By Nicholas Wright

Running through the Central Valley’s patchwork of yellow, green, and brown farmlands is the deep blue of California’s largest river–the Sacramento. Once a much wider river, meandering across the flat valley floor, the Sacramento has been straight-jacketed by steep earthen levees and confined to a more controlled channel. On either side of the river, where once would have stretched seasonal floodplains dotted with gnarled cottonwoods, shrubby willows, and dense tufts of tule grass, there’s now a vast expanse of rice fields. Around 95% of historic floodplain habitat in the Central Valley has been lost to draining and agricultural conversion. There are only a few pockets of floodplain still left in the valley, mostly in the flood bypasses–areas where water from the Sacramento River is intentionally diverted during high-flow years. When river water inundates these floodplains a sort of ecological magic happens.

As the water spreads and slows, terrestrial plant matter is flooded and microbes in the water begin to break it down. This detrital decomposition forms the base of a heterotrophic food web that is extraordinarily productive, fueling the growth of incredible densities of zooplankton. A great diversity of fish species, including juvenile Chinook Salmon, move onto the floodplains to feed on the zooplankton and grow rapidly in the warmer, slow-moving waters (Jeffres et al 2022). The incredible productivity of floodplains is the exact reason why their loss has been so damaging to freshwater ecosystems. Today most of the Sacramento River offers slim pickings for hungry fish, leaving juvenile salmon migrating downstream badly short on food.

There is one exception to that trend though–a rare few stretches of river receive pulses of water draining off floodplains that are dense with zooplankton. Research has shown that in wet years when floodplains like the flood bypasses are inundated, juvenile salmon in the Central Valley have more food in their stomachs than in dry years (Sturrock et al 2022). But the flood bypasses are relatively small and the majority of fish in the Central Valley have no access to floodplains. And here is where the rice fields, surprisingly, offer a potentially huge conservation opportunity. 

In winter, rice fields are flooded to decompose rice stubble and provide habitat for waterfowl, forming a sort of surrogate floodplain habitat. If that turbid, food-rich rice field water was drained back into the river, could it provide hungry fish with a floodplain food web subsidy? To test that idea California Trout and the Dr. Rachel Johnson-Dr. Carson Jeffres research group at the UC Davis Center for Watershed Sciences partnered to implement a landscape-scale experiment seeking to answer an important question–could rice field floodplain food webs be harnessed as subsidies to provide food to salmon in rivers? Salmon enclosure cages were set in the Sacramento River above (as a control) and at one-mile intervals below an export drain pumping in rice field waters. The cages were set for almost two months and each week at each site during that period zooplankton abundance was sampled, all of the fish were measured, and a few salmon were removed for stomach content analysis.

The results of this study told a simple but powerful story. We found that all sites receiving the rice field subsidy had more zooplankton than the upstream site, but that zooplankton densities diminished at the sites further downstream from the export drain. We also found that the upstream salmon almost exclusively ate insects, like chironomids, while the salmon at the export drain almost exclusively ate zooplankton (CalTrout 2021). Even at the furthest site, six miles downstream, zooplankton still made up a large proportion of the fish diet, showing that this floodplain subsidy extends many miles from its point source. When we looked at the sulfur isotope values of the stomach contents we found that the fish eating at the downstream sites had a strong floodplain signature while the upstream fish had a clear river signature, definitively proving that the downstream fish were eating from subsidy-derived food webs (Bell-Tilcock et al unpublished data).

Most importantly, we found that fish receiving the subsidy grew much faster than the upstream fish. The salmon at the export pump grew in weight an average of 12x faster than the fish not receiving the subsidy, and even the fish six miles downstream grew around 6.5x faster. Managed agricultural subsidies can mimic natural wet year flood patterns and create a food-rich flood pulse during dry years, when juvenile salmon face a barrage of environmental threats and experience high mortality rates. These subsidies can help mitigate the damage of the bad years in the boom-and-bust cycle California Chinook salmon are undergoing, which are projected to become even more severe under climate change.

California Trout is leveraging these data to support their Fish Food on Floodplain Farm Fields project, which is expanding the number of rice farms exporting zooplankton-rich water into the Sacramento River. This project is a a reminder that conservation opportunities exist even in heavily modified working landscapes, but for management strategies in these systems to work they need to bring together diverse stakeholders and find multiple coexisting uses for the same areas of land. By using rice fields to restore a land→floodplain→river trophic subsidy that used to occur widely throughout the Central Valley, we can help juvenile salmon grow big enough to survive their dangerous downstream migration.

Nicholas Wright is junior specialist in the Johnson-Jeffres research group.

Further Reading:

Bell-Tilcock, M.N. et al. (2020) Advancing diet reconstruction in fish eye lenses. Methods in Ecology and Evolution. 12(3), 449-457.

California Trout (2021) Fish food from floodplain farm fields: 2021 Annual report of experimental results. 

Jeffres, C.A (2011) Frolicking fat floodplain fish feeding furiously. https://californiawaterblog.com/2011/06/02/frolicking-fat-floodplain-fish-feeding-furiously/

Jeffres, C., Holmes, E. and Rypel, A.L. 2019. Fish are born free, but are everywhere in cages this spring, https://californiawaterblog.com/2019/04/28/fish-are-born-free-but-are-everywhere-in-cages-this-spring/

Jeffres, C.A., Holmes E.J., Sommer T.R., Katz J.V.E. (2020) Detrital food web contributes to aquatic ecosystem productivity and rapid salmon growth in a managed floodplain. PLoS ONE 15(9): e0216019. https://doi.org/10.1371/journal.pone.0216019

Katz, J.V.E. et al. (2017) Floodplain farm fields provide novel rearing habitat for Chinook Salmon. PLoS ONE 12(6): e0177409. https://doi.org/10.1371/journal.pone.0177409

Mount, J., A.L. Rypel, and C. Jeffres. 2023. Nature’s gift to nature in early winter storms. https://californiawaterblog.com/2023/01/15/natures-gift-to-nature-in-early-winter-storms/

Nguyen, Megan. (2017) Yolo Bypass: the inland sea of Sacramento. https://californiawaterblog.com/2017/02/20/yolo-bypass-the-inland-sea-of-sacramento/

Rypel, A.L., D.J. Alcott, P. Buttner, A. Wampler, J. Colby, P. Saffarinia. N. Fangue, and C.A. Jeffres. 2022. Rice and salmon, what a match! https://californiawaterblog.com/2022/02/13/rice-salmon-what-a-match/ 

Sturrock, A.M., Ogaz, M., Neal, K., Corline, N.J., Peek, R.A., Myers, D., Schluep, S., Levinson, M., Johnson, R.C, and Jeffres, C.A. 2022. Floodplain trophic subsidies in a modified river network: managed foodscapes of the future? Landscape Ecology. 37, 2991–3009.

Torres, F., M. Tilcock, A. Chu, and S. Yarnell. Five “F”unctions of the Central Valley floodplain. https://californiawaterblog.com/2022/05/08/five-functions-of-the-central-valley-floodplain/

Wright, N. 2023. Hidden links between aquatic and terrestrial ecosystems: part 1 – Sierra Nevada lakes. https://californiawaterblog.com/2023/07/16/hidden-links-between-aquatic-and-terrestrial-ecosystems-part-1-sierra-nevada-lakes/