Striped Bass: An Important Indicator Species in the Delta

Peter and an inconvenienced bass

by Peter Moyle

The striped bass is a favorite sport fish in the San Francisco Estuary (SFE), especially the Delta, because of its large size, sporting qualities, and tasty flesh. Historically, it supported major commercial and sport fisheries but the commercial fishery was shut down long ago and the sport fishery is in long-term decline. The decline of adult fish is reflected in decline of juvenile striped bass as well. Juvenile abundance in the major fish surveys of the SFE track the decline of delta and longfin smelt well. These declines are a good indication that major changes have taken place in the pelagic (open water) environment in the upper SFE, creating problems for pelagic fishes in general.  Nevertheless, adult striped bass, which are voracious predators, have been accused of causing the declines through predation, although there is little evidence for this (see and

The factors causing the decline of many fish and fisheries in the upper SFE have made their management controversial, usually because of the correlation of declines with increased water exports from the Delta and upstream of the Delta, as well as with invasions of ‘ecosystem engineers’ such as overbite clams. To address this problem better, the California Fish and Game Commission is developing new policies for managing Delta fish and fisheries, with a special focus on striped bass.  The commission sets policy that guides management actions of the state Department of Fish and Wildlife. The proposed policies essentially require fishing regulations to be based on scientific studies and on ecosystem-based management. They give CDFW considerable flexibility in setting regulations and management actions.

The striped bass angling community is passionate about their fishery and is largely in agreement (as far as I can tell) with the basic policies.  However, they also want special emphasis to be placed on increasing striped bass abundance, so as to restore some of the former glory to the fishery and to be assured that management regulations and actions will not perpetuate the decline.  The anglers are well aware of efforts in the past that were made, unsuccessfully, to make non-native striped bass the ‘scapefish’ for declines of native fishes.

In part because of my past blog posts, striped bass anglers asked me to address the commission on October 9th, 2019, on issues related to the importance of striped bass in the SFE ecosystem.  I had five minutes to talk as one of numerous speakers.  What follows is a slightly modified version of my remarks.

I appreciate the efforts of the commission to develop a holistic fisheries management policy for the Delta and for striped bass in particular.  I encourage continuation of the policy that treats striped bass as an important member of the SFE ecosystem, including the Delta, and to avoid actions designed to further reduce its declining abundance even further. In fact, I encourage that steps be taken to increase striped bass numbers naturally because it would reflect an improvement in conditions in the Delta ecosystem for native fishes as well.

I write this as an academic researcher who has studied fishes of the estuary for nearly 50 years, including establishing a Suisun Marsh monitoring program that has sampled fish monthly since January, 1980. One of the principal fishes captured in our samples over the decades has been striped bass; this has given me an appreciation for their importance to the estuary’s ecosystem.  For example, we have determined that Suisun Marsh is a major nursery area for Delta fishes, especially striped bass, and that the juvenile striped bass decline is not as evident there as it seems to be elsewhere in the estuary. We have also found that in the turbid water of the marsh, adult striped bass feed largely on sticklebacks, gobies, and sculpin: not salmon, not smelt.

In the past, my attitude towards striped bass has been ambiguous because they are a non-native species and much of my research has focused on conserving native species.  However, striped bass are also one of the best studied fish species in the Delta, whose population fluctuations have a mostly downward trend. They are a good indicator of the ‘health’ of the estuary, including its ability to support native fishes.  Long-term fisheries monitoring in the Delta and estuary started when fish sampling programs were established in the 1950s and 1960s to explicitly track impacts of the State Water Project and the Central Valley Project on striped bass fisheries (e.g., Fall Midwater Trawl Survey, Summer Tow Net Survey). These surveys are still ongoing. Importantly, they have also been the principal source of status information on other species such as delta smelt. In fact, until recently the trends in juvenile striped bass numbers closely followed those of endangered Delta smelt and longfin smelt.  This indicates that these species have a similar response to the major changes that have taken place in the Delta in the past couple of decades.

I recognized this in my 2002 book Inland Fishes of California where I concluded the striped bass account with:

“The striped bass is a very resilient species and is now a permanent part of the California fish fauna….The best thing that can be done for striped bass is to restore the estuary to a condition that allows it to support more fish of all kinds, but especially native species (p  362).”

