Colin J. Brauner’s research while affiliated with University of British Columbia and other places

Freshwater salinization is increasing globally through seawater intrusion, road de-icing, and changes in anthropogenic land uses. Concurrently, freshwaters are browning with the rise in dissolved organic carbon (DOC) concentrations, while water pH is falling. Elevations in external major ion concentrations (Na⁺ or Ca²⁺) and low pH, independently disturb osmoregulatory homeostasis in freshwater organisms. Several studies have demonstrated that DOC often mitigates osmoregulatory stress responses to acidic pH. However, the interactive effects of these three water quality parameters together have been relatively understudied. Transepithelial potential (TEP), the electrical gradient across the gills between the animal and the external water, can be used as an index of osmoregulatory stress. We investigated whether DOC and exposure to elevated major ions interact with TEP responses at circumneutral and low environmental pH in the freshwater rainbow trout. Two natural DOCs, one allochthonous and the other autochthonous, were used. To aid interpretation, three model compounds of known chemical structure were also employed (tannic acid, sodium dodecyl sulfate, bovine serum albumin), based on the criteria that they structurally resemble or functionally behave like certain chemical moieties of humic or fulvic acids, major components of DOC. The Multi-Ion Toxicity Model predicts that a disturbance in absolute TEP is indicative of salt toxicity; however, recent studies have shown that ΔTEP (the change in TEP relative to the baseline) may be more predictive. Our data followed a pattern that could be described by the Michaelis–Menten equation. Therefore, considering Michaelis–Menten constants (Km and ΔTEPmax), absolute TEP and ΔTEP, we used a weight of evidence approach to predict how DOC and pH will influence Na⁺ or Ca²⁺ toxicity. We conclude that key chemical moieties of DOC will likely play pH-dependent roles in both Na⁺ and Ca²⁺ toxicity. Graphical Abstract

Climate change is impacting river ecosystems, underlining the need for water management strategies to protect native species within these ecosystems. Here, we evaluate the impact of climate change and water management on the physiology of white sturgeon (Acipenser transmontanus) in the Nechako River, British Columbia (Canada). Using the CEQUEAU hydrological–thermal model, we simulated daily water temperatures from 1980 to 2099 under two climate scenarios (SSP2-4.5 and SSP5-8.5). We assessed thermal exposure risk (Te) for different developmental stages of white sturgeon, focusing on the warmest 6-month period. Our findings show that embryos and yolk-sac larvae exhibit resilience, with Te values consistently <1 under both scenarios, signifying low thermal stress. In contrast, feeding larvae and juveniles experience elevated Te values, indicating significant future thermal stress. For feeding larvae, Te values exceeded 1 under both scenarios, reaching up to 1.5 by the mid-century (2050s) and up to 1.8 by the end of the century (2090s) under SSP5-8.5. Juvenile white sturgeon also faced increased thermal risks, with Te values rising >1 during July and August, reaching 1.4 and 1.8 by the 2050s and 1.8 and 2.0 by the 2090s under SSP5-8.5, compared to the 1980s. These results underscore the need to evaluate the existing water management programme to better accommodate the projected changes in thermal conditions associated with climate change. Additionally, regulated river discharge, which can both increase and decrease downstream temperatures, offers a strategic opportunity to mitigate some climate impacts through strategic dam discharge management.

Gill regeneration in fish varies inter and intra-specifically. The latter may be associated with myriad factors including capacity of energy metabolism. This study investigated whether mitochondrial respiration capacity influences the degree of gill regeneration, and features of mitochondria in regenerated tissue by feeding fish an experimental diet aimed at modulating mitochondrial efficiency. Fish reared on control and experimental diet were subjected to 50% filament resection on a subset of filaments on the ventral and dorsal regions of the first gill arch. Mitochondrial respiration and citrate synthase activity (CSA) were measured in the resected tips of filaments (week-0) and then in the regenerated tissue at week-20 post-resection. The degree of filament regeneration was measured at week-20 post-resection (week-20). The experimental diet reduced CSA and respiratory control ratio (RCR), and increased proton leak at week-0 which was associated with a 30% reduction in tissue regeneration compared to fish on standard diet. While CSA increased in the regenerated tissue at week-20, there was a decline in state 3 respiration, proton leak, complex IV activity, and RCR as compared to week-0 irrespective of diet. Overall, mitochondrial respiration efficiency at week-0 was positively correlated with the degree of subsequent gill tissue regeneration. Additionally, state 3 respiration and proton leak at week-20 were positively correlated with tissue regeneration, whereas CSA exhibited a negative relationship. Our results indicate that capacity of mitochondrial respiration may at least partially explain the inter-individual variations in tissue regeneration, but mitochondrial function in the regenerating tissue may be limited.

