The cardiorespiratory response of Atlantic salmon and Nile tilapia to similar acute thermal and hypoxic challenges: the complete story.

Abstract

We used two diverse teleost species (Atlantic salmon and Nile tilapia) to examine the extent to which the regulation of central venous pressure [CVP; and thus, stroke volume (V_S) and cardiac output ([Formula: see text])] and blood oxygen extraction (EO₂) may differ in fishes when exposed to individual, and combined, stressors. These included: 1) acute warming under normoxia, during simultaneous hypoxia, or when heart rate (f_H) was reduced using zatebradine and 2) acute cooling in tilapia. When normoxic: both species increased EO₂ and [Formula: see text] (by ∼2.5- and 1.75-fold, respectively) to meet their aerobic demands (ṀO₂) when warmed; although [Formula: see text] was primarily mediated via changes in f_H, increases in CVP were critical for maintaining V_S, especially in salmon at high temperatures; and CVP was generally negative in tilapia, but decidedly positive in salmon. Salmon showed a strong positive relationship between EO₂ and ṀO₂ under all conditions, but could not elevate EO₂ when [Formula: see text] plateaued at high temperatures. In contrast, tilapia were able to increase ṀO₂ as they approached their critical thermal maximum (CT_Max) because EO₂ continued to rise after [Formula: see text]_Max was reached. Finally, although the CT_Max of both species when normoxic was ∼15°C above their acclimation temperature, only the salmon's CT_Max was constrained by hypoxia or zatebradine (by >5°C and 2°C, respectively). These data provide three novel insights: 1) changes in CVP are both species- and temperature-dependent; 2) the contribution of CVP to V_S, and EO₂ to ṀO₂, are very different between these species; and 3) there are important gaps in our understanding of fish cardiorespiratory physiology.NEW & NOTEWORTHY Central venous pressure (CVP) is temperature-dependent and species-specific in fishes. Negative versus positive CVP values in tilapia versus salmon, respectively, and a greater scope for CVP in salmon, reflect differences in cardiac filling and pumping capacities. Hypoxia exposure and zatebradine administration show that tissue oxygen extraction can help meet the oxygen requirements of some fishes when cardiac output is constrained. Tilapia, but not salmon, were able to preserve their upper thermal tolerance under all conditions.