by Dylan K. Stompe

Striped bass are an iconic and recreationally important fish species throughout the United States, including within their native range on the Atlantic Coast. Based on their value as a sport fish and as table fare, striped bass were one of the early introductions to the San Francisco Estuary (SFE). Their life-history and abundance within the SFE has been studied as much or more than any other fish present in the system, with only Chinook salmon, and more recently Delta smelt, approaching the same level of interest. Given the historical resources dedicated to monitoring and studying striped bass in the SFE, the question must be asked; why don’t we know more about what they’re doing in the Pacific Ocean?

The magnitude of striped bass migrations along the North American Atlantic Coast is well-known. Believed to be cued by increased ocean temperatures during summer months, a large proportion of Atlantic Coast striped bass migrate north every summer in search of productive feeding grounds. This behavior not only provides valuable food resources to migrants, it also exposes individuals to mortality risk from recreational and commercial fisheries. In California, striped bass’ introduction to the SFE occurred in 1879, and the first recorded catch was just one year later in Monterey Bay (Smith 1895). In relatively rapid order, the species became fully established and supported a large commercial fishery (Scofield 1931). Likewise, multiple permanent and transient populations have established via coastal migrations in estuaries up and down the Pacific Coast. One such population, in Coos Bay, Oregon, became large enough to support a commercial fishery for several decades before it collapsed, apparently due to inbreeding (Waldman et al. 1998).

Limited research has gone into understanding what conditions are responsible for striped bass venturing into the Pacific Ocean. Past studies have relied mainly on catch data from commercial party fishing boats that fish within the SFE and just outside of the Golden Gate. These data show that catch of striped bass in the Pacific Ocean increases during periods of unusually warm sea surface temperatures (Radovich 1963, Bennett and Howard 1997). Unusually warm, however, has shifted in meaning as climate change progresses. For example, extreme El Niño events that bring warm water to the Pacific Coast are forecasted to increase in frequency as the global climate warms (Wang et al. 2017).

Increased catch of striped bass in the Pacific Ocean during El Niño events, the establishment of Oregon populations, and the annual Atlantic Coast migrations indicate that Pacific Coast migrations have and will continue to occur. However, the specific dynamics and repercussions of this behavior for the SFE population are largely unknown. Along the Atlantic Coast, it is well-established that large female striped bass migrate. These same individuals also produce the highest number and quality of eggs. Thus, if female mortality is high in the Pacific Ocean, or if individuals permanently emigrate to other estuaries, a loss of juvenile production may occur. Bennett and Howard (1997) speculated that these dynamics may at least partially explain the long-term decline of striped bass populations within the SFE.

While the results from party boat data are compelling, these data are hard to interpret given changing effort and fishing gear over time. In addition, these results do not provide much information outside of the heavily fished areas near the Golden Gate and only a single data point for any given individual. My colleagues and I started the UC Davis Ocean Striped Bass Project with the goal of better understanding what large female striped bass are doing within the SFE and along the Pacific Coast. Part of this project involves tracking movements of individuals using a combination of acoustic telemetry and otolith microchemistry.

Acoustic telemetry is a technical term for what is essentially tracking animals using tags that emit coded high-pitched sounds. The tags used in this study emit a signal at a frequency far higher than what a human can hear, and which can effectively transmit a unique identification code through both freshwater and saltwater. When a tagged fish swims by an acoustic receiver (hydrophone), of which there are many deployed throughout the SFE and the rivers that flow into it, its tag code is recorded along with the time and date. Using this technology, we can track tagged fish for up to seven years throughout the SFE and upstream rivers.

But what about coastal migrations? As a part of the Ocean Striped Bass Project, a total of 22 acoustic receivers have been deployed from zero to three nautical miles offshore at Point Reyes and Point San Pedro, just north and south of the Golden Gate, respectively (Fig. 3). These receivers act as “gates” to migratory individuals, listening for tagged fish as they pass on their way north or south. If an individual is recorded only once at either of these gates, we know it either died or emigrated to another estuary and can be counted as “lost” to the SFE population.

The other component of the study, otolith (‘ear stone’) microchemistry, relies on the unique chemical signals incorporated into otoliths as a fish grows. Otoliths are small calcified structures located in the inner ear of fishes. They serve as an excellent structure for scientific analysis because they continually grow and are not resorbed during periods of stress. By comparing the chemical signatures trapped within different layers of the otolith to the unique chemical signatures of different water bodies, we can effectively retrace the migratory history or even source of an individual. Using these techniques, we should be capable of estimating origin of birth for individual striped bass captured outside of the SFE.

Why do we care what happens to striped bass in the Pacific Ocean? Striped bass are an extremely important recreational species within the SFE and upstream rivers; they are a cultural, consumptive, and economic resource for thousands of people. They are also an important indicator species because they require many of the same environmental conditions as vulnerable native species, such as longfin smelt and delta smelt, that now exist at population levels too low to effectively track. By not knowing the coastal migration dynamics of large female striped bass, we limit our ability to identify drivers of population declines more broadly. Finally, while the effects of predation by striped bass in the SFE and upstream rivers is hotly debated, they may be a more tangible threat to native species in the small estuaries up the California and Oregon coasts. Striped bass are truly novel to these estuaries, and if climate change increases their colonization potential, native species declines may occur.

If you would like to know more about the Ocean Striped Bass Project please contact me, Dylan Stompe, at dkstompe@ucdavis.edu. This project is funded by the California Department of Water Resources.

Dylan Stompe is a Ph.D. student based at the Center for Watershed Sciences.

Further Reading

Bennett, B., and L. Howard. 1997. El Niños and The Decline of Striped Bass. IEP Newsletter 10(4):17–21.

Radovich. 1963. Effect of ocean temperature on the seaward movements of striped bass, Roccus saxatilis, on the Pacific coast. California Fish and Game 49(3).

Scofield, E. C. 1931. The Striped Bass of California (Roccus lineatus). California Fish and Game Fish Bulletin 29.

Smith, H. M. 1895. A Review of the History and Results of the Attempts to Acclimatize Fish and Other Water Animals in the Pacific States. U.S. Fish Commission Bulletin 15.

Sturrock, A. and C. Phillis. 2018. New paths to survival for endangered winter run Chinook salmon. California WaterBlog, https://californiawaterblog.com/2018/01/07/new-paths-to-survival-for-endangered-winter-run-chinook-salmon/

Waldman, J. R., R. E. Bender, and I. I. Wirgin. 1998. Multiple population bottlenecks and DNA diversity in populations of wild striped bass, Morone saxatilis. Fishery Bulletin 96:614–620.

Wang, G., W. Cai, B. Gan, L. Wu, A. Santoso, X. Lin, Z. Chen, and M. J. McPhaden. 2017. Continued increase of extreme El Niño frequency long after 1.5 °C warming stabilization. Nature Climate Change 7:568–572.