Science Stories: Adventures in Bay-Delta Data

By Rosemary Hartman, Brian Mahardja, and Vanessa Tobias

We are now half-way through Water Year 2021 (which started in October of 2020) and it seems likely that California will experience another dry year. The frequency of prolonged drought events is projected to increase due to climate change and the San Francisco Estuary system is expected to have more wild swings between floods and droughts. This was in the mind of a handful of IEP scientists back during the last California drought of 2012-2015. They were wondering, which fish species are negatively impacted by drought, and would they rebound in numbers after the drought is over? Brian Mahardja, Vanessa Tobias, Shruti Khanna, Lara Mitchell, Peggy Lehman, Ted Sommer, Larry Brown, Steven Culberson, and J. Louise Conrad published the results of their study in a recent journal article.

The Delta’s fish community is unique, with a melting pot of native fish and introduced fish. The native fish evolved with California’s regular cycles of droughts and floods. Some of the non-native fish come from ecosystems that are usually less volatile, so may have a harder time resisting the negative effects of drought and bouncing back. However, other introduced fish are considered “weedy” species that take advantage of disturbance. They might be particularly good at bouncing back after a drought.

The IEP researchers used data collected covering previous droughts in California to test these hypotheses. They compared fish abundance as measured by the CDFW Fall Midwater Trawl and the USFWS Delta Juvenile Fish Monitoring Program Beach Seines before, during, and after the droughts of 1976-1977, 1987-1994, 2001-2002, 2007-2010 and 2012-2016 (Figure 1).

Figure 1. Periods of drought in the Sacramento San-Joaquin Delta over the past 50 years. Figure is copied from Mahardja et al. 2021.

Though it sounded simple, the process ended up not being very straightforward. Even the definition of what a “drought” was turned out to be complicated. If you’re from the East Coast of the US, five months without rain is a “drought”. But in California, we would just call that “summer”. For Californians, we need lower-than-normal precipitation for multiple years in a row before we start worrying. Drought is often defined by its impact on people – dry soil conditions, low water storage, and water supply problems – but what does drought mean if you are a fish? The team had to make decisions. They also had to decide which components of the ecosystem to look at. At first they wanted to look at everything IEP monitors – water quality, phytoplankton, zooplankton, and fish, but decided that just doing fish was hard enough!

It was a big team working on the project, and they each brought skills to the analysis. Brian, Lara, Vanessa, and Shruti had a lot of experience with complex statistics and data analysis. Lara was “an R Wizard” who streamlined the data processing. Ted and Larry brought their fish expertise and deep knowledge of the system. Steve and Peggy provided hydrologic and water quality knowledge. Louise was the lead of the IEP synthesis program and organized the overall drought synthesis effort. Even though several authors changed jobs midway through the project, the great thing about working in the IEP community is that the new employers were supportive of seeing the project through, since they knew it would help the IEP community as a whole.

The team found that many fish species declined in abundance during the drought (they had “low resistance to disturbance”), particularly pelagic species such as Delta Smelt, American Shad, and young Striped Bass (Figure 2). Many of these species had the ability to “bounce back” after the drought (they had “high resilience”), however some species never returned to their former abundance (such as the Delta Smelt). Other fish species didn’t decline during the droughts (they had “higher resistance”). These included many of the fish of the littoral habitat, including Largemouth Bass, Redear Sunfish, and Chinook Salmon. A few of the non-native littoral species even increased in abundance during some of the droughts, particularly Mississippi Silversides and Bluegill.

Climate change is likely to increase the frequency and severity of droughts. If these results hold true, we could see a shift in the fish community away from native pelagic fish and towards more non-native littoral fish. Because freshwater flow is a tightly managed commodity, particularly during drought conditions, finding a balance of water use for people and water use for native species will become increasingly difficult.

Figure 2. Fish populations have different responses when it comes to drought. Many of the pelagic species decline, but bounce back. Some don't bounce back quite as well. Many littoral fish don't decline, or even increase in population during droughts. Fish photos from US Bureau of Reclamation.

Learn More

• Durand, J. R., F. Bombardelli, W. E. Fleenor, Y. Henneberry, J. Herman, C. Jeffres, M. Leinfelder–Miles, J. R. Lund, R. Lusardi, A. D. Manfree, J. Medellín-Azuara, B. Milligan, and P. B. Moyle. 2020. Drought and the Sacramento-San Joaquin Delta, 2012–2016: Environmental Review and Lessons. San Francisco Estuary and Watershed Science 18(2). DOI: 10.15447/sfews.2020v18iss2art2
• Department of Water Resources Drought Activities
• Kimmerer, W., F. Wilkerson, B. Downing, R. Dugdale, E. S. Gross, K. Kayfetz, S. Khanna, A. E. Parker, and J. K. Thompson. 2019. Effects of Drought and the Emergency Drought Barrier on the Ecosystem of the California Delta. San Francisco Estuary and Watershed Science 17(3). DOI: 10.15447/sfews.2019v17iss3art2
• National Integrated Drought Information System.
• Mahardja, B., V. Tobias, S. Khanna, L. Mitchell, P. Lehman, T. Sommer, L. Brown, S. Culberson, and J. L. Conrad. 2021. Resistance and resilience of pelagic and littoral fishes to drought in the San Francisco Estuary. Ecological Applications, 31(2): e02243. DOI: 10.1002/eap.2243
• Singer, G. P., E. D. Chapman, A. J. Ammann, A. P. Klimley, A. L. Rypel, and N. A. Fangue. 2020. Historic drought influences outmigration dynamics of juvenile fall and spring-run Chinook Salmon. Environmental Biology of Fishes 103(5):543-559. DOI: 10.1007/s10641-020-00975-8

By: Rosemary Hartman (CA Department of Water Resources), Pascale Goertler (Delta Science Program), Brian Mahardja (US Bureau of Reclamation), and Ted Sommer (CA Department of Water Resources).

