DELTA LEAD SCIENTIST REPORT: Salmon predation in the Delta; Fact sheets on steelhead trout and chinook salmon; and the activities of the Delta Science Program

At the September meeting of the Delta Stewardship Council, Delta Lead Scientist Dr. Laurel Larsen spotlighted an article on salmon predation, highlighted two new fact sheets on salmonids now available, and gave an update on the activities of the Delta Science Program.

Fact sheets on steelhead trout and chinook salmon

Delta Lead Scientist Dr. Laurel Larsen kicked off her Lead Scientist Report by spotlighting two fact sheets developed by Delta Science Program staff member Dr. Pascale Goertler on chinook salmon and steelhead trout, two listed species under the Endangered Species Act.  These species are referred to as salmonids.

Steelhead trout and Chinook salmon have similar life cycles described as anadromous, meaning that they complete part of their lifecycle in the ocean. They spawn in tributary streams of the Central Valley rivers, and after the eggs hatch, the fish spend one or two years in those streams before migrating through the Delta to the Pacific Ocean.  While migrating, they are referred to as smolts. They then spend two to five years in the ocean before returning to their natal streams to spawn, shortly after which they die.

The steelhead trout fact sheet goes into detail on what makes the Central Valley steelhead subpopulation unique.

The Chinook salmon fact sheet details the predation that chinook salmon face as smolts pass through the Delta to the ocean.

Presumably, predation has increased over historical levels due to the introduction of non-native predators, including striped bass, largemouth and smallmouth bass, and white catfish,” said Dr. Larsen.  “We know that predation hotspots tend to develop in areas of modified habitat such as areas with invasive aquatic weeds and areas of modified flows.  However, there hasn’t been a systematic effort to delineate these hotspots and quantify the overall impacts of predation.

Predation is a topic on the Science Action Agenda and is a highly ranked potential action from the workshop for the Science Action Agenda update, and most recently at the recent Collaborative Adaptive Management Team meeting, she said.

Article spotlight: Coldwater fish in a warm water world: Implications for predation of salmon smolts during estuary transit

The paper, Coldwater fish in a warm water world: Implications for predation of salmon smolts during estuary transit, was written by Matthew Nobriga with the US Fish and Wildlife Service along with colleagues at the National Marine Fisheries Service and the Center for Watershed Sciences at UC Davis and was just published in the journal, Ecology and Evolution.

The study uses multiple methods to quantify predation on Chinook salmon smolts by largemouth bass, which is found in the vegetated margins of the Delta, and striped bass, which are found in the deeper water parts of the Delta as they move through the estuary.

One of the foundations for the paper was the observation that when water temperatures in the Delta reach 20 degrees Celsius, which corresponds to 68 degrees Fahrenheit, there’s nearly complete mortality of the smolts.  This observation is decades old, though; it’s from a 1989 study. So the author’s first objective was to see whether it still holds, given decades of more recent acoustic telemetry data. And the data did affirm that threshold.

A second objective was to get at why there is the mortality of smolts at warmer temperatures. It’s a basic tenet that an organism’s metabolism speeds up with temperature, so one hypothesis has been that predators have a greater need for food when water temperatures are warm, so they hunt more frequently.

Additionally, the authors hypothesize that predation would increase at warmer temperatures, given the presence of invasive largemouth bass in the vegetated margins of the Delta.  So they propose that encounters with largemouth bass in areas that were formerly shelters for the smolts would essentially chase the smolts into the open water, where they would then be subject to predation by striped bass; they would have been less likely to encounter a largemouth bass had a largemouth bass not been searching for food in the vegetated regions in the first place.

They tested this hypothesis through a combination of field studies using videotape tethered fish at different distances from shore and at different temperatures and by using models to deduce parameters such as fish attack rate and predation success rate by trying to get the best match of the model outputs to the monitoring data.

Based on these suites of studies, they first found that there’s a small effect of temperature and distance from shore on largemouth bass predation rate, but there’s no effect of either variable on striped bass predation rate,” said Dr. Larsen.  “Overall, this temperature effect is small enough that it doesn’t provide support to the hypothesis that predation rate multiplies at warmer temperatures, and it does not provide a sufficient explanation of why we see extreme mortality at water temperatures greater than 68 degrees Fahrenheit.”

Now, the model results produced a similar conclusion but also provided a further clue into what might be going on,” she continued.  “Though they showed very little effect of temperature on overall predator attack rate, the only way the model results could match the monitoring data was if the salmon smolts swimming speed, which governs the success rate of each attack, decreased at those warmer temperatures. This is not something that’s seen in captive fish, but the authors hypothesize that it might arise from an increased prevalence of disease at warmer temperatures in the field or from effects of contaminants during low water warm periods when those contaminants are highly concentrated.”

Dr. Larsen acknowledged that the need to study the effects of contaminants across the Delta is emerging as a possible priority science action in drafting the revised science action agenda, so the study highlights a significant research gap.

