Science Stories: Adventures in Bay-Delta Data

By Rosemary Hartman

Pop quiz – what are the three longest-running monitoring programs in Sacramento Delta? The Summer Townet Survey started in 1959 to monitor young-of-year striped bass, the Fall Midwater Trawl survey started in 1967 to monitor juvenile striped bass after the Summer Townet finished for the year, and the Adult Striped Bass Survey started in 1969 to monitor adult striped bass (males reach sexual maturity at 2 to 4 years old, when they are about 11 inches long, and females at 4 to 8 years old, when they are 21-25 inches long). Data from Summer Townet and Fall Midwater Trawl have been used for tons of other projects, and are now used to monitor many species of native and non-native pelagic fish (Kimmerer 2002, Mac Nally et al. 2010, Sommer et al. 2011, Bever et al. 2016, Mahardja et al. 2021, Smith et al. 2021, Tempel et al. 2021), but not many people know about the Adult Striped Bass Dataset – and it’s just recently been published online (Stompe and Hobbs 2024)!

Why has so much of the monitoring in the Delta started for striped bass? Well, the striped bass – Morone saxatilis – was introduced into the Sacramento River in 1879 (Stevens et al. 1987) and is a popular sport fish. It originally came from the East Coast of North America, where it was a food source for the Indigenous people of the region and has been a favorite of fishermen after colonization as well. Overfishing has caused a decline in abundance on the East Coast (though the fishing rate has been reduced to better manage the stock)(Richards and Rago 1999, Fabrizio et al. 2017), but how are they doing in California?

The young-of-the-year fish picked up in the Summer Townet and Fall Midwater Trawl have declined a lot since the 1980s, with a particularly sharp downturn in the early 2000s (Sommer et al. 2007), though they have not declined as sharply as native pelagic fishes (Mac Nally et al. 2010, Nobriga and Smith 2020).

Figure 1. Population index for young-of-year striped bass in the Fall Midwater Trawl (FMWT) and Summer Townet (STN) from 1959-2023. Black circles and solid lines are FMWT index, dashed blue lines and triangles are STN index.

The adult striped bass data are a little harder to work with. There have been a lot of changes over the course of the survey, so comparing the data from 1970 to that of 2020 isn’t an apples-to-apples comparison. Analyses of the adult striped bass survey data from 1985 showed that the adult population had declined by 75% from 1970 to 1982, with droughts, overfishing, lower food supplies, contaminants, and water diversions implicated in the decline (Stevens et al. 1985) . Extending this analysis through 1995 revealed the decline continued, with food limitation partially to blame (Kimmerer et al. 2000, Lindley and Mohr 2003), but the population recovered somewhat in the late 1990s (Loboschefsky et al. 2012), probably because of the string of wet years, and numbers of age-3 fish in the early 2000s were higher than would be predicted from the age-0 trawl surveys in recent years (Nobriga and Smith 2020).

Figure 2. A) Adult striped bass population estimates from 1969-2004. Reproduced from Loboschefsky et al. 2012, with permission. B) ratio of age 3 adult striped bass to the fall midwater trawl index from three years prior. Reproduced from Nobriga and Smith 2020, with permission.

Looking at more recent data, we can’t calculate abundance in the same way. The older datasets used to use the Bay-Delta Creel Survey to recapture tagged fish, and that program stopped in the early 2000s. However, we can still pick up a few basic trends. First of all, the average length of the fish has declined over time (Figure 3). This is common in populations where the larger fish are removed from the population by fishing (Law 2000). There is often selective pressure to mature at a smaller size to make sure they reproduce before being caught by a fisherman. This hasn’t been studied specifically in California striped bass, but it might be part of the reason behind the change in size. Another factor contributing to the decreased size of fish is the decreasing proportion of female striped bass in the catch. There are always more male fish than female fish, but the percentage of female fish has been declining over time (Figure 4). Why? We don’t really know, but capture of larger fish might be part of the story.

Figure 3. Average fork length of all striped bass caught by the Adult Striped Bass Survey from 1969 to 2022. Green area represents the standard deviation in length, and black line shows the trend.

Figure 4. Percent of annual striped bass catch that are male or female over time from 1969-2022.

We do know that female fish are always bigger than male fish – even at the same age. You can see with the trend line in Figure 5 that when they are young – two or three years old – they are about the same size. However, by age four females are a bit larger, and by age 6 females are consistently 8-12 cm (2-4 inches) larger than males, on average. This is pretty common in fish, since females need more resources to produce eggs (Parker 1992). It's possible that the larger females are being taken by fishermen at a higher rate, which may cause the change in sex ratios.

Figure 5. Fork length of all fish caught by the adult striped bass survey versus age (as determined from scales) for female fish (pink circles and solid pink line) and males (blue triangles and dashed blue line).

While we can’t calculate abundance indices like the ones used in the first part of the project’s history, we can determine the number of fish caught per hour in our fish traps. Since we began tracking how much effort is being spent on fishing, we’ve seen a slight increase in number of adult striped bass (Figure 6), but it’s highly variable, and may be due to changes in sampling methods and locations. However, the survey tagged less fish in later years, and we have seen a decline in the number of tagged fish that we’ve recaptured (Figure 7). We use the number of fish we've tagged and the number we've recaptured to estimate population size, but due to a reduction in funding and changes to management, we no longer have enough recaptures for accurate population estimates. Despite the changes, the sport fish monitoring programs have provided valuable insights into Delta’s ecology and the role of striped bass within it.

