by Andrea Schreier, Peter B. Moyle, Nicholas J. Demetras, Sarah Baird, Dennis Cocherell, Nann A. Fangue, Kirsten Sellheim, Jonathan Walter, Myfanwy Johnston, Scott Colborne, Levi S. Lewis, and Andrew L. Rypel
Sturgeons belong to an ancient family of fishes that once lived alongside dinosaurs. This resilient group of fishes survived a meteor strike, shifting seas and continents, and the onset of the Anthropocene. In California, sturgeon populations have persisted through periods of extreme overfishing, sedimentation and mercury contamination from hydraulic mining, species invasions, and alteration of rivers by dams and levees (Zeug et al. 2014, Gunderson et al. 2017, Blackburn et al. 2019). However, sturgeons remain highly vulnerable to human activities due to their long lifespans, late age-at-maturity, periodic reproduction, and long migrations between freshwater rivers and the ocean.
Suddenly, the future of these ancient fish does not seem so secure. Between late August and early September, 2022, hundreds of sturgeon perished in the San Francisco Estuary. According to Jim Hobbs, program manager for the Interagency Ecological Program at the California Department of Fish and Wildlife (CDFW) Bay Delta office, “the white sturgeon carcass count total will be over 400 and the total for green was 15. ” Because dead sturgeon tend to sink rather than float, the total number of perished individuals is almost certainly much greater. The dead fish that were found were mostly adults and subadults (Figures 1-3), likely taking advantage of abundant and productive food resources in these habitats. Concurrent with the fish kill, the San Francisco Estuary was experiencing a bloom of the ‘red tide’ alga Heterosigma akashiwo, which has been implicated as a possible cause of death of the sturgeon. H. akashiwo produces toxins dangerous to fishes and also reduces the oxygen available in the benthic habitat preferred by sturgeon. Poisoning, asphyxiation, or both could have contributed to the mass mortality. Never before has such a massive kill of sturgeons been recorded in our estuary.
Unfortunately, mass mortality events of sturgeon in human-dominated environments are not altogether unusual. In fact, other sturgeon mortality events were reported this summer in Idaho, Canada and Europe. The frequency and severity of major mortality events for sturgeon and other fishes is predicted to increase substantially in the future, especially as effects of extreme heat waves become more prevalent in aquatic ecosystems (Till et al. 2019, Tye et al. 2022).
Unfortunately, of the 27 species of sturgeon alive today, all are considered by the IUCN to be in danger of extinction in the wild. For most species there are major gaps in our knowledge of life-histories to improve conservation (Jarik and Gessner 2018). In California, we have two species of sturgeon: green sturgeon (Acipenser medirostris) and white sturgeon (Acipenser transmontanus). Sacramento River green sturgeon are listed as ‘threatened’ under the US Endangered Species Act (ESA), and thus research on that species has increased, even recently (e.g., Colborne et al. 2022; Thomas et al. 2022). Ironically, although green sturgeon are much less abundant, we seem to currently know more about the ecology of this species than white sturgeon, which is not ESA-listed. Beginning in the late 1860’s, white sturgeon in the San Francisco Bay-Delta estuary have supported a burgeoning commercial fishery for both caviar and meat. However, the fishery declined precipitously and commercial harvests were banned in 1917 by the State of California. The white sturgeon population in the San Francisco Estuary was not deemed to have recovered enough to support a sport fishery until 1954. Since, white sturgeon have been abundant enough to support a popular recreational fishery, in which fish weighing over 100 pounds and over 100 years in age are caught (but see Blackburn et al. 2019). They are also the basis of pioneering aquaculture operations in the state. Yet despite their cultural, ecological, and economic importance, we still know relatively little about the life-history of white sturgeon in our waters (but see Walter et al. 2022). This is primarily due to the long life span and motile life-history of the species, which makes it difficult to track abundances over long periods of time. Recent work using fin ray microchemistry to reconstruct migratory histories of individual fish suggest high variation in migratory behaviors, with some spending most of their time in freshwater and others residing almost their entire lives in a brackish (estuarine) environment (Sellheim et al. 2022).
Sturgeons are the redwoods of the San Francisco Estuary. This past summer, the H. akashiwo (red tide) bloom spread like a wildfire and wiped out a huge and still unknown fraction of the estuary’s old-growth fishery. Although white sturgeon have proven resilient in the past, there is no reason to be sanguine about their future now, especially in California. Here, white sturgeon live at the southernmost edge of their geographic range, making them especially vulnerable to climate change. And because California white sturgeon don’t reproduce until they are 10-16 years old (Moyle 2002), and their offspring don’t survive well in drought years, it will likely take decades to replace the adult fish lost to this mass mortality event. Continued harvest at current rates will delay, or possibly prevent, recovery of this ancient species (Blackburn et al. 2019). Action needs to be taken now to protect California white sturgeon to assure this ancient population survives long into the future. Given known population trends, combined with the scope of this event, future ESA listing of white sturgeon is plausible. The authors of this blog are collectively hoping white sturgeon avoid the same fate as those before it. Some possible actions to arrest such a future include:
1. Consider temporarily making fisheries for white sturgeon catch-and-release, while recruiting sturgeon anglers as citizen scientists to help with life-history investigations. A conservative strategy makes sense here given the large uncertainty surrounding how many fish actually perished in the mass mortality event. The current mortality estimate of hundreds likely represents only a proportion of the total number killed, as dead sturgeon in aquaculture have been observed to sink rather than float. Temporary catch-and-release fisheries have been enacted in other regions with valuable fisheries that have quickly declined, with the option to be reopened once the population improves.
2. Provide transparent updates to stakeholders and the public on the causes of the kill, number of fish killed as a proportion of the total population size, and possible management actions.
3. Continued support and expansion of existing long-term sturgeon monitoring efforts, to include all life-history stages and habitats, in order to determine population size and dynamics, and life-history requirements. In particular, how does management of the San Francisco Estuary and water resources more generally affect the populations? What are the ecological and physiological thresholds and tolerances for green and white sturgeon? While monitoring is notoriously expensive, it is in the long run, much cheaper than trying to recover an ESA-listed species.
4. Determine the causes of all sturgeon kills, major and minor, in part by expanded water quality and harmful algal bloom monitoring throughout the estuary. Funding may also be needed for rapid responses to mass mortality events including robust carcass surveys and necropsies to verify cause of death. This would include more research into the causes of the red-tide blooms in the San Francisco Estuary.
Andrea Schreier is an Adjunct Associate Professor and Director of the Genomic Variation Lab at University of California Davis. Nicholas Demetras is an Associate Specialist at the University of California Santa Cruz and NOAA Fisheries. Sarah Baird is a Staff Research Associate in the Department of Wildlife, Fish & Conservation Biology at University of California Davis. Dennis Cocherell is a Lab Manager and Staff Research Associate in the Department of Wildlife, Fish & Conservation Biologyat University of California Davis. Nann A. Fangue is a Professor and Chair of the Department of Wildlife, Fish & Conservation Biology at University of California Davis. Kirsten Sellheim is a Science Operations Manager and Senior Scientist at Cramer Fish Sciences. Jonathan Walter is Senior Researcher at the Center for Watershed Sciences at University of California Davis. Myfanwy Johnston is a Senior Scientist at Cramer Fish Sciences. Scott Colborne is a postdoctoral researcher at University of California Davis. Levi S. Lewis is a Researcher and PI of the Otolith Geochemistry & Fish Ecology Laboratory at University of California Davis. 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.
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