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Hyperoxia has been shown to expand the aerobic capacity of some fishes, although there have been very few studies examining the underlying mechanisms and how they vary across different exposure durations. Here, we investigated the cardiorespiratory function of yellowtail kingfish (Seriola lalandi) acutely (~20 h) and chronically (3–5 weeks) acclimated to hyperoxia (~200% air saturation). Our results show that the aerobic performance of kingfish is limited in normoxia and increases with environmental hyperoxia. The aerobic scope was elevated in both hyperoxia treatments driven by a ~33% increase in maximum O2 uptake (MO2max), although the mechanisms differed across treatments. Fish acutely transferred to hyperoxia primarily elevated tissue O2 extraction, while increased stroke volume-mediated maximum cardiac output was the main driving factor in chronically acclimated fish. Still, an improved O2 delivery to the heart in chronic hyperoxia was not the only explanatory factor as such. Here, maximum cardiac output only increased in chronic hyperoxia compared with normoxia when plastic ventricular growth occurred, as increased stroke volume was partly enabled by an ~8%–12% larger relative ventricular mass. Our findings suggest that hyperoxia may be used long term to boost cardiorespiratory function potentially rendering fish more resilient to metabolically challenging events and stages in their life cycle. [ABSTRACT FROM AUTHOR]

Salmonid fish include some of the most valued cultured fish species worldwide. Unlike most other fish, the hearts of salmonids, including Atlantic salmon and rainbow trout, have a well-developed coronary circulation. Consequently, their hearts' reliance on oxygenation through coronary arteries leaves them prone to coronary lesions, believed to precipitate myocardial ischemia. Here, we mimicked such coronary lesions by subjecting groups of juvenile rainbow trout to coronary ligation, assessing histomorphological myocardial changes associated with ischemia and scarring in the context of cardiac arrhythmias using electrocardiography (ECG). Notable ECG changes resembling myocardial ischemia-like ECG in humans, such as atrioventricular blocks and abnormal ventricular depolarization (prolonged and fragmented QRS complex), as well as repolarization (long QT interval) patterns, were observed during the acute phase of myocardial ischemia. A remarkable 100% survival rate was observed among juvenile trout subjected to coronary ligation after 24 wk. Recovery from coronary ligation occurred through adaptive ventricular remodeling, coupled with a fast cardiac revascularization response. These findings carry significant implications for understanding the mechanisms governing cardiac health in salmonid fish, a family particularly susceptible to cardiac diseases. Furthermore, our results provide valuable insights into comparative studies on the evolution, pathophysiology, and ontogeny of vertebrate cardiac repair and restoration. NEW & NOTEWORTHY Juvenile rainbow trout exhibit a remarkable capacity to recover from cardiac injury caused by myocardial ischemia. Recovery from cardiac damage occurs through adaptive ventricular remodeling, coupled with a rapid cardiac revascularization response. These findings carry significant implications for understanding the mechanisms governing cardiac health within salmonid fishes, which are particularly susceptible to cardiac diseases. [ABSTRACT FROM AUTHOR]

Hyperoxia occurs when water oxygen (O2) levels exceed normal atmospheric pressure (i.e., >100% air saturation). Fish can experience hyperoxia in shallow environments due to photosynthesis or in aquaculture through O2 supplementation. This review provides a comprehensive synthesis of the effects of hyperoxia on fish, spanning influences on cardiorespiratory function, acid‐base balance, oxidative stress and whole animal performance (e.g., thermal tolerance and growth). Fish hypoventilate in hyperoxia, but arterial and venous blood oxygenation increases in spite of reduced convection. Persistently high levels of blood oxygenation in hyperoxia do not commonly result in reduced blood O2 carrying capacity, but assessments in undisturbed fish are required to clarify this. Hypoventilation also causes the retention of carbon dioxide, hence respiratory acidosis. Another consequence of hyperoxia is increased levels of oxidative stress and concomitant changes to antioxidant defence systems. Despite these changes, however, the bulk of evidence shows no effect of hyperoxia on growth. Hyperoxia does impact the aerobic metabolic rate of fish with either no effect or elevated resting metabolic rate and substantial increases in maximum metabolic rate. There is also evidence that hyperoxia increases aerobic capacity improves cardiac performance and mitigates anaerobic stress during acute warming. Along with improved upper thermal tolerance in some species, these findings collectively suggest that hyperoxia might provide fish a metabolic refuge during acute warming. Since hyperoxia occurs in shallow aquatic habitats, further research establishing the ecophysiological implications of concomitant heat stress and hyperoxia is pertinent, particularly with a changing climate. [ABSTRACT FROM AUTHOR]

Decreased oxygen (O2) availability (hypoxia) is common in rock pools and challenges the aerobic metabolism of fishes living in these habitats. In this study, the critical O2 tension (Pcrit), a whole animal measure of the aerobic contribution to hypoxia tolerance, was compared between four New Zealand triplefin fishes including an intertidal specialist (Bellapiscis medius), an occasional intertidal inhabitant (Forsterygion lapillum) and two exclusively subtidal species (F. varium and F. malcolmi). The intertidal species had lower Pcrit values than the subtidal species indicating traits to meet resting O2 demands at lower O2 tensions. While resting O2 demand (standard metabolic rate; SMR) did not show a major difference between species, the intertidal species had higher maximal rates of O2 consumption ( M ˙ O 2 , max ) and higher aerobic metabolic scope (MS). The high O2 extractive capacity of the intertidal species was associated with increased blood O2 carrying capacity (i.e., higher Hb concentration), in addition to higher mass-specific gill surface area and thinner gill secondary lamellae that collectively conveyed a higher capacity for O2 flux across the gills. The specialist intertidal species B. medius also had higher glycogen stores in both white muscle and brain tissues, suggesting a greater potential to generate ATP anaerobically and survive in rock pools with O2 tensions less than Pcrit. Overall, this study shows that the superior Pcrit of intertidal triplefin species is not linked to a minimisation of SMR, but is instead associated with an increased O2 extractive capacity of the cardiorespiratory system (i.e., M ˙ O 2 , max , MS, Hb and gill O2 flux). [ABSTRACT FROM AUTHOR]