Estuary News

February 2022

Let's Not Forget Suisun Marsh

By Peter Moyle

I started sampling the fishes of Suisun Marsh in 1979 because one of my UC Davis graduate students was looking for a place to study tule perch, a live-bearing native fish. We found not only a lot of tule perch in the marsh, but also an abundance of other native fishes. Clearly, this was a good place to study species for which we had little information at that time.

Two things helped with our new project. First, sampling boats could be launched less than an hour’s drive from campus. Second, the California Department of Water Resources needed a study to examine effects of new tidal gates on fish. The gates are designed to retain fresher water in the marsh to benefit waterfowl, for hunting. They also keep marsh channels brackish, favoring estuarine fishes such as striped bass and splittail.

Peter Moyle noting conditions in Suisun Marsh. Photo: Amber Manfree

Now, 42 years later, Suisun Marsh is still the subject of a monthly sampling program. Over all this time, the team, now led by UC Davis’ John Durand with sampling supervised by Teejay O’Rear, has come to a number of broad conclusions. After reviewing the list of stories that made it into this special issue, many of which I suggested, I couldn’t let it publish without sharing some of the team’s findings about the importance of Suisun Marsh to fish in the San Francisco Estuary.

First, despite its small size relative to the Delta, Suisun Marsh is an extremely important rearing area for juvenile estuarine fishes. For native splittail, it is now the most important rearing area, as well as being a refuge for older fish. This benefits the entire estuary.

Second, fish habitat quality is closely tied to water management through operation of the tidal gates. In addition, managers of duck hunting clubs and wildlife areas regulate tidal flooding of marsh areas to benefit waterfowl. The fish also benefit from the abundant food produced by this exchange of water.

Third, the fishes of the marsh form a novel fish assemblage of native and non-native species that behaves remarkably like a natural fish assemblage. Composition, however, can change, reflecting changes in water quality and other factors, such as invasions of new species. The first collections of shimofuri goby and alligator weed in the Estuary came from the marsh, for example, and non-native invertebrates such as Black Sea jellyfish and Siberian prawns are now important players in the Suisun (and estuarine) ecosystem.

Fourth, much of the marsh is composed of small sloughs that don’t mix very much with the large sloughs (Montezuma and Suisun), deterring invasive species (such as clams) while supporting native and pelagic fishes. Understanding this highly unusual feature could lead to improved management and control of invasive species.

Fifth, the sampling program is designed to accommodate volunteers, so over the decades, hundreds of students and others (including reporters) have been able to experience the fishes and marsh sampling. We like to think that better-informed citizens can play an important role in protecting Suisun Marsh and its fishes today. 

Lastly, the disappearance of Delta smelt from marsh samples in the 1980s sparked an investigation to see if the population crash was widespread. It was. The smelt was listed as threatened in 1993.

Projected flooding of Suisun Marsh due to sea level rise by 2100. Map: Amber Manfree

Years of studying Suisun Marsh have demonstrated its importance to the San Francisco Estuary as a whole. Its importance goes way beyond fish, as discussed in our 2014 book Suisun Marsh: Ecological History and Possible Futures. The book uses maps to show how the marsh will change under various management scenarios and sea-level rise. It also shows how local communities such as Suisun City, on the marsh’s edge, face flooding. Indeed, the marsh may serve as a landscape-scale levee to some extent, offering protection to adjacent urban areas. Communities that benefit from protection of their neighboring marshes, however, must also remember to protect the marsh as an ecosystem that supports a diversity of life, including native fishes.

In short, Suisun Marsh is a novel ecosystem that can serve as a laboratory to help us better understand the Delta and Estuary. For me personally, it has always been a great place to “hang out” and to be amazed that such a wild, open place exists in an urbanized region.


Suisun Marsh: Ecological History and Possible Futures, UC Press

Special Issue on Fishes of the San Francisco Estuary

Related Posts

subisdence on 4 delta islands map

Asked to suggest an appropriate future for four Delta islands owned by the Metropolitan Water District, it’s difficult not to want the moon.

Why not richer rice fields, more wetlands, better boat launches, extended trails, even eco-tourism? ? A new survey for the Delta Islands Adaptation Project, funded by a Prop 1 Watershed Restoration Grant, wants the public’s opinion on the importance of 10 possible land-use objectives leading to the selection of one...
chart by Hutton

An examination of 50 years of records reveals an apparent seasonal bias in estimates of freshwater flow from the Delta to the San Francisco Estuary.

The seasonal bias suggests flows were overestimated during the summer months and underestimated during the winter. Estimates of Delta outflow use a measurement called net Delta outflow index (NDOI), which is determined by taking the amount of Delta inflow, from sources such as the Calaveras, Sacramento, and San Joaquin rivers,...
diagram of fish net trawl

Fish monitoring surveys in the San Francisco Estuary net different numbers of different fish species depending upon when and how they sample.

According to a new study published in San Francisco Estuary and Watershed Science, even surveys that target the same part of the water column can come up with significantly different catches. The study’s authors analyzed decades worth of data from the California Department of Fish and Wildlife (CDFW) Fall Midwater...