Green Sturgeon Research Articles

The Value of Information is Context Dependent: A Demonstration of Decision Tools to Address Multispecies River Temperature Management Under Uncertainty

Trade‐offs among objectives in natural resource management can be exacerbated in altered ecosystems and when there is uncertainty in predicted management outcomes. Multi‐criteria decision analysis and value of information (VOI) are underutilized decision tools that can assist fisheries managers in handling trade‐offs and evaluating the importance of uncertainty. We demonstrate the use of these tools using a case study in the Sacramento River, California, USA, where two imperiled species with different temperature requirements, winter‐run Chinook Salmon Oncorhynchus tshawytscha and Green Sturgeon Acipenser medirostris, spawn and rear in the artificially cold Shasta Dam tailwater. A temperature‐control device installed on Shasta Dam maintains cool water for Chinook Salmon; however, uncertainties exist related to the effects of temperatures on the spawning and rearing of both species. We consider four alternative hypotheses in models of early life‐stage dynamics to evaluate the effects of alternative temperature management strategies on Chinook Salmon and Green Sturgeon management objectives. We used VOI to quantify the increase in management performance that can be expected by resolving hypothesis‐based uncertainties as a function of the weight assigned to species‐specific objectives. We found the decision was hindered by uncertainty; the best performing alternative depends on which hypothesis is true, with warmer or cooler alternative management strategies recommended when weights favor Green Sturgeon or Chinook Salmon objectives, respectively. The value of reducing uncertainty was highest when Green Sturgeon was slightly favored, highlighting the interaction between scientific uncertainty and decision makers' values. Our demonstration features multi‐criteria decision analysis and VOI as transparent, deliberative tools that can assist fisheries managers in confronting value conflicts, prioritizing resolution of uncertainty, and optimally managing aquatic ecosystems.

Effects of low temperature on growth and metabolism of larval green sturgeon (Acipenser medirostris) across early ontogeny.

Southern Distinct Population Segment (sDPS) green sturgeon spawn solely in one stretch of the Sacramento River in California. Management of this spawning habitat is complicated by cold water temperature requirements for the conservation of winter-run Chinook salmon. This study assessed whether low incubation and rearing temperatures resulted in carryover effects across embryo to early juvenile life stages on scaling relationships in growth and metabolism in northern DPS green sturgeon used as a proxy for sDPS green sturgeon. Fish were incubated and reared at 11°C and 15°C, with a subset experiencing a reciprocal temperature transfer post-hatch, to assess recovery from cold incubation or to simulate a cold-water dam release which would chillrearing larvae. Growth and metabolic rate of embryos and larvae were measured to 118days post hatch. Reciprocal temperature transfers revealed a greater effect of low temperature exposure during larval rearing rather than during egg incubation. While 11°C eggs hatched at a smaller length, log-transformed length-weight relationships showed that these differences in developmental trajectory dissipated as individuals achieved juvenile morphology. However, considerable size-at-age differences persisted between rearing temperatures, with 15°C fish requiring 60days post-hatch to achieve 1g in mass, whereas 11°C fish required 120days to achieve 1g, resulting in fish of the same age at the completion of the experiment with a ca. 37-fold difference in weight. Consequently, our study suggests that cold rearing temperatures have far more consequential downstream effects than cold embryo incubation temperatures. Growth delays from 11°C rearing temperatures would greatly increase the period of vulnerability to predation in larval green sturgeon. The scaling relationship between log-transformed whole-body metabolism and mass exhibited a steeper slope and thus an increased oxygen requirement with size in 11°C reared fish, potentially indicating an energetically unsustainable situation. Understanding how cold temperatures affect green sturgeon ontogeny is necessary to refine our larval recruitment estimations for this threatened species.

An integrated population model and sensitivity assessment for a data-poor population of green sturgeon

Conservation planning requires understanding population sizes and trajectories. As with many imperiled species, green sturgeon ( Acipenser medirostris) census data are limited, making it difficult to contextualize current threats and promote quantitative conservation goals. We combined spawning census, growth, and demographic data for the southern population of green sturgeon to build a population size and trajectory estimate. We generated a distribution of population size estimates and trajectories reflecting uncertainty from multiple sources. We then propagate these estimates through a demographic model to assess the potential impact of fishing bycatch. Our model suggests the population is below the recovery goal of 3000 adults. The most probable current total population estimate (including juveniles) is approximately 10 000 fish (5300–18 400 95% high density interval (HDI)), with 2400 adults (2197–2624 95% HDI). Simulated fishing bycatch pressure on the adults and subadults reduced abundance by a median value of 0.4% per year, which could be an impediment to recovery. Fisheries bycatch is one of many threats this population faces; this integrated framework may be used to assess how other threats may affect this population.

