Food supplies and temperature refuges are some of the ways habitat restoration can help salmon adapt to climate change, says Dr. Ted Sommer
As California’s climate changes, concern grows for the survival of salmon, but research shows that habitat restoration may be one of the key tools for supporting salmon in the years ahead, says Dr. Ted Sommer, DWR’s Lead Scientist. He gave this presentation earlier this fall at the California Salmon and Climate Variability Symposium convened by the UC Davis Coastal and Marine Sciences Institute Center for Coastal Ocean Issues and the Delta Science Program.
Dr. Ted Sommer began by saying that habitat restoration is one of the better ways that we can reintroduce some of the resiliency that we need to support salmon through the changes that are expected to occur with climate change, such as changes in the hydrograph, changes in ocean circulation, and changes in stream temperature. “My message today is that restoration is one of the key tools that we can use to help support salmon through these changes,” he said. “It’s not the only tool, but I think it’s really kind of a key one.”
Prior to human development, the Delta was a historical flood basin with broad habitat complexity; there was a lot of seasonal floodplain, emergent marshes, natural levees and so forth, and salmon were adapted to deal with this variable landscape, but now it has been oversimplified, he said. “For salmon, this basically means we’ve completely homogenized things and there’s no habitat,” he said. “There’s very little shallow water habitat available in a trapezoidal channel and it doesn’t change much as you increase flows, and that’s really different than historically. That doesn’t mean that flow has no effect on survival; in fact, we do see higher survival at higher flow rates, but probably what is happening is that the fish are just moving really quickly down this biological desert at higher flows, so you’re basically moving the problem further downstream.”
On a landscape scale, flow can have a significant effect on rearing habitat, such as in the Yolo Bypass, he said. “We have a managed floodway based on the historical floodplain where when the Sacramento River gets high, it spills out on this broad floodplain and a lot of rearing habitat is generated, “ he said. “This is something that we found over the years is incredibly important for young salmon. Work out in the bypass for the past 16-17 years or so, one of the things that we have found is that all of the four major races tend to use the floodplain when it spills out into the bypass. … You’ve also probably largely seen the idea that salmon grow really well on the floodplain.”
Dr. Sommer said they have also done a lot of tag studies to better understand the dynamics of what fish are doing on the floodplain and what drives their success. “When the floodplain is inundated, the fish really spend a lot of time there, unlike the Sacramento River where they move through relatively quickly,” he said. “Out here in the bypass when we tag fish and release them, they are typically out here for a month or more. When we have longer duration flood events, we get longer residence time, so there’s a nice correlation between the duration of floodplain inundation and residence time. And finally, there’s a really good correlation we find between residence time and bigger fish, and that’s a key issue for resilience.”
However, the Yolo Bypass is a managed floodplain is designed first as a floodway, so it drains really quickly, he said. “Typically we see things drain in about a week, so bottom line here is that fish get a lot of benefits out from this habitat, but they don’t get to stay very long.”
So what can be done about this? “Our agency is solution-oriented; we’re looking for ways we can help improve the status of salmon and in the process, hopefully provide some resilience against climate change,” he said. “One of the first things that we’re working on is providing greater connectivity with the adjacent Sacramento River. The Sacramento River only floods into the system at very high flows, so one thing we can do is provide a notch at the top at Fremont Weir so that it’s just not at the peak flood events that the bypass is flooding. We think that will make a big difference.”
He then displayed the modeling results for simulation of an inundated area for a range of flows, noting that scale on the x axis is up to 50,000 cfs, but large events can be on the order order of 100,000 or 500,000 cfs; however, it doesn’t take a large flood to reap the benefits. “We get a lot of the surface area benefit of flooding just at the low end, 10,000 – 20,000 cfs, so if we have a notch to provide greater connectivity in the floodplain, we can get a lot of the benefits for salmon at relatively low flows and for longer periods and we need that in the face of climate change.”
