmon: evidence of enhanced growth and survival. Can J Fish Aquat Sci 58:325–333

Kelley, Robert (1989), Battling the Inland Sea: Floods, Public Policy, and the Sacramento Valley, University of California Press, Berkley, CA.

Jeffres, C., Opperman J.J.., & Moyle P. B. (2008).  Ephemeral floodplain habitats provide best growth conditions for juvenile Chinook salmon in a California river. Environmental Biology of Fishes. 83(4),

Katz, J., Jeffres C., Conrad L., Sommer T., Martinex J.., Brumbaugh S., et al. (2017).  Floodplain farm fields provide novel rearing habitat for Chinook salmon. PLoS ONE. 12(6),

Megan Nguyen (2017), Yolo Bypass: the inland sea of Sacramento, 20 February,

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About jaylund

Professor of Civil and Environmental Engineering Director, Center for Watershed Sciences University of California - Davis
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