Understanding predation impacts on Delta native fishes


Native threespine sticklebacks pumped from the stomach of a single 22 inch striped bass. The bass was feeding in water being drained from a duck club in Suisun Marsh, The sticklebacks were abundant, concentrated, and confused so were easy prey for the bass. Sticklebacks continue to be one of the most abundant fish in Suisun Marsh. Photos by Teejay O’Rear April 2010.

By Peter Moyle, Andrew Sih, Anna Steel, Carson Jeffres, William Bennett of University of California, Davis.

Will endangered fishes, such as Chinook salmon, delta smelt, and longfin smelt, benefit from control of predators, especially of striped bass? This question is of interest because if the answer is ‘yes’, then predator control might increase the benefits of other actions, such as provision of environmental water for native species. In this blog we express our skepticism of large-scale predator control as a conservation tool, based on eight principles.

  1. Predation ‘problems’ do not have simple solutions.
    Predation is one of many stressors affecting declining species. In ecosystems such as the Delta, predator-prey relationships are complex. Many predators forage opportunistically on whatever prey species are most abundant and accessible at any time and place. As a result, predator control can have unintended consequences.  For example, reducing striped bass populations might cause an increase in important prey species, such as Mississippi silverside, that prey on delta smelt eggs and larvae. In other words, controlling striped bass may backfire and increase predation on delta smelt.  Grossman et al. (2013) have written a good overview of predator-prey dynamics in the Sacramento River. This review provides a basis for the above statements and the conclusion that predator control in the Delta will likely create more problems than it solves. This conclusion can be applied broadly to predator control programs, such as those for invasive mammals.  However, more research could provide a better understanding of predation as a stressor of fish populations, provided that such studies are linked with modeling, focusing on predator-prey interactions in the Delta (similar to work done for the lower Columbia River).
  1. The best long-term strategy for increasing populations of small fish (prey) is to improve the ability of the ecosystem to support them.
    In a healthy ecosystem, multiple predators and multiple prey typically co-exist in dynamic fashion. Prey species such as delta smelt have highly effective predation defense mechanisms that operate best in an environment similar to the one in which they evolved. For the Delta, we suggest that ecosystem recovery efforts should focus on the arc of habitat that includes the Yolo Bypass, the Lindsey-Cache slough region, the Cosumnes-Mokelumne river region, the Sacramento River, Sherman Island and Suisun Marsh (similar to the String of Pearls concept for Chesapeake Bay).  This region is tied together (the string) by the interaction of Sacramento River flows with tidal flows and contains the highest concentrations of native fishes in the upper estuary.
  1. Bypassing problem areas can reduce predation impacts.
    Increasing flows from the Sacramento River down the Yolo Bypass in winter could carry large number of juvenile salmon from upstream areas to productive habitat in the Yolo Bypass. Such flows can also attract fishes such as splittail, and perhaps smelt from the Delta into the Bypass. Fish using the Bypass avoid the rip-rapped channels and likely high predation areas in the western Delta and lower Sacramento River. A similar strategy might work for the San Joaquin River and southern Delta if fish (except smelt) were directed towards the pumping plants and then trucked past predation hot spots in the Delta.  This strategy will only work if predation on trucked fish is reduced by modifying the pumping facilities and adopting different release strategies (#4, #5).
  1. Changing release strategies of captive fish can reduce predation mortality.
    Salmon and other fishes are most vulnerable to predation when they are transported to a release site, usually by truck, and then dumped into the water in large numbers in one place. This release strategy, used by the pumping plants in the South Delta and by many hatcheries, caters to predator behavior, because predators are attracted to concentrations of prey, especially prey that are confused following release. Release strategies need to be developed and carefully monitored, such as slow releases from barges towed at random times of day and night, which do not habituate predators to concentrations of prey. Similar release strategies are needed for hatchery salmon releases as well (#8).
  1. The solution to reducing effects of predation ‘hot spots’ is to move prey around them (see #3) or to reduce their attractiveness to predators.
    Predatory fishes such as striped bass move around a lot.  Therefore, predator control on a hot spot has to be continuous and intensive, because as predators are removed new ones are likely to move in.  However, each hotspot has its own problems that have to be dealt with individually.  For example, Sabal et al. (2016) found striped bass consumed 8-29% of juvenile salmon passing through Woodbridge Irrigation District Dam on the Mokelumne River and reducing the numbers of adult striped bass could temporarily reduce predation rates. It helped that the ‘hot spot’ was some distance upstream from the Delta, where most bass reside. Their conclusion was not that universal striped bass control was needed but that “ …it is important to consider habitat alterations and interactive effects when estimating large-scale predation impacts and when planning local management strategies (p 318).”This conclusion applies to Clifton Court Forebay, which is well-documented as one of the hottest of the predation hotspots.  Striped bass and other predators concentrate there to feed on small fish drawn towards the giant pumps at the state pumping plant. Modifying its structure or operation should be the best way to reduce predation impacts in the forebay. In this light, the National Marine Fisheries Service is currently requiring that both long-term and interim measures to reduce predation on endangered fishes be implemented (letter from Maria Rea to Carl Torgersen, January 22, 2016).  Essentially, NMFS is saying that just studying the problem is no longer a sufficient response to the documented high predation rates at this facility.
  1. Striped bass are not the problem.
    Striped bass get blamed for declines of native fishes because they are an abundant, voracious, non-native predator. Yet striped bass have been part of the Delta ecosystem for nearly 150 years, plenty of time for co-adaptation of predator and prey.  In periods when delta smelt, longfin smelt, and salmon were abundant in the past, striped bass were much more abundant than they are today, suggesting that the same factors that drive native fish declines are also driving striped bass populations.  As generalist, wide-roaming predators, they feed on the most abundant prey available, which is often the result of ‘ringing the dinner bell’ release strategies of captive fish (see #4, above). If striped bass regulate populations of any other fishes, their effects will be mostly on small, consistently abundant prey fishes such as Mississippi silverside and threadfin shad that may compete with or prey on smelt and juvenile salmon.  By reducing competition or predation by silversides or shad on smelt, striped bass might actually have a net positive effect on smelt.  Indeed, other managers have found, to their distress, that reducing top predators has backfired because of this ‘enemy of my enemy is my friend’ effect.  Repeating this error in our system would be unfortunate. All this indicates that programs aimed at direct striped bass control are as likely to have no or negative effects, as to have positive effects, on populations of desirable fishes.
  1. Having a prey species in a predator’s diet does not mean the predator controls the prey’s populations.
    Dietary studies of predators in the Delta have often concentrated in areas where predation is perceived to be a problem, such as predation by striped bass near water diversion structures on salmon in the Delta and Sacramento River or below hatchery release sites. It is not surprising that prey are seen in predator stomachs in those situations. Prey fish have evolved strategies to minimize the effect of predators. For example, a natural predation-reduction strategy of juvenile salmon is to migrate to the ocean in pulses, usually when river flows are high and muddy from run-off. Striped bass and other predators might have stomachs full of juvenile salmon at this time but the percentage of total population is likely to be low. Granted, such strategies may no longer be fully effective under conditions of drought, warm winters and reduced population sizes; however, reduction of overall striped bass predation will likely increase predation by other organisms, taking advantage of whatever increase in prey the absence of striped bass might cause.. In short, a predator control program based mainly on dietary studies is too simplistic to serve as a basis for management to increase prey populations.
  1. Hatchery-reared salmon are exceptionally vulnerable to predation.
    Hatchery salmon start life packed together in cement troughs, with food pellets raining down from above.  This does not give the fish much chance to learn how to avoid predators. They are then either released directly into a river or trucked to a release point in the estuary.  It is scarcely surprising that predators take advantage of these naïve and fat-laden prey, gorging themselves.  Many of these salmon die of stress and other causes. They are then scavenged by unlikely predators such as white catfish.  Studies on the Yolo Bypass indicate that about 30% of hatchery salmon die within a day or two after release into food rich, nearly predator-free environments, in which most wild salmon thrive (Jacob Katz, unpublished data).   Release of hatchery fish into rivers in large numbers mimics, to a certain extent, the predator-swamping strategy used by wild fish.  But the rivers are rarely high and muddy during the release and the fish lack the behavior to avoid predation in clearer water, so predation rates are high.  In short, heavy predation on juvenile hatchery salmon is more a reflection of hatchery practices than of un-natural rates of predation by striped bass and other predators.