“The problem is that we’re still a long ways from being able to construct this sort of notch that we need to do that,” he acknowledged. “There are all kinds of permitting issues; this is a floodway … so we’re looking at what we can do in the near term to provide some support to Chinook salmon.”
There are a couple of different approaches they can take, he said. “The first is when we do get those Sacramento River events, can we do something to topography to slow things down? It drains off really quickly so can we provide some topographic complexity. The other thing that we’re looking at is strategic use of tributaries. The Sacramento River flows really are the main story out here, but there are smaller westside streams, such as Putah Creek, Cache Creek, and Knight’s Landing Ridge Cut, so we’re strategically looking at ways to use those tributaries when the Sacramento River stops flowing to keep the habitat inundated.”
Dr. Sommer noted that the tributaries can provide a lot of benefit, presenting a modeled example of surface area, noting that the yellow is the flow from Cache Creek. “The inundated surface area in white on the modeled output inundates about a quarter to a third of the Yolo Bypass, so the tributaries alone really can provide a fair bit of support. We’ve been trying to use that to our advantage in some recent studies that we’re doing.”
They have been working in the northern Yolo Bypass over the past several years evaluating whether targeted tributaries can be used to support salmon during drier periods, such looking at Knight’s Landing Ridge Cut flows which originally came from the Sacramento River, and doing work out on fallow rice fields and other fallowed grasslands to see if salmon are supported by managed flooding. There are many agencies and organizations partnering with DWR on this, such as UC Davis, Cal Trout, a lot of helpful local landowners, and the Bureau of Reclamation.
When the lands in the Yolo Bypass are inundated using tributary flows, it generates a huge food web. “It’s basically zooplankton soup out there,” he said, presenting a chart showing densities of copepods, cladocerans, and ostracods in a Knaggs Ranch test inundation using tributary flows as compared to the adjacent Sacramento River in natural flood events and the Yolo Bypass. “The densities of zooplankton really compare quite favorably and the fish respond quite well to this.”
“We typically get about a millimeter of growth per day over several years of study when we do tributary-based inundation and that compares favorably to natural flood events out in Yolo Bypass; it also compares favorably to Cosumnes floodplain, and it is substantially better than fish that are out in the Sacramento River.”
Survival rates have been somewhat mixed, he acknowledged. “These are very small fish we are working with so we don’t expect fabulous survival. In 2013, there was pretty meager survival; we started late, it was really hot. The other years suggest that we can get decent survival rates in this sort of system.”
As to the idea of using topography to help enhance flooding, they are hoping for a wetter year this coming winter. “What we’re planning to do in 2016 is take four sites out in the floodplain and do simple modifications to topography so that it doesn’t drain off like a bathtub so quickly,” he said. “Once the Sacramento overtops, we’ll use topography out here in these managed areas to keep things flooded; we’ll be studying the wild fish that have moved in to these habitats, and we’ll also be supplementing each of the test areas with hatchery tagged fish as well to give us some of the experimental comparison, so exciting study, got my fingers crossed. If this works, this is something we could potentially do on a management basis relatively soon and consistently.”
It’s not just floodplain resilience against climate change. “We do have some tidal slough habitat lower down; at Liberty Island, some of my colleagues have done sampling, and we do consistently get salmon down there. There’s a lot of work from the Northwest of course, showing the benefits of emergent marsh, most of it gone.”
During drought, salmon are able to take advantage of this. “In the wetter years, fish can get out into the floodplain, but in drier years, they can use some of the tidal slough habitat,” Dr. Sommer said. “If they migrate down the Sacramento River, there are smaller channels, and there’s also the base of the bypass which is this complex tidal slough habitat, and what we’re finding is that large numbers of fish, even in drought years, are able to locate this and find it.”
