Green Sturgeon in California: Hidden Lives Revealed From Long-Term Tracking

By Scott F. Colborne, Lawrence W. Sheppard, Daniel R. O’Donnell, Daniel C. Reuman, Jonathan A. Walter, Gabriel P. Singer, John T. Kelly, Michael J. Thomas, and Andrew L. Rypel

Fig. 1. An adult Green Sturgeon *Acipenser medirostris*. CDFW photo, taken by Mike Healy.

You gotta respect fishes that have been around since the dinosaurs, such as the 27 described sturgeon species. Unfortunately, the majority of these fishes currently face a high risk of extinction. Extinctions happen to these gentle giants. The Chinese Paddlefish went extinct following a long-term decline – finished off by the Three Gorges Dam (Zhang et al. 2020). California has two sturgeon species: White Sturgeon and Green Sturgeon. Recently, Schreier et al. 2022 published a blog on the status of White Sturgeon in California focused on impacts from a mass die off during a red tide event in the summer of 2022. Here we focus on the southern distinct population segment (DPS) of Green Sturgeon (Fig. 1), a federally ‘threatened’ species under the US Endangered Species Act. Green Sturgeon are fully anadromous and live primarily in the ocean as adults, but spawn in freshwater rivers in the spring. High-quality habitat is essential for the life-history of native fishes, especially in California (Sass et al. 2017, Hause et al. 2022). Yet despite their large size and historical importance to many communities, there is still much we don’t know about Green Sturgeon biology in California.

Fig. 2. Acoustic receivers through San Fransisco Bay and the Sacramento-San Joaquin watershed 2006-2018.

Acoustic telemetry is a method for tracking animals using ultrasonic tags that transmit unique identification codes via sound waves in the water, combined with listening devices, called receivers, that decode these sounds and record when they are heard. Acoustic telemetry has become commonplace for studying aquatic animal movements because GPS tags are not well suited for animals that do not frequently surface, particularly because GPS signals do not transmit well through water. In California, acoustic telemetry has been used to study fish movements since at least 2001 with construction and maintenance of a large array of receivers throughout the Sacramento River and Sacramento-San Joaquin Delta (Fig. 2). In total, more than 450 km of the Sacramento River have been covered in acoustic receivers. This array was supported by several state and federal agencies in California that foresaw the great potential of telemetry approaches to advance understanding and management of our native fish populations. A team of researchers at UC Davis, led by A. Peter Klimley, originally developed and maintained this array for many years.

We recently published a paper (Colborne et al. 2022) synthesizing 12 years of acoustic telemetry data for Green Sturgeon in the Sacramento River and Sacramento-San Joaquin Delta. It had been observed that enough telemetry data had accumulated that a synthesis science effort could contribute additional value beyond the original purposes of previous studies. Our team included academic researchers from across the US, international collaborators, and leaders from state and federal agencies in California. This research advances our understanding of the life-history and specifically migration timing of Green Sturgeon in several ways. First, we showed that during spring months (Fig. 3, left), there is a single pulse of upriver migrants towards spawning grounds in the upper Sacramento River, consistent with other observations (Steel et al. 2018). The upriver migration occurs during March and April, but fluctuates within that time range across years. Second, Green Sturgeon return to the ocean in two groups separated in time by several months (Fig. 3, right). Earlier returning sturgeon begin migrating downriver during mid-June; however, the second group over-summers in the river where they remain until mid-December.

Fig. 3. Distributions of (left) swim-up and (right) swim-down dates for Green Sturgeon.

Fig. 4. A Green Sturgeon sampled during the fall in Suisun Bay. Photo by Davis Dominquez of USFWS.

It’s not yet known if these two migration groups reflect divergent life-history strategies, sexual differences in migration timing, or fish becoming increasingly trapped upriver by low summer flows. The timing of outmigration in both groups is significantly related to river flow conditions such that early migrants are cued by higher discharge values in the spring/early summer whereas late migrants begin outmigration when the hydrograph begins to rise in the fall. Interestingly, downriver migrants show some fidelity to the timing of downriver migrations, such that late migrants are statistically more likely to be late migrants during their second monitored spawning year. When migrants did change the timing of outmigration,they tended to migrate late when flow rates during the early period of the year were low compared to the years when they migrated early. Combined, our findings suggest water managers may have a capacity to help sturgeon migrate, potentially by strategically releasing pulse flows from dams. The idea of sturgeon pulse flows may ostensibly seem new, but we are already accustomed to managing flows for Pacific salmon, which also have distinct life-history variations based on run timing.