Striped bass were introduced into California in 1879 with explosive success. They have become naturalized, adapting, over 140 years, to an estuary that bears little resemblance to the one into which they were introduced. For example, there are 23 other non-native fish species permanently established in the estuary, as well as 150-200 non-native invertebrate species.  Today’s Delta ecosystem is best termed a novel ecosystem because of the strong presence of the non-native species from all over the world and because of the extensive alteration of its physical structure.  Striped bass remain one of the best species for monitoring this novel system because they use the entire estuary to complete their life cycle with different life stages having different ecological requirements. It is highly likely that adaptations of striped bass to this estuary now have a genetic basis, as has been shown for American shad, introduced at about the same time, with a similar life history.

In 2019, I was part of an Independent Scientific Advisory Panel which wrote a report for the Delta Science Program on Developing Biological Goals for the Bay-Delta Plan (Dahm et al. 2019)In this report, we recommended getting away from using just endangered fishes as the principal species to monitor ecological conditions in the Delta. These species are becoming so rare that they have limited value in determining the quality of habitat for a spectrum of native and other desirable fishes. We recommended instead that new metrics be developed that integrate information from multiple species, native and non-native, including striped bass.  The importance of striped bass stems from our extensive knowledge of its life history and the fact that its population tracks the condition of the pelagic portions of the ecosystem well.

What all this means is that regulations for managing striped bass should not be aimed at reducing its population but rather at increasing, or at least stabilizing, it.  We especially need management actions that reduce removal of large fish from the system.  The largest fish are females that produce the most and highest quality eggs that ultimately become the juvenile fish that are particularly sensitive to annual changes in estuarine conditions. As these juvenile fish progress through their life cycle, their abundance and health at each stage reflect the impacts of multiple factors that stress the ecosystem, from contaminants to altered foodwebs. The striped bass should be treated as a species that not only supports a valuable fishery but as an important indicator of the ability of the San Francisco Estuary, especially the Delta, to support a diverse and vibrant ecosystem.

Thank you for listening. I appreciate your considerable efforts to design management strategies that are flexible and science- based.

Further reading.

Dahm, C., W. Kimmerer, J. Korman, P. B. Moyle, G. T. Ruggerone, and C.A.Simenstad. 2019. Developing Biological Goals for the Bay-Delta Plan: Concepts and Ideas from an Independent Scientific Advisory Panel. A final report to the Delta Science Program. Sacramento: Delta Stewardship Council. Report,\

Hasselman, D.J., P. Bentzen, S.R. Narum, and T. P. Quinn. 2018. Formation of population genetic structure following the introduction and establishment of non-native American shad (Alosa sapidissima) along the Pacific Coast of North America. Biological invasions 20(11): 3123-3143.

Mount, J., B. Gray, K. Bork, J. E. Cloern, F. W. Davis, T. Grantham, L. Grenier, J. Harder, Y. Kuwayama, P. Moyle, M. W. Schwartz, A.Whipple, and S.Yarnell. 2019.  A Path Forward for California’s Freshwater Ecosystems.  San Francisco: Public Policy Institute of California. 32 pp.

Moyle, P. B.  2002.  Inland Fishes of California. Revised and Expanded. Berkeley: University of California Press. 502 pp.

Moyle, P.B.  and W.A. Bennett, 2011. Striped bass control: cure worse than disease? California WaterBlog, January 31.

Moyle, P.  A. Sih, A. Steel, C. Jeffres, and W. Bennett. 2016.  Understanding predation impacts on Delta native fishes. UC Davis Center for Watershed Sciences California WaterBlog. May 22.

Stompe, D. and P. Moyle, 2018. Striped bass in the San Francisco Estuary: insight into a forgotten past.  California WaterBlog, November 18,

O’Rear, T. and P. Moyle 2019. Remarkable Suisun Marsh: a bright spot for fish in the San Francisco Estuary.

Peter Moyle is a Distinguished Professor, Emeritus, with the Department of Wildlife, Fish, and Conservation Biology and an Associate Director with the Center for Watershed Sciences at the University of California – Davis.

About jaylund

Professor of Civil and Environmental Engineering Director, Center for Watershed Sciences University of California - Davis
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3 Responses to Striped Bass: An Important Indicator Species in the Delta

  1. tryg sletteland says:

    Professor Moyle makes compelling arguments for taking action to restore striped bass to the Sacramento-San Joaquin Delta and S.F. Bay. That would include the State and Federal water projects providing as much water for the estuary as used to be received back in the 1980s.

  2. George wight says:

    Very interesting article. As I have been fishing for stripers going on 50 years,I have seen the there numbers decline along with all species in the delta. These fish coexisted fine for more than 100 yrs. We all know what the problem is. Water diversions to the the greedy mid state corporate farms.

  3. Thomas Correa says:

    Great article. Will fellow up by reading some of the books you listed.

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