Identification of upper thermal thresholds during development in the endangered Nechako white sturgeon with management implications for a regulated river

Downscaled climate change threats to United States freshwater finfish aquaculture

Climate change threatens food production, yet gaps remain in our understanding of these threats to aquaculture, the fastest growing food production subsector. To build climate-resilient practices and policies we need to quantify and map current and future climate threats to aquaculture. Here, we explore how downscaled climate change [SSP 2 (eq. RCP 4.5) and SSP 5 (eq. RCP8.5), CMIP6] threats - including water scarcity, flooding, and increasing temperature - may directly affect United States (US) freshwater farmed fish (N = 7) based on their biological thermal tolerances and indirectly challenge the operations required for production, including to the human workforce. Aquaculture in the US is dominated by catfish, trout, and tilapia production and is widespread, with some form of finfish aquaculture present in every state and nearly half of all counties across the country. Given the current location of catfish, tilapia, bass, and carp in the US and their tolerance to warmer conditions, we find increasing temperatures are less likely to biologically impact these species negatively. In contrast, current trout, sturgeon, and perch production will be biologically threatened by rising temperatures. With respect to operational needs for facilities, increases in 'wet bulb' temperatures in the Southeast will regularly challenge human physiological limits and constrain worker capacity. Drought in the Southwest will also limit an intrinsically water dependent system, affecting nearly all taxa. While current areas of aquaculture will tend to become increasingly challenging for farmed fishes, new potential habitats will open up for nearly all species. Overall, in the absence of immediate greenhouse gas mitigation, there are several non-mutually exclusive climate adaptations, yet these adaptations can be extremely costly. Ultimately, freshwater aquaculture in the US is going to be under intense climate pressure, which may drive out small operations and cause the country to further increase dependence on international aquatic food imports.

Too Hot to Handle: A Meta-Analytical Review of the Thermal Tolerance and Adaptive Capacity of North American Sturgeon

Understanding how ectotherms may fare with rising global temperatures and more frequent heatwaves is especially concerning for species already considered at-risk, such as long-lived, late-maturing sturgeon. There have been concerted efforts to collect data on the movement behavior and thermal physiology of North American sturgeon to enhance conservation efforts; thus, we sought to synthesize these data to understand how sturgeon respond to thermal stress and what capacity they have to acclimate and adapt to warming. Here, we combined a systematic literature review and meta-analysis, integrating field-based observations (distribution and spawning) and laboratory-based experiments (survival, activity, growth, metabolism, and upper thermal limits) for large-scale insights to understand the vulnerability of North American sturgeon to rising global temperatures. We summarized the preferred thermal habitat and thermal limits of sturgeon in their natural environment and using meta-analytical techniques, quantified the effect of prolonged temperature change on sturgeon whole-animal physiology and acute upper thermal limits. While acclimation did not have significant effects on physiological rates or survival overall, there were positive trends of activity and metabolism in young-of-the-year sturgeons, likely offset by negative trends of survival in early life. Notably, North American sturgeon have a greater capacity for thermal tolerance plasticity than other fishes, increasing upper thermal limits by 0.56°C per 1°C change in acclimation temperature. But with limited laboratory-based studies, more research is needed to understand if this is a sturgeon trait, or perhaps that of basal fishes in general. Importantly, with these data gaps, the fate of sturgeon remains uncertain as climate change intensifies, and physiological impacts across life stages likely limit ecological success.

The carryover effects of embryonic incubation temperature on subsequent growth and thermal tolerance in white sturgeon

Environmental variation experienced during early periods of development can lead to persistent phenotypic alteration, known as carryover effects. Such effects increase concern for threatened or endangered species such as the white sturgeon (Acipenser transmontanus), particularly considering expected thermal changes due to climate change. We evaluated how temperature during embryonic development affects physiological parameters such as larval and early juvenile growth and thermal tolerance. Nechako River white sturgeon embryos were incubated at different environmental temperatures (Te) of 12 °C (the natural spawning temperature of this population), 15 °C (the hatchery incubation temperature), and 18 °C (representing potential increases in river temperatures given global climate change). After hatch, fish were reared at a common 15 °C for 80 days post-hatch (dph). Individuals from each temperature treatment were tested for thermal tolerance using the critical thermal maximum method (CTmax), euthanized, and measured. Fish were examined at regular intervals from 13 to 80 dph, which bridged the time from the start of exogenous feeding through the transition into early juveniles. We found carryover effects of high embryonic Te in the short term for both thermal tolerance and growth. Fish that developed at 18 °C had the lowest thermal tolerance during the start of exogenous feeding. However, differences in thermal tolerance were small for early juveniles and were unlikely to be ecologically relevant in the longer term. Fish that developed at 18 °C were smallest over the observation period, indicating a possible cost for survival from increasing environmental temperatures during embryonic development. This research represents a window into a critical period of development during which fish are particularly vulnerable to climatic variation, and shows that cooler temperatures (12 °C) during incubation are optimal for this population. The results can inform environmental managers on the best strategies to help conserve current white sturgeon populations across their range.

