Chronic experimental hyperoxia elevates aerobic scope: a valid method to test for physiological oxygen limitations in fish

Abstract

Experimental hyperoxia has been shown to enhance the maximum oxygen uptake capacity of fishes under acute conditions, potentially offering an avenue to test prominent physiological hypotheses attempting to explain impacts of climate warming on fish populations (e.g., gill-oxygen limitation driving declines in fish size). Such benefits of experimental hyperoxia must persist under chronic conditions if it is to provide a valid manipulation to test the relevant hypotheses, yet the long-term benefits of experimental hyperoxia to oxygen uptake capacity have not been examined. Here, the authors measured aerobic metabolic performance of Galaxias maculatus upon acute exposure to hyperoxia (150% air saturation) and after 5 months of acclimation, at both 15 C and 20 C. Acute hyperoxia elevated aerobic scope by 74%-94% relative to normoxic controls, and an elevation of 58%-73% persisted after 5 months of hyperoxia acclimation.When hyperoxia-acclimated fish were acutely transitioned back to normoxia, they maintained superior aerobic performance compared with normoxic controls, suggesting an acclimation of the underlying metabolic structures/processes. In demonstrating the long-term benefits of experimental hyperoxia on the aerobic performance of a fish, the authors encourage the use of such approaches to disentangle the role of oxygen in driving the responses of fish populations to climate warming.

Supplemental oxygen does not improve growth but can enhance reproductive capacity of fish

“…gill remodelling [50]) that divert resources away from somatic growth, leading to a flawed understanding of the role of oxygen in the TSR under normoxic conditions. We echo the suggestion of Atkinson et al [24] that mild levels of hyperoxia can better reveal whether ectotherm life in normoxia may be oxygen limited, especially given that supplementary oxygen can improve cardiorespiratory capacities of fish with no apparent cost to maintenance oxygen requirements [29] (figure 2).…”

Fish gill surface area can keep pace with metabolic oxygen requirements across body mass and temperature

“…Rather, the gills of fish appear to have the capacity to develop in ways that accommodate an individual's oxygen requirements. While declining aquatic oxygen levels may cause issues for the survival and energy budgets of fish in a changing world (Audzijonyte et al., 2022; Schmidtko et al., 2017), our results suggest that oxygen limitation does not explain body size reductions in fish with warming, and thus, alternative mechanisms such as optimal resource allocation strategies should receive further attention (Kozłowski et al., 2004; Skeeles & Clark, 2023).…”

“…Metabolic data have been reanalysed from Skeeles et al. (2022), which assessed the effects of acclimated and acute hyperoxia, at cool and warm temperatures, on mass‐standardised metabolic rates. Here, we use the raw metabolic measurements of the same individuals at their acclimated conditions and pair these with new measurements of GSA.…”

“…Detailed methods pertaining to metabolic measurements can be found in Skeeles et al. (2022) but briefly, size‐selected individuals were removed from their rearing tank, exercised to exhaustion for 2 min in acclimated temperature and oxygen conditions and immediately placed in a size‐appropriate sealed respirometer to capture their maximum oxygen uptake rate (; ≈MMR). This was repeated for a total of seven fish per trial, after which respirometers were left to intermittently flush and seal for ~18 h (15:30 min flush:seal cycle) to record each individual's SMR, deemed the average of the three lowest measurements of .…”

Respiratory acidosis and O2 supply capacity do not affect the acute temperature tolerance of rainbow trout (Oncorhynchus mykiss)

“…6 ), it seems unlikely that this would prevent hyperoxia from having beneficial impacts on CT max . Duration of exposure to hyperoxia could play a significant role in physiological acclimation, as chronic hyperoxia exposure can result in changes in underlying metabolic systems, such as increased mitochondrial densities in tissues ( Mustafa et al., 2011 ), which improve aerobic scope when acutely re-exposed to normoxia ( Skeeles et al., 2022 ) and could facilitate increased CT max when compared with fish acutely exposed to hyperoxia. Despite this, the length of hyperoxia exposures in our study compared to McArley et al.…”