A new synthesis study indicates Striped Bass could start swimming up the Sacramento River earlier in the year as climate change progresses. You probably know “stripers” as popular game fish, and die-hard fishermen will tell you all about the best times and places to try and catch a big one. However, Striped Bass do not stay put. Stripers are “anadromous”, meaning that just like salmon, they begin their life in fresh water, migrate to the estuary and ocean, and eventually return to freshwater as adults to spawn. This study, by IEP Scientists Pascale Goertler, Brian Mahardja, and Ted Sommer, looked at one of our oldest data sets to examine fish migration patterns. Scientists in other systems have found a number of changes to migration timing of many species linked to climate change, and Pascale, Brian, and Ted were curious whether there had been any changes in our system. They found that adult Striped Bass migrated into the Central Valley later in the year when there was higher Delta outflow and cooler sea surface temperatures (Figure 1).

Figure 1. Diagram of how environmental conditions influence Striped Bass Migration. In the top panel, warm ocean temperatures and low freshwater flow correlate with earlier upstream migration. In the bottom panel, lower ocean temperature and higher flow mean Striped Bass hang around in the ocean or estuary for longer before migrating.

In order to figure this out, the team started with data from the CDFW Adult Striped Bass Survey. This survey has captured and tagged Stripers using gill nets and fyke traps throughout the Delta and the Sacramento River since 1969 (Figure 2). Having a really long-term data set let them correlate migration to climate factors, which has not been studied very often in migratory fishes, but it also caused problems. This is a “presence-only” data set, which means the researchers didn’t know whether there was no record for a given date because the monitoring crews didn’t sample, or because they sampled, but failed to catch any fish. The monitoring crews also didn’t record how long they were out in a consistent way. Did they only catch one fish because there weren’t many fish? Or because they only put the net in the water for five minutes instead of an hour? They didn’t have the crews from the 1970s to ask.

Figure 2. CDFW scientists have been monitoring Striped Bass populations using fyke traps such as this one since the 1960s. CDFW photo.

While the patterns Pascale and her colleagues found have held true over the past forty years of the data set they analyzed, migration timing could change in the future. Based on trajectory of climate change, we expect warmer sea surface temperature and lower outflow during late spring and early summer (as snowpack level decreases), which could mean earlier bass migration. Striped Bass are a voracious predator, so adult fish entering the estuary earlier in the year could mean a head-on collision with juvenile salmon moving out of the estuary. While Striped Bass have coexisted with salmon for the past 150 years, they are not native to California, and changes in predation patterns could affect already stressed salmon populations.

Learn More:

• Goertler, P., B. Mahardja, and T. Sommer. 2021. Striped bass (Morone saxatilis) migration timing driven by estuary outflow and sea surface temperature in the San Francisco Bay-Delta, California. Scientific Reports 11(1):1510.
• Morris Jr., J. A., R. A. Rulifson, and L. H. Toburen. 2003. Life history strategies of striped bass, Morone saxatilis, populations inferred from otolith microchemistry. Fisheries Research 62:53-63.
• Nobriga, M., and F. Feyrer. 2008. Diet composition in San Francisco Estuary striped bass: does trophic adaptability have its limits? Environmental Biology of Fishes 83(4):495-503 (PDF).
• 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(2):309-320.
• Sommer, T., F. Mejia, K. Hieb, R. Baxter, E. Loboschefsky, and F. Loge. 2011. Long-term shifts in the lateral distribution of age-0 striped bass in the San Francisco Estuary. Transactions American Fisheries Society 140:1451-1459.
• Stompe, D. and P. Moyle. Striped Bass in the San Francisco Estuary: Insight Into a Forgotten Past. California WaterBlog. UC Davis Center for Watershed Sciences. 11/18/2018.

IEP is best known for long-term monitoring surveys. You can find data, maps, and descriptions of monitoring elements dating back since the 1950s! But IEP is much more than just counting fish and recording salinity. We often use targeted research to answer specific questions, such as:

• Can we use genetics to tell the difference between runs of salmon?
• Does electrofishing work better than beach seines for looking at fish communities?
• Can we use cameras instead of nets to monitor fish?
• Do baby salmon get eaten by bass in some areas more than others?

We also periodically look back on the data we have collected by long-term monitoring and special studies and “synthesize” the results to answer questions that are too big to answer with one data set by itself. Recent synthesis projects have included:

• How does Delta outflow impact Delta Smelt habitat?
• How quickly does the system recover after a major drought?
• What are the consequences of climate change for the ecosystem?

In this space, we will be highlighting the work being done by these special studies and synthesis projects at IEP. Stay tuned!