Estimating spawning green sturgeon (Acipenser medirostris Ayres, 1854) abundance in the Sacramento River using side-scan sonar and N-mixture models

Current estimates of the threatened southern distinct population segment of the North American green sturgeon ( Acipenser medirostris) combine a plot-sampling density estimator with Dual frequency IDentification SONar (DIDSON) and adaptive resolution imaging sonar (ARIS) sonar data. From 2020 to 2022, we annually collected images of all known green sturgeon aggregations and compared the established method to an N-mixture model using side-scan sonar images. We compared 18 different N-mixture model combinations and chose an overdispersed Poisson model that produced estimated abundances of 742, 1286, and 1208 for 2020–2022, respectively. These numbers are ∼2 times greater than the previous method and, if sustained, would fulfill a key criterion for green sturgeon recovery. N-mixture models are known to be sensitive to violations of assumptions, such as the highly dispersed data from our study that caused serious issues, and we recommend practitioners make judicious use of overdispersion and goodness-of-fit tests and be able to identify parameter confounding between detectability and abundance estimates. For our green sturgeon, we recommend simpler population estimates and to focus future energy on reducing variability in the data collection process.

Transitional Strategies of Juvenile Green Sturgeon from a Riverine to a Brackish Water Environment

Information on habitat transitions is critical to understand whether efforts to manage the freshwater phase of juvenile green sturgeon (Acipenser medirostris) rearing habitats are feasible or beneficial. We implanted microacoustic transmitters in naturally produced age-0 juvenile green sturgeon in the Sacramento River to evaluate residency, general spatial distribution, movement rates, and downstream migration patterns over 300 river kilometers. Furthermore, we investigated whether changes in three environmental variables (discharge, turbidity, and water temperature) were associated with the beginning of movements resulting in the transition from riverine to brackish waters during their outmigration to the Sacramento-San Joaquin Delta Estuary. We captured 183 juvenile green sturgeon by trawl of which 88 were implanted with microacoustic tags. An additional 10 individuals were collected and tagged from the Red Bluff Diversion Dam rotary-screw traps. Linear mixed models showed that individuals caught by trawl had longer upper river residence times, post-tagging, than those caught by trap. Intergate velocity models showed that trap-caught individuals relocated at higher upper river reach velocities than trawl-caught fish, but there were no differences between trap- and trawl-caught fish within the middle or lower reach. Detections showed that juvenile green sturgeon began the downstream transition to brackish water habitat when discharge and turbidity increased. Temperature was not found to significantly influence the initiation of downstream migration. Both continuous and stepped downstream migration patterns were observed in each of the four cohorts, with smaller individuals being more likely to exhibit stepped migration. These data provide information that was previously unknown about the life history of the southern distinct population segment of the North American green sturgeon and can be utilized to assist with water resource management and recovery of this threatened fish species.

Effects of acclimation temperature and feed restriction on the metabolic performance of green sturgeon.

Green sturgeon (Acipenser medirostris) are an anadromous threatened species of sturgeon found along the Pacific coast of North America. The southern distinct population segment only spawns in the Sacramento River and is exposed to water temperatures kept artificially cold for the conservation and management of winter-run Chinook salmon (Oncorhynchus tshawytscha). Past research has demonstrated costs of cold-water rearing including reduced growth rates, condition and survivorship of juvenile green sturgeon. Our research investigates how the stressors of water temperature and food limitation influence the metabolic performance of green sturgeon. We reared green sturgeon at two acclimation temperatures (13 and 19°C) and two ration amounts (100% and 40% of optimal feed). We then measured the routine and maximum metabolic rates (RMR and MMR, respectively) of sturgeon acclimated to these rearing conditions across a range of acute temperature exposures (11 to 31°C). Among both temperature acclimation treatments (13 or 19°C), we found that feed restriction reduced RMR across a range of acute temperatures. The influence of feed restriction on RMR and MMR interacted with acclimation temperature. Fish reared at 13°C preserved their MMR and aerobic scope (AS) despite feed restriction, while fish fed reduced rations and acclimated to 19°C showed reduced MMR and AS capacity primarily at temperatures below 16°C. The sympatry of threatened green sturgeon with endangered salmonids produces a conservation conflict, such that cold-water releases for the conservation of at-risk salmonids may constrain the metabolic performance of juvenile green sturgeon. Understanding the impacts of environmental conditions (e.g. temperature, dissolved oxygen) on ecological interactions of green sturgeon will be necessary to determine the influence of salmonid-focused management.

