By Elsie Platzer & Lynette Williams Duman
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A friendship forged in mud
On an unseasonably warm day in February of 2022, after being acquainted for approximately eight hours, we found ourselves canoeing across Big Break near Antioch, California. As we paddled against the wind we were busily dodging bass boats and floating logs. We watched a Peregrine Falcon dive at the water in front of us and fly off holding an American Coot who never saw the assault coming. With our canoe full of nets, water quality equipment, and zooplankton supplies, we were taking the long way around from one tidal slough mouth to another. Although it functioned as intense workplace bonding – and the beginning of a long friendship – we were out there to catch thousands of nearshore fish at the brand-new tidal restoration at Dutch Slough.
Complementary projects and a lot of fieldwork
Nearly 20 years ago, a US Fish and Wildlife Service (USFWS) Biological Opinion mandated the creation of new tidal wetland habitat across the California Delta and Suisun Marsh, for the stated purpose of benefitting the now-critically endangered Delta smelt (Hypomesus transpacificus; USFWS 2008). Since then, a total of 32,000 acres of tidal wetland restoration have been implemented or planned across the region (Chapple et al 2025). These wetlands are meant to provide fishes with shallow-water habitat, now scarce in the highly altered San Francisco Estuary, which can be utilized for rearing and foraging (Herbold et al 2014). Now, as these sites develop, researchers seek to understand how restored tidal wetlands are functioning, and whether they are providing the intended benefits to target fish populations.
We led two studies evaluating the aquatic ecosystems of restored tidal wetlands in the Delta and Suisun Marsh (Williams Duman et al. 2026 and Platzer et al. 2026, respectively). Both sampled water quality, zooplankton densities, and nearshore fish assemblages, and compared some or all of these metrics within tidal restorations to values observed in nearby reference sites and alternative habitat types. Our results contribute to our complicated understanding of tidal restoration’s impact on native fishes. Generally, restored sites reproduced the conditions present in reference habitats: native fish were outnumbered by non-natives, and zooplankton production in sloughs was lower than in pond-like habitats. However, tidal sites do increase habitat for fish, particularly when the land-use prior to restoration was not aquatic. They can also support a higher diversity of native species, as compared to leveed-off wetlands, due to their connectivity with the greater aquatic landscape.
Study 1: Restorations in the Delta – a fish in a slough is a fish in a restoration?
At Dutch Slough, located in the western freshwater reaches of the Sacramento-San Joaquin Delta (Figure 1), we were interested in whether the different slough designs impacted fish communities at the restoration, and whether the fish communities outside of the restoration reflected the communities on the inside (Williams Duman et al. 2026).
The Delta, which is managed as freshwater habitat to support water exports, has a mixture of non-coevolved native and non-native species, and a main goal of restorations in the region is to enhance habitat for natives (California Department of Water Resources 2026). We sampled a terminal slough (Gilbert), a non-terminal slough with a freshwater input from Marsh Creek (Emerson), and an open-water borrow-pit (Embayment). In the months immediately after breaching, the site was filled with nonnative Mississippi silverside (Menidia audens). We suspect that breaching during a drought created ideal conditions for silverside proliferation and colonization. As time since breaching increased, silverside numbers decreased, and we did not see their recovery for the remainder of the study. Nor did we see other species from other guilds (vegetation-dependent, benthic, edge-associated, or pelagic species) increase in the absence of silverside. At the Embayment, pelagic species were abundant in the first year and then decreased as submerged vegetation established. The silverside decline we saw at Dutch Slough was not reflected in our reference sites at Antioch Dunes, Sherman Island, or Franks Tract (see Figure 2).
Our study at Dutch Slough led to a few key insights regarding tidal restoration in the freshwater reaches of the San Francisco Estuary.
First, the fishes that are in the surrounding bodies of water are likely to colonize new restorations. We found that the fishes of Dutch Slough largely reflected the communities of nearby mainstem sampling sites. The major difference was that reference areas had more fish caught in general, and more native species. Hydrological connectivity at reference sites likely facilitated access by higher numbers of native species, implying that increasing connectivity at restoration sites could open them up to a greater diversity of fishes.
We also found that in the absence of silversides, other species did not increase, suggesting that other species were limited by landscape-level features, rather than by interspecific interactions. This means that landscape features determine what fish species were present, and restorations should be designed with features that enhance nearshore fish communities. At Dutch Slough, that means restricting the establishment of submersed aquatic vegetation, which we believe greatly degrades nearshore fish habitat.
Study 2: Restorations in Suisun – Silverside Gold Mine?
Unlike the Delta, where most historical wetlands were drained for agriculture, Suisun Marsh has retained much of its original wetland area as “duck clubs” – managed wetlands that are gated and seasonally dried out, their hydrology carefully controlled for the purpose of supporting waterfowl. When tidal wetlands are created in Suisun, they are often “converted” from managed wetlands by removing those operating gates. In this study, we attempted to compare tidal restorations with both managed wetlands and sloughs–the deep, fast-flowing channels of Suisun Marsh that serve as “reference” tidal habitat (Platzer et al. 2026).
