Where the sturgeon go: how age-related differences in habitat use shape exposure of white sturgeon to population risks

By Jon Walter, Gabriel Singer, Scott Colborne, Andrew L. Rypel, and Erin Tracy

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White sturgeon are a prominent member of the sturgeon family in North America. They are generally considered the largest fish found in North American freshwaters (up to 12 feet long) and are quite long-lived (reaching 100 years or more). In California, a population of white sturgeon spawns in the Sacramento River, and these fish spend much of their lives in the San Francisco Estuary (SFE). White sturgeons’ long lives and immense size might make them seem invincible, but in reality, their populations face grave risks: currently, white sturgeon are a candidate for listing under the California Endangered Species Act, underscoring the need for research to support decision-making and management.

As white sturgeon (Figure 1) move between rivers, estuaries, and the Pacific Ocean, they encounter a variety of habitats and a gauntlet of risks, from fishing pressure to harmful algal blooms. A new study sheds light on how white sturgeon movements and habitat use intersect with potential risks across the San Francisco Estuary (SFE) and Sacramento River system, and how these patterns change across life stages.

Side angled view of a white sturgeon on a boat. Photo by Erin Tracy — *Figure 1. White sturgeon. Photos by Erin Tracy.*

Side view of a white sturgeon swimming in a tank at an aquarium. Photo by Erin Tracy — *Figure 1. White sturgeon. Photos by Erin Tracy.*

Habitat use differs across life stages

The study uses long-term acoustic telemetry records to quantify how white sturgeon use different parts of the study area, spanning the mouth of the San Francisco Bay at the Golden Gate Bridge to their freshwater spawning grounds in the Sacramento River. Acoustic telemetry studies implant fish with transmitters that emit sounds representing unique codes for each tagged animal. Underwater receivers (listening devices) are placed at locations throughout the estuary and rivers and detect when a tagged fish swims nearby, allowing researchers to build records of where and when different fish are present in an area. We combined 8 water years of data (2011–2017 water years) from multiple studies to track how sturgeon habitat use changes as sturgeon age, from large juveniles that have yet to reproductively mature through to larger, older, reproductively mature fish.

We documented changes across life stages in the frequency with which white sturgeon are detected in different habitat areas (Figure 2). All life stages were detected most frequently between Carquinez Strait and Decker Island, but for juveniles (7–9 years estimated age, before fish have matured to reproductive adults), they were rarely detected outside this area. Transitional fish (10–15 years est. age; in this age range some fish are reproductively mature but most are not) began to exhibit upriver movements toward spawning areas and may also make more frequent forays into San Francisco Bay than juveniles. Reproductive-age fish (16+ years est. age) were detected most frequently on up-river migrations to spawning sites and also showed some evidence of journeys into the coastal ocean (i.e., were detected at the Golden Gate Bridge).

A manuscript figure: It is a 3 panel map showing where sturgeon habitat use is in the San Francisco Estuary based on whether the fish is in a stage of being juvenile, transitional, or reproductive age. The map is gray and the water is blue and the points are black. Size of points = proportional occupancy. — Figure 2. Habitat use by white sturgeon differs by life stage. Points indicate locations of telemetry receivers used in this study and are scaled so that larger points correspond to areas where sturgeon are detected more frequently. White sturgeon spawn primarily in the Sacramento River upstream of location 15. Figure from Walter et al. 2026, https://doi.org/10.1007/s10641-026-01819-7.

Though not unexpected considering prior knowledge of the white sturgeon life cycle in Central California, our findings clarify patterns of white sturgeon habitat use within the system. Additionally, we found that differences in habitat use between life stages lead to sturgeon at different life stages having different levels of exposure to risks, namely angling and a severe red tide (toxic algal bloom) that caused a substantial fish kill in the SFE during late summer 2022. Because different life stages contribute unequally to long-term population trends, understanding how place-based risks are experienced by different life stage classes can help to understand population changes and inform management practices.

Fishing Exposure

To quantify exposure of white sturgeon to fishing, we used data from the California Department of Fish and Wildlife (CDFW) sturgeon report cards on where anglers reported catching sturgeon. Harvesting white sturgeon is currently illegal; however, during the years of our study, white sturgeon were legally harvestable under specific size slot limits to protect both small and very large fish. The size of legally harvestable fish corresponded to the ‘transitional’ life stage group (10–5 years est. age). Catch-and-release fishing remains legal, and white sturgeon are believed to be robust to catch-and-release despite catches causing physiological stress responses and behavioral changes immediately following release (McLean et al. 2019).