Comparative stable isotope analyses of green sturgeon (Acipenser medirostris) and white sturgeon (A. transmontanus) in the San Francisco estuary.

Green sturgeon (Acipenser medirostris) and white sturgeon (A. transmontanus) are closely related, sympatric species that inhabit the San Francisco estuary. Green sturgeon have a more marine life history but both species spawn in the Sacramento River and reside for some duration in San Francisco Bay. These sturgeons are of conservation concern, yet little is known about their dietary competition when they overlap in space and time. To examine evidence of dietary differentiation, we collected whole blood and blood plasma from 26 green sturgeon and 35 white sturgeon in San Francisco Bay. Using carbon and nitrogen stable isotope analyses, we compared their relative trophic levels and foraging locations along the freshwater to marine gradient. Sampling blood plasma and whole blood allowed comparison of dietary integration over shorter and longer time scales, respectively. Plasma and whole blood δ13 C values confirmed green sturgeon had more marine dietary sources than white sturgeon. Plasma δ15 N values revealed white sturgeon fed at lower trophic levels than green sturgeon recently, however, whole blood δ15 N values demonstrated the two species fed at the same trophic level over longer time scales. Larger individuals of both species had higher δ13 C values than smaller individuals, reflecting more marine food sources in adulthood. Length did not affect δ15 N values of either species. Isotope analyses supported the more marine life history of green than white sturgeon and potentially highlight a temporary trophic differentiation of diet between species during and preceding the overlapping life stage in San Francisco Bay.

Assessment of potential impact of magnetic fields from a subsea high-voltage DC power cable on migrating green sturgeon, Acipenser medirostris

Empirical evidence suggests that marine animals perceive and orient to local distortions in the earth’s natural magnetic field. Magnetic fields (MFs) generated by electrified underwater cables may produce similar local distortions in the earth’s main field. Concern exists that these distortions may impact migration movements of MF-sensitive animals. The Trans Bay Cable (TBC) is a ± 200-kV, 400-megawatt, 85-km high-voltage direct current transmission line buried through San Francisco Bay (37° 56′ 8.81″ N, 122° 27′ 0.19″ W). Detections of adult green sturgeon implanted with acoustic transmitters were used from six cross-bay receiver arrays from 2006 to 2015 to investigate how inbound and outbound migration movements through lower portions of their route to/from upstream breeding grounds are related to the TBC’s energization status (off/on) and other local environmental variables. Here, we assess how these variables impacted transit success, misdirection from the migration route, transit times, and migration path locations within stretches between the Bay’s mouth and the start of the Sacramento River. Overall, there was varied evidence for any effect on migration behavior associated with cable status (off/on). A higher percentage of inbound fish successfully transited after the cable was energized, but this effect was nonsignificant in models including temperature. Outbound fish took longer to transit after cable energization. Inbound and outbound migration path locations were not significantly influenced by cable energization, but results suggest a potential subtle relationship between energization and both inbound and outbound paths. Overall, additional migration-based studies are needed to investigate the impact of anthropogenic cables on marine species.