Resetting thermal limits: 10-year-old white sturgeon display pronounced but reversible thermal plasticity

While many ectotherms improve thermal tolerance in response to prolonged thermal stress, little is known about the lasting effects of warm acclimation after returning to cooler temperatures. Furthermore, thermal stress may disproportionately impact threatened and endangered species. To address this, we repeatedly measured critical thermal maxima (CTmax; °C) and associated stress responses (hematocrit, hemoglobin concentration, plasma cortisol) of endangered subadult white sturgeon (Acipenser transmontanus) in response to control temperature (pre-acclimation; 14°C), after 1 month at either control or warm temperature (acclimation; 14°C or 20°C), and after one smonth following return to control temperature (post-acclimation; 14°C). While control fish demonstrated fairly repeatable thermal tolerance (interclass correlation coefficient = 0.479), warm-acclimated fish experienced a ∼3.1°C increase in thermal tolerance and when re-acclimated to control temperature, decreased thermal tolerance ∼1.9°C. Hematocrit, hemoglobin concentration, and final splenic somatic index (spleen mass relative to whole body mass, collected after post-acclimation CTmax) were not significantly different between control and treatment fish, suggesting no effects of warm acclimation on aerobic capacity. Plasma cortisol was significantly higher in control fish after pre-acclimation and post-acclimation CTmax trials, but importantly, acclimation temperature did not affect this response. Strikingly, final hepatosomatic index (relative liver size) was 45% lower in treatment fish, indicating warm acclimation may have lasting effects on energy usage and metabolism, even after reacclimating to control temperature. To our knowledge, these 10-year-old subadult sturgeon are the oldest sturgeon experimentally tested with regards to thermal plasticity and demonstrate incredible capacity for thermal acclimation relative to other fishes. However, more research is needed to determine whether the ability to acclimate to warm temperature may come with a persistent cost.

Diagnosing the ability of reservoir operations to meet hydropower production and fisheries needs under climate change in a western cordillera drainage basin

Water regulation has contributed to the decline in Pacific salmon in British Columbia (Canada) despite attempts to manage reservoir operations to achieve operational requirements while meeting environmental needs to limit fish thermal stress. The ability of reservoir managers to meet these trade-offs in a changing climate is unknown. Here, we examine the reliability and vulnerability of the Nechako Reservoir to meet hydropower production commitments and fisheries needs under two projected Shared Socioeconomic Pathway scenarios (SSP2-4.5 and SSP5-8.5). While our findings are specific to the operation of the Nechako Reservoir, the issues that emerge are likely common to many reservoirs in areas where reservoir inflow regimes are currently snow-storage dominated. We found that projected changes in the timing of water availability have little to no influence on hydropower generation commitments. However, larger water releases will be required to avoid compromising reservoir safety, possibly endangering downstream fish habitat through scouring. Furthermore, the temperature of water released from the reservoir is projected to more frequently exceed a level, 20°C, that is detrimental to migrating sockeye salmon. Water released is subject to further warming as it travels towards the lower reaches of the Nechako River used by migrating salmon. Hence, there is a need to adapt reservoir operations to ensure reservoir safety and mitigate adverse effects on salmon habitat.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10584-023-03632-y.

Hydrothermal impacts of water release on early life stages of white sturgeon in the Nechako river, B.C. Canada

Water temperature plays a crucial role in the physiology of aquatic species, particularly in their survival and development. Thus, resource programs are commonly used to manage water quality conditions for endemic species. In a river system like the Nechako River system, central British Columbia, a water management program was established in the 1980s to alter water release in the summer months to prevent water temperatures from exceeding a 20 °C threshold downstream during the spawning season of Sockeye salmon (Oncorhynchus nerka). Such a management regime could have consequences for other resident species like the white sturgeon (Acipenser transmontanus). Here, we use a hydrothermal model and white sturgeon life stage-specific experimental thermal tolerance data to evaluate water releases and potential hydrothermal impacts based on the Nechako water management plan (1980-2019). Our analysis focused mainly on the warmest five-month period of the year (May to September), which includes the water release management period (July-August). Our results show that the thermal exposure risk, an index that measures temperature impact on species physiology of Nechako white sturgeon across all early life stages (embryo, yolk-sac larvae, larvae, and juvenile) has increased substantially, especially in the 2010s relative to the management program implementations' first decade (the 1980s). The embryonic life stage was the most impacted, with a continuous increase in potential adverse thermal exposure in all months examined in the study. We also recorded major impacts of increased thermal exposure on the critical habitats necessary for Nechako white sturgeon recovery. Our study highlights the importance of a holistic management program with consideration for all species of the Nechako River system and the merit of possibly reviewing the current management plan, particularly with the current concerns about climate change impacts on the Nechako River.

Heatwave resilience of juvenile white sturgeon is associated with epigenetic and transcriptional alterations

Heatwaves are increasing in frequency and severity, posing a significant threat to organisms globally. In aquatic environments heatwaves are often associated with low environmental oxygen, which is a deadly combination for fish. However, surprisingly little is known about the capacity of fishes to withstand these interacting stressors. This issue is particularly critical for species of extreme conservation concern such as sturgeon. We assessed the tolerance of juvenile white sturgeon from an endangered population to heatwave exposure and investigated how this exposure affects tolerance to additional acute stressors. We measured whole-animal thermal and hypoxic performance and underlying epigenetic and transcriptional mechanisms. Sturgeon exposed to a simulated heatwave had increased thermal tolerance and exhibited complete compensation for the effects of acute hypoxia. These changes were associated with an increase in mRNA levels involved in thermal and hypoxic stress (hsp90a, hsp90b, hsp70 and hif1a) following these stressors. Global DNA methylation was sensitive to heatwave exposure and rapidly responded to acute thermal and hypoxia stress over the course of an hour. These data demonstrate that juvenile white sturgeon exhibit substantial resilience to heatwaves, associated with improved cross-tolerance to additional acute stressors and involving rapid responses in both epigenetic and transcriptional mechanisms.