After eighteen consecutive months of sampling in all three habitat types, we saw managed wetlands sustain higher chlorophyll densities and higher zooplankton biomasses compared to both tidal restorations and slough channels. In terms of fishes, our catch was dominated by two species. Mississippi silverside, a hyperabundant non-native, comprised the majority of catch in both tidal habitats. By contrast, the muted tidal managed wetlands supported large populations of threespine stickleback, a native littoral resident fish. Though we caught sticklebacks somewhat regularly in the tidally restored wetlands and along slough channel banks, we almost never captured silversides in the managed sites. The ratio of catch between habitat types was shocking: over 20,000 individual silversides in the tidally restored sites to exactly 137 in the managed wetlands (Figure 4). Clearly, managed wetlands demonstrated the ability to “filter” the ambient fish community, whereas tidal habitats did not.
Why might silversides struggle to utilize managed wetlands? As the California Water Blog has discussed (Moyle 2023), Mississippi silversides are a littoral/pelagic species that often move offshore into deeper water overnight. The gates on shallow managed wetlands block access to pelagic habitats, which may discourage silverside colonization. At the same time, this indicates that tide gates may also discourage habitat use by other pelagic fishes, including species of concern, like smelt and salmonids–a potential downside of managed habitats.
Implications for restoration and conclusions
A common takeaway from these two studies is a similarity in fish assemblage between restored tidal wetlands and adjacent waterways. Tidal wetlands and sloughs across Suisun Marsh both were dominated strongly by silversides, as were Dutch Slough and nearby reference sites in its early months post-breach. Tidal restorations seemingly do not “filter” for any particular species, meaning that non-native or undesirable fishes can take advantage of the habitat just as easily as natives. Gates habitats create a filter, but threatened native fishes might struggle to directly access gated habitats. This is a tradeoff with no easy solution, though it does highlight the importance of considering restorations in the context of a landscape-level mosaic. If fish from surrounding habitats are the fish that will inhabit restorations, then restoration placement should consider surrounding habitat and the species that are present within that greater habitat matrix.
If we hope to support native fish populations in the coming decades, we must build restoration sites to address fish functional needs. One of those needs is pelagic production, which is poor in tidal sites, but can be enhanced by incorporating gates, weirs, or other tide-dampening structures in site design that would allow practitioners to directly manipulate residence times. These structures can slow tidal flows sufficiently to give plankton time to accumulate during critical periods for fishes, and allow for more effective submersed aquatic vegetation management. Recent studies from Suisun Marsh have shown that these managed sites produce 22 times the zooplankton of tidal sites, and pulses can be strategically timed to coincide with fish migration (Phillips et al. 2026).
It is important to remember that with every tidal restoration we learn something new. Before widespread channelization, draining, and diking, tidal wetlands took thousands of years to form. And, in the San Francisco Estuary, we rarely have any baseline data to work off of when restorations are being planned and implemented. Tidal restorations therefore present a fascinating opportunity to learn about what wetland designs function in what ways. Great lengths have been taken by practitioners to design these restorations as experiments. By creating wetlands as experiments, and the continued support of researchers (like us!) to assess their fish communities, zooplankton production, and water quality, we are able to enhance future restoration efforts and adaptively manage pre-existing restorations. With each restoration, the scientific knowledge grows, much like our friendship, and much like the friendship of the next two twenty-something-year-old PhD students who will work at these sites together. We just hope they have practiced their paddling.
About the Authors
Elsie Platzer is a PhD Candidate in the Aquatic Research Collective. Her research focuses on wetland restoration processes and outcomes in the San Francisco Estuary.
Lynette Williams Duman is a PhD Candidate in the Aquatic Research Collective. She is interested in the food webs of tidal restorations and trophic interactions in the San Francisco Estuary.
Further Reading
Moyle, P.B. (2023). The Rapid Invasion of Mississippi Silverside in California. California Waterblog. March 19, 2023.
California Department of Water Resources. (2026). Dutch Slough Tidal Restoration Project.
Chapple, D.; Moffat, J.; Melcer, R.; Mattson, M.; Griffith, K.; Anderson, K., et al. (2025). Restoring the Heart of a Healthy Estuary: A Review of Restoration in the Sacramento–San Joaquin Delta and Suisun Marsh. San Francisco Estuary and Watershed Science, 23(4).
Herbold, B.; Baltz, D. M; Brown, L.; Grossinger, R.; Kimmerer, W.; Lehman, P., et al. (2014). The Role of Tidal Marsh Restoration in Fish Management in the San Francisco Estuary. San Francisco Estuary and Watershed Science, 12(1).
Moyle, P. B. (2002). Inland fishes of California: revised and expanded. Univ of California Press.
Phillips, K. A., Tung, A. M., Platzer, E., O’Rear, T. A., Lawler, S. P., & Durand, J. R. (2026). Pulsed zooplankton production in seasonally managed wetlands could supplement food availability for a threatened fishery in the San Francisco Estuary. Estuaries and Coasts, 49(3), 69.
Platzer, E., Phillips, K., O’Rear, T. and Durand, J. (2026). Silverside gold mine? Restored tidal habitats host abundant invasive fishes in a novel California marsh (U.S.A.) Restoration Ecology e70452.
US Fish and Wildlife Service (USFWS). (2008). Delta Smelt Biological Opinion
Williams Duman, L., Holst, M., Platzer, E., Phillips, K., & Durand, J. (2026). Fish assemblage at a newly restored tidal wetland reflects surrounding waterways in the Sacramento‐San Joaquin Delta. Restoration Ecology, e70414.
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