Detecting acoustically tagged green sturgeon in the Northeast Pacific Ocean

Identifying critical habitats is challenging for a relatively rare species like green sturgeon (Acipenser medirostris), which spends most of its life at sea. We used a fixed array and an autonomous underwater vehicle (Slocum glider) as monitoring platforms to detect acoustically tagged green sturgeon in the coastal ocean. For 1 month in 2018 during spring (May) and again in fall (September–October), both methods were used simultaneously to detect sturgeon bearing uniquely coded transmitters. For the fixed array, nine acoustic receivers were interspersed with three sentinel transmitters along a single line of latitude off Winchester Bay, OR. For the glider, two 3-week deployments were completed along the coast of Oregon from nearshore to the 200-m depth contour. For both deployments, the glider flew a zig-zag course southward as it profiled the water column and collected water quality information. Tagged green sturgeon and sentinel transmitters were successfully detected by both the fixed array and the glider. The fixed array provided indications of onshore and offshore movement, while the glider indicated along-shore movement. Although more green sturgeon were detected by the fixed array, the glider provided information on potential sturgeon aggregation areas. In addition, this application of the underwater glider may provide a unique opportunity for public engagement, teaching, and outreach.

Experimental evaluation of louver guidance efficiency for green sturgeon (Acipenser medirostris)

AbstractThroughout the world, louver‐bypass systems are a common method for fish protection at water diversion sites. Their performance has been evaluated for pelagic, strong‐swimming teleost fishes, but limited information exists regarding the effectiveness of louvers for guiding sturgeon, despite being a globally threatened taxon. This study used controlled laboratory experiments to quantify louver efficiency for juvenile green sturgeon under a range of conditions. Fish guidance efficiency was influenced most strongly by fish size and approach velocity, with poor efficiency for small fish at high velocities. Low velocities and, to a lesser extent, daylight conditions allowed a greater proportion of fish to remain upstream of the louver at all sizes. High velocities resulted in the greatest contact rate between the sturgeon and louver face. Louvers can be effective for benthic, weak‐swimming fish, but site‐ and species‐specific needs should always be considered in the development or operation of a louver guidance system.

Investigating fishing strategies and habitat differences in late Holocene Oregon Coast sturgeon (Acipenser spp.) through coupled genetic and isotope analyses

Understanding the habitats people were fishing in the past is central to evaluating the relationship between coastal environmental change and human behavior. Researchers often use zooarchaeological identification of fishes and modern ecological data to infer the habitats people fished in the past. However, these inferences assume stable environmental conditions through time and can be hindered by precision issues in identification of archaeological specimens (species vs. genus or family). Here, we integrate genetic and bulk tissue stable isotope data to investigate a late Holocene sturgeon fishery in northern Oregon. Ancient DNA analysis indicated that people were fishing for both green and white sturgeon (Acipenser medirostris and Acipenser transmontanus) in comparable numbers. Stable isotope analyses of these same bones documented distinct isotope values for each species, correlating with species-specific habitat preferences. These findings highlight the value of paired isotope and genetic data to elucidate human fishing strategies and environmental change and provide baseline ecological data for modern fisheries.

Intraspecific variation in migration timing of green sturgeon in the Sacramento River system

AbstractUnderstanding movement patterns of anadromous fishes is critical to conservation and management of declining wild populations and preservation of habitats. Yet, the duration of observations for individual animals can constrain accurate descriptions of movements. In this study, we synthesized over a decade (2006–2018) of acoustic telemetry tracking observations of green sturgeon (Acipenser medirostris) in the Sacramento River system to describe major anadromous movement patterns. We observed that green sturgeon exhibited a unimodal in‐migration during the spring months but had a bimodal distribution of out‐migration timing, split between an “early” out‐migration (32%) group during May–June, or, alternatively, holding in the river until a “late” out‐migration (68%), November–January. Focusing on these out‐migration groups, we found that river discharge, but not water temperature, may cue the timing of migration and that fish showed a tendency to maintain out‐migration timing between subsequent spawning migration events. We recommend that life history descriptions of green sturgeon in this region reflect the distinct out‐migration periods described here. Furthermore, we encourage the continued use of biotelemetry to describe migration timing and life history variation, in not only this population but also other green sturgeon populations and other species.

