Food for Fish: A 2-Dimensional Fate and Transport Model for Zooplankton with Implications for Juvenile Salmon Growth and Water Management Efficiency

Project ID: 23022
Principal Investigator: Jenna Paul
Research Topic: Fish Passage and Entrainment
Funded Fiscal Years: 2023 and 2024
Keywords: None

Research Question

Flood control measures and agricultural development have restricted fish access to the abundant food web that was previously available on the floodplain. Marginal in-river prey densities and poor growth and survival rates necessitate restoration actions in California's Central Valley Rivers. Zooplankton-rich water from farm fields may present a viable source of nutrition for fish when released into the river and is under consideration as a potential mitigation measure. However, it is unclear if the effluent can provide sufficient levels of zooplankton to sustain juvenile salmonids during migration. In-channel concentrations are highly dependent on a river's advective and mixing capabilities, which may flush or disperse a constituent. Therefore, our research question follows:

Can rice field effluent increase prey (food) availability for rearing and migrating salmonids in the main river channel?

Reclamation has two-dimensional river modeling capabilities but is missing the tools to model the fate and transport of water quality constituents, such as zooplankton. The scope of this research includes (1) developing an algorithm to compute zooplankton concentrations in a 2-dimensional, unsteady, non-uniform river system, and (2) testing it on a study reach in the Sacramento River. The algorithm will account for changes in the zooplankton distribution due to advection, dispersion, growth, and decay. Code for the algorithm will be integrated into Reclamation's hydrodynamic model, SRH-2D.

The results from the model will provide time and space-varying values of zooplankton concentration, river velocity, and depth. These values can be used to establish the magnitude and footprint of zooplankton availability from a given rice field release condition, which can be compared against existing background levels of zooplankton in the river.

Need and Benefit

Historically, rearing and migrating fish have benefitted from the habitat provided by the Sacramento River's floodplain. During periods of flood-activation, the floodplain offered refuge from the extreme velocities in the main channel. It also provided an abundant source of food, where juvenile fish could forage, hold, and grow before their migration to the Pacific Ocean. After the dams were constructed in the mid-1900s, river flows became managed for flood control and agriculture. This resulted in diminished peak flows and reduced floodplain activation. Implementation of flood control levees and agricultural berms further limited the availability of floodplain area for fish access. It has been estimated that up to 95% of the Central Valley's original wetland habitat has been lost to grazing and agricultural development (Mount 1995; Isenberg 2005, as cited in Hanak et al. 2011).

With the changes in the Central Valley's landscape and water management practices, rearing and migrating fish have lost access to a significant source of food. The impact may have contributed to diminished growth rates and higher salmonid mortality, as under-nourished juveniles are physically unfit for migration and are susceptible to predation. Yoshiyama et al. (2000) estimate that, since the 1950s, Chinook salmon have decreased to less than 75% of their population. California's recent mega-drought has only exacerbated the poor survival of migratory fish. Michel et al. (2015) showed that average Chinook salmon survival rates were less than 6% in the Sacramento River during low water years. The increasingly harsh conditions for migratory fish call for immediate restoration actions.

Reclamation has worked to improve salmon survival by creating off-channel juvenile rearing habitat, such as the North Cypress side channel on the Upper Sacramento River. Off-channel habitat affords juveniles with the opportunity to eat, rest, and grow during their migration. However, side channel construction is not feasible on the lower reaches of the Sacramento River because of levee confinement. Reclamation needs additional fish-supporting strategies that can be implemented in areas with physical constraints such as these.

Reconnecting the Sacramento River to the food source from the floodplain has the potential to fill this need. Research has shown that flooded rice fields generate an abundant source of zooplankton, and when released in the river, it may increase growth rates in juvenile salmon (California 2019, 2021). Several rice field outfalls have been identified on the leveed reach of the Sacramento River. If strategically managed, sequenced releases have the potential to convey elevated levels of fish food across a broad extent of the river. The zooplankton model will have the capability to compute the magnitude and footprint of increased food availability that results from a water release action.

The immediate benefit of this project will be a new zooplankton module than can be used on any river. Other regions, such as Pacific Northwest, that are looking for strategies to improve salmon growth and survival rates will be able to immediately evaluate design scenarios with this model. They will save time, effort, and funding by using an established zooplankton solver, rather than developing their own algorithm or by performing extensive zooplankton sampling to determine unknown concentrations in the river.

Another immediate benefit of this research will include model results for the available acreage of juvenile drift feeding habitat on the study reach. These results will give Reclamation managers and farmers insight into how the rice field discharge, number of flooded farm acres, and river flow conditions impact the amount of feeding habitat for juvenile salmon. These results are expected to lead to the future benefit of farm field irrigation and release strategies to optimize the availability of fish food during migration.

Contributing Partners

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Research Products

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Last Updated: 6/22/20