He then presented a chart that shows the catch from four years in the tidal slough complex at the base of the Yolo Bypass, noting that 2011 was a wet year, followed by three drought years when the fish weren’t able to access the floodplain from the top. He noted that each of the dots on the graph is a salmon. “Note that the catch of salmon across these different years is high in each year, so during the drought, fish are really actively seeking out this tidal slough habitat and rearing here, and it’s no surprise. There’s no habitat in the Sacramento River, particularly during the drought years.”
He then pointed out that the y axis is the distribution of when the fish are using the habitat. “What we see is the fish are using the habitat over a pretty broad time period, all the way through winter and spring. There is some variation between wet and dry years, but this provides some portfolio effect here, being able to use habitat.”
He lastly noted that the dots on the graph are scaled to fish size. “So little dots, tiny fish; big dots, big fish, so a whole range of fish sizes are using this habitat. Again, this is part of the argument that this is the sort of habitat that can promote the sort of reliance and life history diversity that we need. We also have some evidence that this habitat isn’t a black hole, even in extreme drought years. Studies done releasing fish into the tidal slough habitat during dry years found survival is really good, on the order of 99%+ per kilometer, so that compares favorably, so again that’s an indication that this will have benefits.”
As climate change brings warmer temperatures, a lot of conditions are going to become lethal, he said. “Part of this is a food story, because part of the ability of fish to tolerate higher temperatures is whether they are getting enough food,” said Dr. Sommer. “What we see in the rip rapped Sacramento River, there’s no food there, so of course they are going to be extremely susceptible to high temperatures, but by contrast in the floodplain and other tidal habitats, there is a lot of food there providing some resistance to high temperatures.”
“Having some habitat complexity out there creates some opportunities for temperature refuges,” he said, presenting a graph showing temperatures in both the managed floodplain and the Sacramento River. “It was a drought year and the temperatures are often warmer, but note that there are periods in February and March where there are long periods where temperatures are actually cooler. What we’ve found is that these are periods when there’s a lot of wind out there, and so we get evaporative cooling. The argument here is that if you have some habitat complexity, there are spatial and temporal temperature refuges that fish will find and can make use of.”
Dr. Sommer noted that Chris Enright has a paper showing that a similar mechanism may apply down in the Suisun marsh downstream. “They’ve studied Suisun Marsh and found that marsh temperatures are a really complex interaction of tides, air temperature, and geomorphology, and depending on where things are in the tidal cycle, you can get a similar kind of cooling effect when water inundates the marsh plain there.”
He acknowledged that freshwater conditions won’t affect ocean conditions. “But it matters how big you are when you hit the ocean, and the sort of habitat that we’re talking about here will make a difference in the diversity of sizes of fish that enter the ocean and hopefully improve resiliency.”
Dr. Sommer than gave his conclusions. “We expect a bunch of different climate effects and we think restoration will be one of the key solutions,” he said. “We’re going to see a change in the hydrograph; we’re hoping that through restoration we can help buffer the frequency and duration of rearing habitat that’s available to try and increase habitat and life history diversity. We’re hoping that habitat restoration can help offset some of the temperature effects by increasing resistance to high temperatures through food supply and also by creating temperature refuges, as well as creating some size diversity at ocean entry to help deal with variable ocean conditions.”
“And with that … “
More coverage on Maven’s Notebook from the California Salmon and Climate Variability Symposium …
- Saving salmon in a changing California (Dr. Peter Moyle)
- A regional-scale view of climate impacts on California’s salmon habitats (Dr. Nathan Mantua)
- Juvenile Chinook challenges in the Central Valley: baked, boiled or bass turds? (Dr. Sean Hayes)
- Conserving Chinook salmon at the southern end of their range: Challenges and opportunities (Dr. Rachel Johnson)
For more from the symposium …
Coverage of Maven’s Notebook has focused on the freshwater science presented there, but it’s only a portion of what was covered at the symposium. Many more topics were presented, including a panel discussion with agency officials and stakeholder. I encourage you to explore the rest of the symposium at the link below.
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