Conclusion.  It seems unlikely that a large-scale predator removal program focused on striped bass would have a sustainable, measurable effect on populations of its prey species, specifically protected smelts and salmon. However, if managers deem enough uncertainty exists about the importance of predation as a source of mortality relative to other factors, then an integrated program of empirical studies and modeling should be instituted.   If a control program moves forward despite scientific uncertainty, it should be implemented as an experiment, focusing on data collection and modeling to determine if the program achieves carefully specified objectives.

Further reading

Cannon, T. 2016. Hatcheries Release Salmon Smolts into Low Flows and Warm Water – April and early May, 2016. California Fisheries Blog. May 5, 2016.

Doherty, T.S. and E. G .Richie. 2016.  Stop jumping the gun: a call for evidence-based invasive predator management, Conservation Letters. doi: 10.1111/conl.12251.

Grossman, G., Essington, T., Johnson, B., Miller, J., Monsen, N. and T. Pearsons. 2013. Effects of fish predation on salmonids in the Sacramento River – San Joaquin Delta and associated ecosystems. Report for Cal. Fish Wildlife/Delta Stewardship Council/NMFS. 71pp.

Moyle, P B.   2011. Striped bass control: cure worse than disease?

Sabal, M., S. Hayes, J. Merz, and J. Setka. 2016  Habitat alterations and a nonnative predator, the striped bass, increase native Chinook salmon mortality in the Central Valley, California. North American Journal of Fisheries Management 36:309–320. DOI: 10.1080/02755947.2015.1121938

Sabalow,R. 2016. Should California’s striped bass be vilified as native-fish killers? Sacramento Bee May 6, 2016.

Wunderlich, V. 2015.  Clifton Court Forebay Predation Study.  Bay-Delta Office, California Department of Water Resources.

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6 Responses to Understanding predation impacts on Delta native fishes

  1. Pingback: MAVEN'S NOTEBOOK - Water news

  2. Jiggery Pokery says:

    So, we need some genetically modified striped bass, engineered to have no teeth, to release into the salmon breeding troughs so the little guys can learn to dodge.

    You’re welcome!

  3. Pingback: Time for a Common-Sense Approach to Salmon Recovery - FISHBIO Fisheries Research, Monitoring, and Conservation

  4. Pingback: Scientists: Culling Predator Fish Won’t Save Salmon | Michael Levitin

  5. Pingback: Guest Species – What about the nonnative species we like? | California WaterBlog

  6. Pingback: Killing Native Fishes for Fun and Predator Control | California WaterBlog

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