Movement patterns of juvenile green sturgeon (Acipenser medirostris) in the San Francisco Bay Estuary

Little is known about the distribution and ecology of juvenile green sturgeon throughout their range and specifically within California’s Central Valley and the San Francisco Bay Estuary. The objective of our study was to identify rearing movement patterns of juvenile green sturgeon and their interaction with Delta and Bay habitats. Acoustic biotelemetry was used to record juvenile movements and residence within different regions of this highly modified watershed. We document a diversity of movements exhibited by 31 acoustically tagged juvenile green sturgeon monitored over a 9-month period. The juvenile sturgeon either (1) remained in the Delta or moved (2) into the Carquinez Straits, (3) into San Pablo Bay, (4) into San Pablo Bay but returned to Delta, (5) through the estuary and likely left through the mouth of the bay, (6) or left the estuary only to return later. Spatiotemporal habitat quality varies within and between Delta and Bay habitats. Variation in water year and hydrologic connectivity are likely to influence habitat quality and quantity, and behaviors may be differentially optimized for fitness in response to environmental variation. Our results provide initial evidence supporting multiple behavioral movement patterns and a broad use of regional habitats for green sturgeon in the California Central Valley. This apparent behavioral diversity observed through movement patterns may ensure long-term success of all population segments over time in an extremely variable environment, ultimately supporting overall population stability (i.e., the portfolio effect). Enhanced conservation efforts may be needed to protect juvenile green sturgeon and the habitats they rely upon.

Epidermal cell cultures from white and green sturgeon (Acipenser transmontanus and medirostris): Expression of TGM1-like transglutaminases and CYP4501A.

Using a system optimized for propagating human keratinocytes, culture of skin samples from white and green sturgeons generated epithelial cells capable of making cross-linked protein envelopes. Two distinct forms of TGM1-like mRNA were molecularly cloned from the cells of white sturgeon and detected in green sturgeon cells, accounting for their cellular envelope forming ability. The protein translated from each displayed a cluster of cysteine residues resembling the membrane anchorage region expressed in epidermal cells of teleosts and tetrapods. One of the two mRNA forms (called A) was present at considerably higher levels than the other (called B) in both species. Continuous lines of white sturgeon epidermal cells were established and characterized. Size measurements indicated that a substantial fraction of the cells became enlarged, appearing similar to squames in human epidermal keratinocyte cultures. The cultures also expressed CYP1A, a cytochrome P450 enzyme inducible by activation of aryl hydrocarbon receptor 2 in fish. The cells gradually improved in growth rate over a dozen passages while retaining envelope forming ability, TGM1 expression and CYP1A inducibility. These cell lines are thus potential models for studying evolution of fish epidermis leading to terrestrial adaptation and for testing sturgeon sensitivity to environmental stresses such as pollution.

Organochlorine and Metal Contaminants in the Blood Plasma of Green Sturgeon Caught in Washington Coastal Estuaries

Non-invasive monitoring was used to evaluate the concentrations of 40 contaminants in the blood plasma of the North American Green Sturgeon Acipenser medirostris caught and released from three estuaries in Washington State. The highest contaminant concentrations were found in fish caught in the most urbanized sites. Few statistical differences were found when evaluating contaminant levels according to sex, maturation stage, or distinct population segments of Green Sturgeon. The results indicate that recent exposure to legacy contaminants was reflected in Green Sturgeon plasma. Aldrin, 4,4-DDE, α-HCH, copper, and selenium were the most frequently detected contaminants. This study also explored the challenges of assessing toxicity in threatened species using non-lethal approaches. There is currently a lack of environmental contaminant monitoring data in estuaries frequented by Green Sturgeon and limited plasma to tissue toxicity correlations.

Living Fossils: Sturgeon of the San Francisco Estuary

Sturgeon are fish that are considered living fossils. Their ancestors date back over 200 million years, to the same time as dinosaurs. These fish can grow taller than humans (over 2 m), weigh over 160 kg, and live as long as humans. Sturgeon species have special adaptations, such as a vacuum-like mouth and body armor called scutes. There are 27 species of sturgeon worldwide. Two species, green and white sturgeon, are native to California, USA, and are some of the largest animals in San Francisco Bay. Sturgeon populations have declined due to habitat loss, water management, overfishing, poaching, pollution, and climate change. Sturgeon cannot jump over barriers like salmon can, so structures like dams that block water also block sturgeon from reaching their natural spawning habitat farther upstream in the river. Scientists are using new technologies to monitor sturgeon populations and discover the unique behaviors of these dinosaur-era fish in California’s rivers and estuaries.

