ESTUARY PEARLS: Bay-Delta research from the most recent issue of San Francisco Estuary and Watershed Science. Plus: Can otters thrive in San Francisco Bay?

North America’s largest and most ancient freshwater fish species, white sturgeon, hang out in some kinds of Estuary waterways more than others, scientists find.

Researchers with the U.S. Geological Survey found that adult and sub-adult white sturgeon occupy deep open-water channels and shallow open-water shoals in equal measure, but don’t use shallow wetland channels. As a group, white sturgeon are characterized as amphidromous, meaning they regularly migrate between freshwater and the sea, in both directions, but not for the purpose of breeding.
 
According to the study, which appears in the December 2020 issue of San Francisco Estuary and Watershed Science, adults in the local population use coastal habitats to some degree, but typically remain in the Estuary and lower Sacramento and San Joaquin rivers. There they congregate in deep areas with fine-sediment substrate, and are thought to move into shallow subtidal habitats to feed during high tides.
 
The study may help guide management and conservation of the declining population through habitat restoration and other measures, says lead author Veronica Larwood. Future research could address habitat use for rearing.

Reported by Nate Seltenrich for Estuary News Service/Photo: Matthew Young


As agencies wrangle over how best to protect the Delta’s dwindling native fish species, researchers want to see more consideration to the needs of the estuary’s birds.

If we want to restore the ecology of the Delta, we can’t just be looking in the water,” says Kristen E. Dybala of Point Blue Conservation Science.
 
In a paper published in San Francisco Estuary and Watershed Science, Dybala and two co-authors make the case that “birds and their habitat needs are often not addressed in science syntheses, conservation planning, and large-scale restoration initiatives in the Delta.”
 
While some birds use the same sloughs and channels that support such high-profile fishes as Chinook salmon and Delta smelt, other bird species rely on habitat types that fringe the Delta’s waterways. Indeed, while general habitat restoration in the Delta can provide multiple benefits for humans, fish and many birds, the authors observe that numerous bird species “have a specialized set of habitat needs that require particular attention.”
 
Historically, the Delta and the Central Valley provided millions of acres of habitat for uncountable droves of resident and migratory birds. Much has changed: The authors write that “an estimated 97% of historical freshwater emergent wetland in the Delta, 77% of seasonal wetlands, and 77% of riparian forest are now gone, primarily converted to agricultural land.”
 
Still, the Estuary and the farmland fringing it remain a critical refuge for many species, and over time, this importance may amplify. Considering current and future “changes to the composition and configuration of the Delta’s landscape,” as well as expected species range shifts, the authors predict that “the Delta may become even more important to birds under future climate change. Thus, bird conservation in the Delta is more important than ever.” 
 
Reported by Alistair Bland for Estuary News Service/Photo: Blake Barbaree/Point Blue

Filling significant recently identified gaps in monitoring spring-run Chinook is critical to protecting these threatened Central Valley salmon.

There’s no way we can manage them for recovery if we don’t understand the biological processes that govern their dynamics through time and space,” says UC Santa Cruz/NOAA salmon expert Flora Cordoleani, lead author of a study reported in the December 2020 issue of San Francisco Estuary and Watershed Science.
 
Cordoleani and colleagues identified the monitoring gaps while building a model of the spring-run Chinook life cycle. The model accounts for three self-sustaining populations of these at-risk fish, assessing survival of key life stages (eggs, fry, smolts and adults) as well as in key habitats (natal creeks, the Sacramento River, floodplains in the Sutter and Yolo bypasses, the Sacramento-San Joaquin Delta, and San Francisco Bay).
 
It’s a complex model, and we needed data to fine-tune and test it,” Cordoleani explains. “But there wasn’t much.”
 
Major monitoring gaps include how many juveniles each population produces, and how many survive in the ocean.
 
Spring-run Chinook haven’t gotten as much attention because they are less endangered than winter-run Chinook,” she continues. That is beginning to change: The California Department of Water Resources is now working to estimate how many spring-run juveniles make it to the Delta. Identifying trouble spots for young salmon is vital to helping them.
 