Assessing how at-risk species respond to co-occurring stressors is critical for predicting climate change vulnerability. In this study, we characterized how young-of-the-year White Sturgeon (Acipenser transmontanus) cope with warming and low oxygen (hypoxia) and investigated whether prior exposure to one stressor may improve the tolerance to a subsequent stressor through “cross-tolerance”. Fish were acclimated to five temperatures within their natural range (14-22°C) for one month prior to assessment of thermal tolerance (critical thermal maxima, CTmax) and hypoxia tolerance (incipient lethal oxygen saturation, ILOS; tested at 20°C). White Sturgeon showed a high capacity for thermal acclimation, linearly increasing thermal tolerance with increasing acclimation temperature (slope = 0.55, adjusted R2 = 0.79), and an overall acclimation response ratio (ARR) of 0.58, from 14°C (CTmax = 29.4 ± 0.2°C, mean ± S.E.M.) to 22°C (CTmax = 34.1 ± 0.2°C). Acute warming most negatively impacted hypoxia tolerance in 14°C-acclimated fish (ILOS = 15.79 ± 0.74% air saturation), but prior acclimation to 20°C conferred the greatest hypoxia tolerance at this temperature (ILOS = 2.60 ± 1.74% air saturation). Interestingly, individuals that had been previously tested for thermal tolerance had lower hypoxia tolerance than naïve fish that had no prior testing. This was particularly apparent for hypoxia-tolerant 20°C-acclimated fish, whereas naïve fish persisted the entire 15-h duration of the hypoxia trial and did not lose equilibrium at air saturation levels below 20%. Warm-acclimated fish demonstrated significantly smaller relative ventricular mass, indicating potential changes to tissue oxygen delivery, but no other changes to red blood cell characteristics and somatic indices. These data suggest young-of-the-year White Sturgeon are resilient to warming and hypoxia, but the order in which these stressors are experienced and whether exposures are acute or chronic may have important effects on phenotype.

The Amazon basin contains more than 15% of the world’s freshwater fishes, many of ecological and economical importance. Unfortunately, research suggests that tropical species, which have adapted to a thermally stable environment, are most at risk with a long-term increase above current temperatures, which is worrisome when the Amazon could experience an increase in air temperature of up to +6.5 °C within the next century. In this chapter, we review the thermal limits of Amazonian fishes and their capacity to acclimate to warm temperatures. Furthermore, we explore how this thermal tolerance data may be incorporated into prediction models to evaluate Amazonian fish’s vulnerability to thermal stress and highlight the research that is needed to better understand their capacity to acclimate to a warming world.

Climate change is affecting freshwater systems, leading to increased water temperatures, which is posing a threat to freshwater ecological communities. In the Nechako River, British Columbia, a water management program has been in place since the 1980s to maintain water temperatures at 20 °C during the migration of adult Sockeye salmon. However, the program's effectiveness in mitigating the impacts of climate change on resident species like Chinook salmon's thermal exposure is uncertain. In this study, we utilised the CEQUEAU hydrological model and life stage-specific physiological data to evaluate the consequences of the current program on Chinook salmon's thermal exposure under two contrasting climate change and socio-economic scenarios (SSP2-4.5 and SSP5-8.5). The results indicate that the thermal exposure risk is projected to be above the optimal threshold for parr (intermediate juvenile) and adult life stages under both scenarios relative to the 1980s. Under the SSP5-8.5 scenario, these life stages could experience an increase in thermal exposure ranging from two to five times higher by the 2090s compared to the 1980s. This exposure is projected to occur during the months in which these life stages emerge, including the period when the program is active (July 20th to August 20th). Additionally, our study shows that climate change will result in a substantial rise in cumulative heat degree days, ranging from 1.9 to 5.8 times (2050s) and 2.9 to 12.9 times (2090s) in comparison to the 1980s under SSP5-8.5. Our study highlights the need for a holistic approach to reviewing the current Nechako management plan, ensuring that all species in the Nechako River system are considered especially in the face of climate change.

In recent lab‐based experiments, some post‐smolt Atlantic salmon (Salmo salar) held at 3°C for 5 weeks exhibited a range of clinical signs. They became lethargic and swam at the water's surface, developed ulcers to the head and jaw (clinical signs similar to tenacibaculosis in Norwegian salmon aquaculture) and had fin erosion, and this was associated with significant mortalities. In addition, when fish with ‘early’ and ‘advanced’ stages of these different clinical signs were further examined, their livers were found to be large, pale and friable. Fish with this aetiology also had elevated aspartate aminotransferase levels (indicative of liver damage), elevated plasma [Na⁺], [Cl⁻] and osmolality (indicating osmoregulatory impairment), low glucose levels (likely limiting metabolic responses to maintain homeostasis) and high circulating cortisol levels (∼100 ng/mL). This suite of physiological disturbances is very similar to that observed in a condition referred to as ‘Winter Syndrome’ or ‘Winter Disease’ (WS/WD) in cultured gilthead sea bream (Sparus aurata) and other fish species. Thus, it appears that WS/WD described here for the first time in Atlantic salmon, alone or in combination with opportunistic infections, results in lipid deposition in the liver, compromising liver function and osmoregulatory capacity, and metabolic collapse that ultimately results in significant losses.