Abstract
Hypoxia in aquatic ecosystems is becoming increasingly prevalent, potentially reducing fish performance and survival by limiting the oxygen available for aerobic activities. Hypoxia is a challenge for conserving and managing fish populations and demands a better understanding of the short- and long-term impacts of hypoxic environments on fish performance. Fish acclimate to hypoxia via a variety of short- and long-term physiological modifications in an attempt to maintain aerobic performance. In particular, hypoxia exposure during early development may result in enduring cardio-respiratory modifications that affect future hypoxia acclimation capacity, yet this possibility remains poorly investigated. We incubated Atlantic salmon (Salmo salar) in normoxia (~100% dissolved oxygen [DO, as percent air saturation]), moderate hypoxia (~63% DO) or cyclical hypoxia (100–25% DO daily) from fertilization until 113 days post-fertilization prior to rearing all groups in normoxia for a further 8 months. At ~11 months of age, subsets of each group were acclimated to hypoxia (50% DO) for up to 44 days prior to haematology, aerobic metabolic rate and hypoxia tolerance measurements. Hypoxia exposure during incubation (fertilization to 113 days post-fertilization) did not affect the haematology, aerobic performance or hypoxia tolerance of juvenile salmon in later life. Juveniles acclimated to hypoxia increased maximum aerobic metabolic rate and aerobic scope by ~23 and ~52%, respectively, when measured at 50% DO but not at 100% DO. Hypoxia-incubated juveniles also increased haematocrit and haemoglobin concentration but did not affect acute hypoxia tolerance (critical oxygen level and DO at LOE). Thus, while Atlantic salmon possess a considerable capacity to physiologically acclimate to hypoxia by improving aerobic performance in low oxygen conditions, we found no evidence that this capacity is influenced by early-life hypoxia exposure.
Highlights
Aquatic hypoxia is increasing in prevalence throughout marine and freshwater systems, largely due to nutrient enrichment from anthropogenic sources (Diaz and Rosenberg, 2008; Diaz and Breitburg, 2009; Jenny et al, 2016)
Haemoglobin concentration ([Hb]), haematocrit and Mean corpuscular haemoglobin concentration (MCHC) were not affected by the previous incubation treatment at any time-point across both acclimation treatments
Prior to commencing the hypoxia acclimation (0 days), [Hb] (89.3 ± 1.0 g L−1), haematocrit (50.0 ± 0.4%) and body mass (5.84 ± 0.23 g) were similar between the groups destined for normoxia and hypoxia acclimation (P = 0.1403, P = 0.9570 and P = 0.1979, respectively, Fig. 2A and B)
Summary
Aquatic hypoxia is increasing in prevalence throughout marine and freshwater systems, largely due to nutrient enrichment from anthropogenic sources (Diaz and Rosenberg, 2008; Diaz and Breitburg, 2009; Jenny et al, 2016). Once free-swimming, salmonids may again encounter hypoxia in freshwater lakes and marine ecosystems due to expanding hypoxic zones and decreasing global aquatic oxygen content (Diaz and Rosenberg, 2008; Keeling et al, 2010; Jenny et al, 2016, Schmidtko et al, 2017). Oxygen uptake rate (aerobic metabolic rate, M O2) and critical oxygen level (O2crit) measurements are useful tools for assessing the extent to which aquatic animals can endure low DO environments. Energy-demanding activities such as digestion and swimming are dependent on the capacity for the oxygen uptake rate to increase beyond baseline levels (aerobic scope: the difference between minimum metabolic rate, M O2min and maximum metabolic rate, M O2max) (Fry, 1971; Clark et al, 2013; Claireaux and Chabot, 2016). The acclimation capacity of these traits may play some role in predicting population-level responses to increasingly hypoxic environments
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