While results on precisely why mortality goes through this temperature-dependent threshold were a little unclear, one thing that was clear from the study was that there is a significant negative relationship between inflows to the Delta and water temperatures in the Delta – in other words, as inflow goes down, the temperature goes up,” said Dr. Larsen.  “This was important to demonstrate because the common belief has been that flow has very little to do with water temperature in the Delta. And while its impact might indeed be small, managing temperatures through controlling flow might be just sufficient to keep water temperatures below critical thresholds for fish at some places and/or at some times, and the study suggested that flows have the greatest relationship to temperature in April, May, and June.”

The authors conclude the paper with some management implications, including active intervention, such as trucking fish around the Delta when water temperatures are too high; the strategic management of inflows for maintaining suitable temperatures; and manipulating flows to mimic historic flow patterns and trigger smolts to start their migration through the Delta early before temperatures get too warm.  The authors also stressed that any actions that reduce the available habitat for non-native largemouth bass, such as removing nuisance aquatic weeds or filling of ponded areas through restoration, would likely enhance the survival of Chinook salmon.

During the discussion period, Councilmember Don Nottoli asked how the experiment with the tethered fish was conducted.

It was a Chinook salmon smolt that was tethered,” said Dr. Larsen.  “Basically, they were looking at whether predation rate by the striped bass or the largemouth bass would vary as a function of where they put the smolt in terms of distance to the edge, and also as a function of temperature. So they ran these experiments at different temperatures. Now, they acknowledged that one weakness of this study design is that it doesn’t give the smolts a chance to escape, which was a big factor underlying their hypothesis that predation would be multiplicative as temperature goes up, because when the smolts move to get away from a predator, they enter the habitat of another predator, so that particular design was not conducive really to testing the extent to which smolt behavior was consistent with that hypothesis.”

Councilmember Virginia Madueño asked about the relationship between invasive vegetation and warmer temperatures.  “Having served on the Boating and Waterways Commission before, we obviously saw the increase in the invasive water hyacinth that really took over in so many parts of the Delta. Could you elaborate a little bit more?

Invasive aquatic weeds essentially create habitat for the largemouth bass, which is a predator that was not historically in the Delta,” said Dr. Larsen.  “Historically, smolts would frequent areas of native vegetation and the margins of the Delta in order to escape predation by organisms in the open water environment. But now, when they go to areas of vegetative cover, those areas are the same ones frequented by the bass. And so it basically decreases the safety of their refugia for them and creates a new source of predation.”

We’re finding that control of those aquatic weeds is really challenging, and it’s widely acknowledged if we theoretically could control invasive aquatic weeds to a larger extent, it’s likely that predation by largemouth bass would go down. But that’s very difficult to realize because we don’t have a great method for control of aquatic weeds in estuarine environments where you have tides that consistently dilute any chemical treatment you might put in the water.”

Councilmember Madueño agreed, recalling how at one point, there were such large patches of water hyacinth in the Delta near the port of Stockton that some of the ships were actually detecting it as landmass.  “It is a continued problem that I think we’re going to have to become more creative and take a look at other deltas throughout the world to see how they’re taking care of some of these problem invasive species of plants. Hopefully, there are others that maybe have some best practices.”

Dr. Larsen noted that invasive aquatic weeds and other species have been ranked high on the Science Action Agenda.  “So I think we’ll see some science actions on potentially testing experimental controls on invasive aquatic weeds and looking to other systems for some synthesis that might be applicable to the Delta.”

READ THE PAPER HERE: Coldwater fish in a warm water world: Implications for predation of salmon smolts during estuary transit,

Activities of the Delta Science Program

Environmental Justice Brown Bag Series: The environmental justice brown bag series, spearheaded by Dr. Jessica Rudnick, will consist of four talks that focus on water justice, indigenous justice, climate justice, and environmental justice work in the Delta.

  • The first talk on September 8 featured State Water Resources Control Board Member Laurel Firestone and UCLA scholar Dr. Kristen Dobbin discussing the human right to water act in California and how it intersects with local, regional, and state water management. Click here to watch the video.
  • The second talk will be held on November 3 at 12pm and will focus on indigenous justice. The speaker will be Dr. Kyle White from the University of Michigan and a member of the White House Environmental Justice Advisory Council.  

Salinity management workshop:  Karen Kayfetz, Environmental Program Manager with the Delta Stewardship Council, is leading an effort to coordinate a salinity management workshop, which will likely take place in the winter of 2021.  Rather than focusing on the current drought or reinventing an ongoing drought synthesis, the workshop is designed to promote forward-thinking, anticipatory, and collaborative planning for distinct future scenarios.  “We hope that it will lay a foundation for establishing a firm scientific basis for evaluating the full suite of impacts that salinity management alternatives, some of which are still highly experimental or have yet to be tested on human communities within and outside the Delta, and ecosystems, as well as the ability of agencies to meet their regulatory constraints,” said Dr. Larsen.

Delta Lead Scientist Ask Me Anything:  The next session is November 1 with California Sea Grant science fellow Denise Colombano, who is starting a postdoc at UC Berkeley.  Join the session on Instagram Live.  All past sessions can be accessed on the Council’s Instagram page.

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