Prior thermal acclimation gives White Sturgeon a fin up dealing with low oxygen

Abstract

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.

Keywords: fish; global climate change; hypoxia; multiple stressors; temperature; tolerance.

Figure 1

Experimental design set-up and timeline. White Sturgeon were acclimated to five different acclimation temperatures for one month prior to experimental tests (n = 16 per temperature). Half of the fish (n = 8 per acclimation temperature) first underwent thermal tolerance trials (critical thermal maxima; CTmax), prior to hypoxia tolerance trials. All fish underwent hypoxia trials either in a pilot study of hypoxia tolerance at 14°C (n = 8 fish per acclimation temperature; n = 4 naïve, n = 4 previously tested for thermal tolerance), or hypoxia tolerance at 20°C (n = 8 fish per acclimation temperature; n = 4 naïve, n = 4 previously tested for thermal tolerance). The second half of fish underwent thermal tolerance trials after hypoxia trials (n = 8 per acclimation temperature). Finally, fish tested for thermal tolerance prior to hypoxia tolerance were sampled for red blood cell characteristics and somatic indices (n = 8 fish per acclimation temperature).

Figure 2

Critical thermal maxima (CT _max ) of juvenile White Sturgeon acclimated to five different temperatures. Letters denote statistically significant differences as detected by Tukey HSD post-hoc analysis (see Supplementary Table S1 for details). Individual data points (light circles; n = 16 per acclimation temperature) are overlayed with mean ± S.E.M. (dark circles ± error bars).

Figure 3

Hypoxia tolerance of juvenile White Sturgeon acclimated to five different temperatures and measured at 20°C for (A) incipient lethal oxygen saturation (ILOS; % air saturation) and (B) time in hypoxia (in minutes). There were significant main effects of experiment order (whether fish underwent CTmax trials before ILOS trials, or vice versa) and acclimation temperature (see Results for details). Asterisks denote statistically significant differences between experiment order, and lowercase letters denote statistically significant differences between acclimation temperatures as detected by Tukey HSD post-hoc analysis (see Supplementary Table S2 for details). Individual data points (light circles; n = 4 per experiment order per acclimation temperature) are overlayed with mean ± S.E.M. (dark circles ± error bars).

Figure 4

Somatic indices of juvenile White Sturgeon acclimated to five different temperatures. Somatic indices and Cohen’s d values (with 95% confidence intervals) were calculated for hepatosomatic index (A-B), splenic somatic index (C-D), and relative ventricular mass (E-F). Letters denote statistically significant differences between acclimation temperatures for relative ventricular mass (E) as detected by Tukey HSD post-hoc analysis (see Supplementary Table S3 for details). Cohen’s d values demonstrate the magnitude of effect for each acclimation temperature as compared to the control temperature of 14°C (where effect size is small if d = 0.2, medium if d 731 = 0.5, and large if d = 0.8). For somatic indices, individual data points (light circles; n = 8 per acclimation temperature) are overlaid with mean ± S.E.M. (dark circles ± error bars).