Filling Knowledge Gaps for a Threatened Species: Age and Growth of Green Sturgeon of the Southern Distinct Population Segment

AbstractThe Green Sturgeon Acipenser medirostris is an anadromous, long-lived species that is distributed along the Pacific coast of North America. Green Sturgeon is vulnerable to global change because of its sensitive life history (e.g., delayed maturation) and few spawning locations. The persistence of Green Sturgeon is threatened by habitat modification, altered flows, and rising river temperatures. In 2006, because of persistent stressors, the U.S. Endangered Species Act listed the southern distinct population segment as threatened. Despite increased research efforts on this species after the listing, substantial gaps in basic population information for Green Sturgeon remain. We present the only published information on age structure and growth of a threatened population of Green Sturgeon. By analyzing archived fin rays collected from 1984 to 2016, we revealed highly variable growth among individuals. We detected several age classes from 0 to 26 y and found similar growth rates of southern distinct population segment Green Sturgeon compared with northern population Green Sturgeon. Although limited, this analysis is an important first step to understanding Green Sturgeon population dynamics and highlights critical research needs.

Correction to: Spatio-temporal distribution of Green Sturgeon (Acipenser medirostris) and White Sturgeon (A. transmontanus) in the San Francisco Estuary and Sacramento River, California

Green Sturgeon (Acipenser medirostris) and White Sturgeon (A. transmontanus) are closely related species inhabiting the San Francisco Estuary (SFE) and Sacramento River. Both spawn in the Sacramento River. White Sturgeon typically remain within SFE throughout their lives while subadult and adult Green Sturgeon make marine migrations. These broad-scale patterns are known. However, the reach-specific timing of spawning migrations is not, and little is known about seasonal movements of juveniles and subadults. We tagged 41 Green and 160 White Sturgeon with acoustic transmitters (2010–2012). Analyses also included 167 previously tagged sturgeon. Over 300 receivers deployed throughout the Sacramento River, SFE, and nearshore Pacific Coast detected movements. Adult Green Sturgeon moved quickly though SFE to their spawning reach, using the mainstem through the Delta. 5% of spawning Green Sturgeon remained on the spawning reach for over 300 days. White Sturgeon spent significantly less time in their spawning reach before exiting (32 ± 48.2 days vs. 197 ± 97.3 days) and instead selected northern Delta sloughs over the mainstem for pre-spawning migrations. Even at earlier life stages, Green Sturgeon had more marine distributions than White Sturgeon. Juveniles and subadults of both species moved between the San Francisco Bay and Delta throughout the year, but only White Sturgeon overwintered in the Delta. Additionally, we refine several life history parameters for sturgeon in California. We provide a comprehensive characterization of Green and White Sturgeon movements within the SFE and Sacramento River across life stages. These analyses will inform reach-specific management of each species.

Spatio-temporal distribution of Green Sturgeon (Acipenser medirostris) and White Sturgeon (A. transmontanus) in the San Francisco Estuary and Sacramento River, California

Spatio-temporal distribution of Green Sturgeon (Acipenser medirostris) and White Sturgeon (A. transmontanus) in the San Francisco Estuary and Sacramento River, California

Estimating the energetic savings for green sturgeon moving by selective tidal stream transport

As fish move, they incur an energetic cost of transport (COT) from the use of aerobic muscles. Water currents are an integral component of the physical world of fishes, and if water currents are present, fish may pay higher costs fighting the currents, or may use the currents to facilitate movement and reduce COT. Some fish use “selective tidal stream transport” (STST) to move efficiently through tidal regions, swimming into the water column when the current is favorable, and returning to the bottom during opposing tides. This behavior has been reported in marine fish migrating through tidal habitats, but it is also likely of value in daily movements of fish residing in those habitats. It is extremely difficult to directly measure COT in wild fish; however, it is possible to combine analysis of field telemetry data of individual swimming efforts and measurements of metabolic costs of swimming from respirometry in order to estimate the COT in the wild, and to calculate the costs or savings of swimming with or against currents. We describe this novel analytical approach and demonstrate it using data from two green sturgeon tracked in San Francisco Bay, California. In this analysis, when moving at the surface and employing STST, the fish benefited from the current, swimming within 85.5% of optimal efficiency. When conducting non-STST movements near the bottom, swimming was less efficient, with a COT similar to swimming directly into the current. These results suggest that green sturgeon may opportunistically utilize stream transport in daily movements, swimming at the surface and orienting with currents to achieve substantial energetic savings.