Then you know where to focus management so the salmon can recover,” Cordoleani says.”We’re trying to fill the gap between the data we have and what we really need—we can then go to stakeholders and say, ‘This is where we should put our efforts to protect and conserve spring-run Chinook.’”
 
Reported by Robin Meadows for Estuary News Service/Graphic: Key life stages and habitats included in the spring-run Chinook life cycle model. Courtesy of Flora Cordoleani

Human activity could undermine the success of efforts to reintroduce sea otters to San Francisco Bay.

Although past studies have found that the Bay could support 1.5 times the entire current southern sea otter population, a new study from the Estuary & Ocean Science Center at San Francisco State University and published by PeerJ last November indicates that anthropogenic risks like contaminants, vessel traffic, and oil spills may constrain the otter’s ability to gain a foothold.
 
The study, led by Jane Rudebusch, at the time graduate student at SFSU, looked at the types of human stressors present and ranked them according to factors like temporal overlap (frequency of its interaction with otters), intensity (consequence of that interaction), and management effectiveness (how well the stressor is mitigated by human regulation). The study concluded that high-speed vessel traffic, such as commuter ferries, was of primary concern.
 
However, Rudebusch found that “the risk from human disturbance is really not evenly spread throughout the Bay. You can look at pockets of the Bay where there is lower risk and use those to focus a reintroduction effort.”
 
Sites like China Camp on the Marin bayshore are particularly appealing because of their protected status and relative lack of human stressors. Rudebusch says it was important to capture a picture of the entire area because “although sea otters occupy a small home range, they can also travel great distances. Where you put them is not necessarily where they are going to stay.”
 
San Francisco Bay presents an interesting reintroduction site because of the success of the population at Elkhorn Slough—another estuarine habitat in Monterey Bay—and otters’ inability to naturally expand their range north of Pigeon Point, where they are prey for great white sharks. Returning otters to their original pan-California range would improve the health of the coastline and estuaries.
 
Sea otters are a keystone species, this singular glue that holds an ecosystem together,” says Rudebusch. “For over a hundred years, [many] ecosystems in California have been missing this key link.”
 
Reported by Michael Hunter Adamson for Estuary News Service/Graphic: Results of the Habitat Risk Assessment Cumulative risk scores ranged from 0 to 10.7, out of a possible maximum cumulative risk score of 21. Credit: Rudebusch et al. 2020

The increasing flow of microplastics entering San Francisco Bay from trash, fleece clothing, car tires, and myriad other sources is likely being trapped by a surprising filter: native eelgrass (Zostera marina).

Miniscule polymer pieces the size of a sesame seed or tinier, microplastics pose a growing pollution threat to marine environments worldwide. To understand how microplastics accumulate in nearshore, urbanized environments, researchers quantified the prevalence of microplastics in and around the Zostera marina meadows of Deerness Sound, in the Orkney Islands of Scotland.
 
Mark Hartl and colleagues at Heriot-Watt University found that microplastic flakes, fibers, and fragments were twice as concentrated in the water above eelgrass meadows as in adjacent control areas of sandy sediments. Sediments within the meadows contained 40% more microplastics than in the sandy areas. The scientists also found plastics attached to every one of the 60 blades of eelgrass they examined; in fact, microplastics were 20% more abundant atop eelgrass than in control areas.
 
Eelgrass meadows are prized for their ability to absorb wave energy and increase rates of sedimentation; the researchers suspect this talent for slowing the travel of water helps floating microplastics settle out of the water column onto meadow areas.
 
Meanwhile, biofilms such as algae and the microscopically rough surface of seagrass blades help microplastics adhere to the surface of the plants. The prevalence of these contaminants on eelgrass itself—a source of food for marine grazers, as well as detritivores such as amphipods that eat dead plant matter—indicates eelgrass may be a route for microplastics to enter the marine food web. Indeed, the Scottish scientists found microplastics in more than 80% of the snails and other eelgrass grazers they sampled.
 
Whether the ability of eelgrass to trap microplastics is a boon or bane to the Bay remains an open question in the burgeoning field of marine microplastics research.
 
Reported by Kathleen M. Wong for Estuary News Service
 
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