A researcher getting water-quality data from a managed wetland at Suisun Marsh. Courtesy John Durand.

ESTUARY NEWS: Suisun Marsh: A Bastion for Fish

By Nate Seltenrich

Rising seas are coming for Suisun Marsh, and without careful management some of its most critical habitats could be lost forever. The  largest remaining contiguous, brackish-water marsh in western North America, Suisun Marsh is also  a critical refuge for many native fishes of the highly modified, highly invaded San Francisco Bay-Delta Estuary – but for how much longer?

[Sea-level rise] really has the potential to change the marsh into a much more homogeneous, much less interesting system that offers a lot less refuge and food production for fishes,” says UC Davis senior researcher John Durand, who since 2015 has led the 42-year-old Suisun Marsh Fish Study, which involves monthly surveys of fish and invertebrates.

Suisun Marsh and adjacent Grizzly Bay occupy a key position in the San Francisco Estuary, sitting squarely between the Bay and Delta. To the west, separated by Carquinez Strait and the cities of Vallejo and Benicia, is salty San Pablo Bay. To the east are the lower reaches of the Sacramento and San Joaquin rivers and the myriad channels composing the Delta proper. Freshwater also flows into Suisun Marsh from the north via a number of creeks.

For the past 30 to 35 years, the abundance of native fishes living in the marsh has remained relatively steady, Durand says — with the exception of Delta smelt, which spend much of their lives in more open waters and began declining in the 1980s. But for many less-famous fish like tule perch, threespine stickleback, and particularly Sacramento splittail, the marsh remains an essential rearing area and, in some cases, a refuge for older fish. All of that could change if rising seas inundate intertidal habitats in Suisun Marsh and convert them to more static subtidal zones, Durand says.

Any expansion of open water in the Estuary is probably moving toward less productive habitat,” he explains. “The reason for that is open-water habitats of the Bay and Delta tend to be full of invasive organisms, both submerged aquatic plants and clams. Where that happens, we tend to see a decline in productivity. Aquatic plants … shade out a lot of phytoplankton production, while the clams tend to outcompete other animals in the foodweb.

The marsh is also unique among remaining intact Baylands in that much of it is privately owned and managed by hunting clubs for the benefit of ducks and other waterfowl – a situation that presents both a challenge and some potential advantages. Patches of public land include the 3,710-acre San Francisco Bay National Estuarine Research Reserve and the 12,900-acre, non-contiguous Grizzly Island Wildlife Area. The California Department of Fish and Wildlife also manages the smaller Hill Slough and Point Edith wildlife areas to the north and south of the marsh and adjoining bays.

Across this patchwork of owners, priorities, and habitats, a wide variety of land-use approaches are practiced. In addition to natural tidal wetlands, the complex includes managed ponds, artificially diked marshes, and extensive systems of levees, pumps, and waterways.

The preservation or relocation of Suisun’s most valuable habitat will likely depend on proactive, continuous, and coordinated management by all stakeholders, Durand says. “One of the nice things about the managed wetlands is that there’s an opportunity for a public-private partnership. We have landowners who are interested in conservation, and they have the wherewithal and the capacity to be able to help in this struggle to manage the marsh of the future.” This could take the form of redesigning and reinforcing levees, he notes, as well as altering flooding and water-exchange patterns within managed ponds.

Some private landowners will simply abandon their property and let it flood, Durand believes. Others are already looking to sell. But for those who are interested in staying the course, a more holistic view of marsh productivity will be required if the entire system is to withstand future sea-level rise. “It becomes more sustainable if we can offer people a way to stay involved in the landscape, and manage it actively” – not only for waterfowl and duck hunting, Durand says, but for all desirable species including the many native fishes that rely on the marsh for their survival.

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