Figure 6. Catch per unit effort of striped bass in the fyke trap on the Sacramento River from 1994-2022, within linear fit line shown in blue.

Figure 7. Percentage of tagged fish recaptured per year from 1969 to 2022 with trend line shown in blue.

References and further reading

• NOAA Fisheries Species Profile on striped bass.
• CDFW Species Profile on striped bass
• Bever, A. J., M. L. MacWilliams, B. Herbold, L. R. Brown, and F. V. Feyrer. 2016. Linking hydrodynamic complexity to Delta Smelt (Hypomesus transpacificus) distribution in the San Francisco Estuary, USA. San Francisco Estuary and Watershed Science. 14 (1).
• Fabrizio, M. C., T. D. Tuckey, and S. Musick. 2017. A Brief Guide to Striped Bass Ecology & Management in Chesapeake Bay.
• Hartman, R., E. Stumpner, C. Burdi, D. Bosworth, A. Maguire, and IEP Drought Synthesis Team. 2024. Dry me a river: Ecological effects of drought in the upper San Francisco Estuary. San Francisco Estuary and Watershed Science. 22 (1).
• Kimmerer, W. 2002. Physical, biological, and management responses to variable freshwater flow into the San Francisco Estuary. Estuaries. 25(6B):1275-1290.
• Kimmerer, W. J., J. H. Cowan, Jr., L. W. Miller, and K. A. Rose. 2000. Analysis of an estuarine striped bass (Morone saxatalis) population: influence of density dependent mortality between metamorphosis and recruitment. Canadian Journal of Fisheries and Aquatic Sciences. 57 (2):478-486.
• Law, R. 2000. Fishing, selection, and phenotypic evolution. ICES Journal of Marine Science. 57 (3):659-668.
• Lindley, S. T., and M. S. Mohr. 2003. Modeling the effect of striped bass (Morone saxatilis) on the population viability of Sacramento River winter-run chinook salmon (Onchorhynchus tshawytscha). Fishery Bulletin. 101 (2):321-331.
• Loboschefsky, E., G. Benigno, T. Sommer, K. Rose, T. Ginn, A. Massoudieh, and F. Loge. 2012. Individual-level and population-level historical prey demand of San Francisco Estuary Striped Bass using a bioenergetics model. San Francisco Estuary and Watershed Science. 10 (1):25.
• Mac Nally, R., J. R. Thomson, W. J. Kimmerer, F. Feyrer, K. B. Newman, A. Sih, W. A. Bennett, L. Brown, E. Fleishman, S. D. Culberson, and G. Castillo. 2010. Analysis of pelagic species decline in the upper San Francisco Estuary using multivariate autoregressive modeling (MAR). Ecological Applications. 20 (5):1417-1430.
• 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, 02216 p.
• Nobriga, M. L., and W. E. Smith. 2020. Did a Shifting Ecological Baseline Mask the Predatory Effect of Striped Bass on Delta Smelt? San Francisco Estuary and Watershed Science. [accessed 2023 Feb 13]. 18 (1).
• Parker, G. A. 1992. The evolution of sexual size dimorphism in fish. Journal of Fish Biology. 41 (sB):1-20.
• Richards, R. A., and P. J. Rago. 1999. A Case History of Effective Fishery Management: Chesapeake Bay Striped Bass. North American Journal of Fisheries Management. 19 (2):356-375.
• Smith, W. E., L. Polansky, and M. L. Nobriga. 2021. Disentangling risks to an endangered fish: using a state-space life cycle model to separate natural mortality from anthropogenic losses. Canadian Journal of Fisheries and Aquatic Sciences. 78 (8):1008-1029.
• Sommer, T., C. Armor, R. Baxter, R. Breuer, L. Brown, M. Chotkowski, S. Culberson, F. Feyrer, M. Gingras, B. Herbold, W. Kimmerer, A. Mueller-Solger, M. Nobriga, and K. Souza. 2007. The collapse of pelagic fishes in the upper San Francisco Estuary. Fisheries. 32 (6):270-277.
• Sommer, T., F. H. Mejia, M. L. Nobriga, F. Feyrer, and L. Grimaldo. 2011. The spawning migration of delta smelt in the upper San Francisco Estuary. San Francisco Estuary and Watershed Science. 9 (2):16 pages.
• Stevens, D., H. Chadwick, and R. Painter. 1987. American shad and striped bass in California's Sacramento-San Joaquin River system. American Fisheries Society Symposium. 1:66-78.
• Stompe, D. K., and J. A. Hobbs. 2024. California Department of Fish and Wildlife Adult Striped Bass Study, Sacramento-San Joaquin Watershed, California, 1969-2022 Environmental Data Initiative. ver. 1.
• Tempel, T. L., T. D. Malinich, J. Burns, A. Barros, C. E. Burdi, and J. A. Hobbs. 2021. The value of long-term monitoring of the San Francisco Estuary for Delta Smelt and Longfin Smelt. California Fish and Wildlife. Special CESA Issue:148-171.

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.