Coming Home: Helping Endangered Fish Return to Suisun Marsh

The Suisun Marsh is the largest brackish water wetland on the West Coast, where salt water from the San Francisco Bay meets fresh water from the Sacramento-San Joaquin Delta. The site, a sprawling estuary of grassland and sloughs, sits south of the city of Fairfield, and has been a focus for habitat restoration by the Department of Water Resources (DWR) for more than a decade due to its importance for native fish and wildlife.

DWR is currently overseeing five habitat restoration projects in Suisun Marsh. In October 2019, one of these projects, the Tule Red Tidal Habitat Restoration Project – which converts approximately 600 acres of existing managed wetland into tidal habitat – is expected to finish construction.

Tule Red was the first among a series of Delta-area habitat restoration projects to break ground following a 2008 federal requirement that the state of California restore 8,000 acres of tidal habitat in the Delta and Suisun Marsh. The project was completed by the State and Federal Contractors Water Agency with funding from DWR's State Water Project (SWP).

That 2008 federal mandate followed the continued decline of the Delta smelt, a fragile, tiny fish, whose population has plummeted to the point that it stopped appearing on several key population surveys. Scientists at DWR and partnering agencies have sought ways to reverse this trend and have taken innovative approaches in Suisun Marsh. 

DWR launched a pilot project last year that directed more fresh water flow into Suisun Marsh. The action involved opening salinity control gates in the summer months instead of during fall and winter, as is customarily done to reduce salinity in the marsh for migrating ducks and other waterfowl. The Delta smelt relies on low-salinity water – opening the salinity control gates allowed the smelt to enter the marsh from the Sacramento River, where it can access greater amounts of food and shelter.

Extinction looms so closely over the Delta smelt population that the project could have been considered a success even if it didn’t lure any countable Delta smelt to the marsh, said DWR Lead Scientist Ted Sommer. Just creating the conditions that allow smelt to thrive – that is, low salinity levels, lots of food, and high turbidity or muddy water that magnetizes smelt – would have been a cause for celebration. 

But in this case, the flow experiment attracted modest numbers of Delta smelt. When attempting to save a species like the Delta smelt, it’s important to play the long game, Sommer said.

“By opening the salinity gates of the vast Suisun Marsh in the summertime, we’ve discovered a new tool in our toolbox,” he said. “If we’re going to recover the Delta smelt, we’re going to need use every tool in that toolbox.” 

Other tools in practice by DWR include multiple habitat restoration projects to attract smelt and other endangered species; use of upstream habitats as a “food bank” for Smelt; and removal of aquatic weeds.

Last year’s salinity control gate action was widely supported by water contractors and agencies involved in water management and regulation. DWR hopes to repeat the action following regulatory approval. 

For more information on the Suisun Marsh salinity control gates action, read our past blog “Researchers Test New Approach to Improve Fish Habitat in Suisun Marsh”