By combining sturgeon harvest data with our data on sturgeon habitat use, we showed that juvenile white sturgeon and reproductive adult white sturgeon experienced greater exposure to angling than transitional life stage fish (Figure 3). To us, this was an intriguing but puzzling result. Could harvestable fish have avoided areas that anglers frequented to evade capture? This would not be the first time it has been proposed that fish can learn to avoid areas with fishing pressure (e.g., Koeck et al. 2020). However, we were unable to identify a plausible reason why this response would be stronger in one age class than another, and the cause of this pattern remains unclear. Though the future of white sturgeon harvest in California is uncertain, understanding how sturgeon fishing impacts the population is key to effective management.

A manuscript figure: It is a histogram. x-axis has 3 categories of age (juvenile, transitional, reproductive) and the y-axis is the "fishing capture index". The juvenile bar is highest, the reproductive one is second highest. — Figure 3: An index of exposure of white sturgeon to capture by fishing differs by lifestage. The fishing capture index takes into account prevalence of sturgeon catch at different locations and lifestage-specific patterns of habitat use. Figure from Walter et al. 2026, https://doi.org/10.1007/s10641-026-01819-7.

Red Tide Exposure

To estimate white sturgeon’s risk of exposure to toxic red tide, we focused on white sturgeon detections in two portions of the study area strongly affected by the red tide, San Francisco Bay and San Pablo Bay, during the late-summer period when the 2022 event occurred. While red tide and other toxic algal blooms are not unheard of in the SFE, the 2022 event was especially severe and widespread. Although our data predates the event, we found that historically (2010–2014), more than half of reproductive-aged white sturgeon detections occurred in regions ultimately affected by the 2022 red tide (Figure 4). Taken together with the >850 sturgeon carcasses recovered following the red tide, our findings paint a picture of substantial population-level impacts from the red tide.

A manuscript figure: It is a histogram. x-axis is the day of the year. y-axis is the proportion. there are 6 panels to this figure – the top 3 panels are for the San Francisco Bay region, the bottom 3 panels are for the San Pablo Bay region. The far left panels are showing juveniles. The middle panels are showing transitional aged fish. The far right panels are showing reproductive aged fish. — Figure 4: Occupancy of regions affected by the 2022 red tide and fish kill by white sturgeon differs by life stage. Each panel shows the proportion of white sturgeon detections over time occurring in one of two areas (San Francisco Bay and San Pablo Bay) where red tide effects were strongest, by lifestage group. Grey dashed lines indicate the approximate time period of the red tide. Figure from Walter et al. 2026, https://doi.org/10.1007/s10641-026-01819-7.

Implications and Conclusions

Native fishes are struggling in California, with at least 83% of native species vulnerable to extinction if present trends continue (Rypel & Moyle 2024), and white sturgeon are no exception. The rise of catastrophic disturbances like red tides would add a new threat necessitating wise stewardship of this fascinating species and the habitats it depends on. It’s important to note that in our study, we did not quantify population losses or mortality rates associated with angling or the red tide. Thus, our research does not indicate precisely how these risks impact the numbers or population dynamics of white sturgeon. Rather, we demonstrate how patterns of habitat use affect their potential exposure to different sources of risk, and that the relative exposure to these risks tends to change across life stages. Future research may leverage additional data streams to more directly investigate these questions and to project their consequences for the long-term health of the white sturgeon population in Central California. Synthesis studies like this one show how large volumes of ecological data arising from prior investments in science can be used for new discoveries aiding conservation and natural resource management.

About the Authors

Jon Walter is a Senior Researcher at the Center for Watershed Sciences at UC Davis.

Gabriel Singer is a Senior Environmental Scientist at the California Department of Fish and Wildlife.

Scott Colborne is an Assistant Professor at the Quantitative Fisheries Center at Michigan State University.

Andrew L. Rypel is Director of the School of Fisheries, Aquaculture, and Aquatic Sciences at Auburn University. He is a former professor in the Department of Wildlife, Fish, & Conservation Biology at UC Davis and was the previous Director of the Center for Watershed Sciences.

Erin Tracy is a Postdoctoral Researcher at the Center for Watershed Sciences at UC Davis.

Further Reading

Walter, et al. (2026) Habitat use differences mediate population threat exposure in white sturgeon. Environmental Biology of Fishes 109:51. https://doi.org/10.1007/s10641-026-01819-7

**Note: if you are unable to download the manuscript using this link, contact the lead author for access.

McLean, et al. (2019) Immediate physiological and behavioural response from catch-and-release of wild white sturgeon (Acipenser transmontanusRichardson, 1836). Fisheries Research 214, 65-75. https://doi.org/10.1016/j.fishres.2019.02.002

Koeck, et al. (2020) Behavioural adjustment of fish to temporal variation in fishing pressure affects catchability: an experiment with angled trout. Canadian Journal of Fisheries and Aquatic Sciences 77:188-193. https://doi.org/10.1139/cjfas-2019-0064
Rypel, A.L. and P.E. Moyle (2024) Watching native fishes vanish. California WaterBlog https://californiawaterblog.com/2024/09/01/watching-native-fishes-vanish/