Our work exposes how synthesis science of telemetry data specifically has important utility in today’s matrix of California water science activities. Broad-scale collaborative efforts like these deliver actionable science to decision makers concerned with the decline of sturgeon and other native species. Few studies in California have leveraged prior large and long-term investments in the core array, tags, and various specific studies. Individual telemetry studies are exceedingly expensive, considering the costs of tags, personnel, receivers, transportation, etc. Oftentimes, there is “more juice in the orange,” and investments in synthesis and team science groups allows full value extraction from prior investments. Furthermore, the high expense of these studies may limit the intellectual capacity that can be applied to these problems by restricting access to data and hypothesis testing (Nguyen et al. 2017). Open science and team science help solve these problems. And we are not the only ones – there are synthesis science efforts popping up all around the California water world. This is a great thing. Hopefully, using all our efforts and ways of knowing, we can generate information to keep sturgeon and other native species alive and well in California for a long time to come.

Acknowledgements. This synthesis work was supported by a grant from the Delta Stewardship Council, Delta Science Program, and the US Bureau of Reclamation (award: 18204; Synchrony of native fish movements). We recognize A.P. Klimley for his leadership and efforts in organizing the initial “core 69 kHz array,” and CDFW for funding it. We also recognize the contributions of numerous people in the UC Davis Biotelemetry Laboratory over the years that made this extensive Green Sturgeon dataset a possibility. The number of people involved with these projects across multiple agencies and institutions is too numerous to list everyone here, but we express a deep gratitude to anyone that has collected data, turned on tags, tagged fish, maintained receivers, downloaded data, curated data etc.

Scott F. Colborne was a postdoctoral researcher at University of California Davis and is currently a Research Specialist at the Quantitative Fisheries Center at Michigan State University. Lawrence W. Sheppard is a Research Scientist at the Marine Biological Association in the UK, Daniel R. O’Donnell was a postdoctoral researcher at University of California Davis and is currently a Senior Scientist at CDFW. Daniel C. Reuman is a Professor in the Department of Ecology and Evolutionary Biology at the University of Kansas and a Senior Scientist at the Kansas Biological Survey. Jonathan A. Walter is a Senior Researcher at the Center for Watershed Sciences at University of California Davis, Gabriel P. Singer is a Senior Environmental Scientist at CDFW, John T. Kelly is a Senior Environmental Scientist and Statewide Sturgeon Coordinator at the CA Department of Fish and Wildlife, Fisheries Branch, Michael J. Thomas was a Staff Researcher at University of California Davis and is currently a Fish Biologist at U.S. Army Corps of Engineers. Andrew L. Rypel is a Professor and the Peter B. Moyle and California Trout Chair in Coldwater Fish Ecology at University of California Davis, and the Director of the Center for Watershed Sciences.

2 Responses to Green Sturgeon in California: Hidden Lives Revealed From Long-Term Tracking

parackal oommen thomas says:

February 12, 2023 at 6:49 am

WHEN the upriver migration occurs during March and April, can some of them be channeled into man made ponds to be reared and retuned later to the sea.
such a programme may ensure that these species do not become exitinct.

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- Andrew Rypel says:
  
  February 12, 2023 at 1:32 pm
  
  There are already broodstock programs for the species in hatcheries as a back up. In wild populations, the key really seems to be using science to understand the habitats needed to complete the life-cycle and engender fitness. Then, using conservation science to support the sturgeon through each phase of that cycle. Hard long-term work, but there are many examples for how this can work successfully. Lake Sturgeon in Lake Winnebago is one particularly great example. They are so well managed (and thus abundant) that there is actually a sustainable spear fishery on the population. Takes great science, monitoring and management though, including lots of collaboration and public support!
  
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