Antarctic emerald rockcod have the capacity to compensate for warming when uncoupled from CO2‐acidification

Increases in atmospheric CO2 levels and associated ocean changes are expected to have dramatic impacts on marine ecosystems. Although the Southern Ocean is experiencing some of the fastest rates of change, few studies have explored how Antarctic fishes may be affected by co‐occurring ocean changes, and even fewer have examined early life stages. To date, no studies have characterized potential trade‐offs in physiology and behavior in response to projected multiple climate change stressors (ocean acidification and warming) on Antarctic fishes. We exposed juvenile emerald rockcod Trematomus bernacchii to three PCO2 treatments (~450, ~850, and ~1,200 μatm PCO2) at two temperatures (−1 or 2°C). After 2, 7, 14, and 28 days, metrics of physiological performance including cardiorespiratory function (heart rate [fH] and ventilation rate [fV]), metabolic rate (M˙O2), and cellular enzyme activity were measured. Behavioral responses, including scototaxis, activity, exploration, and escape response were assessed after 7 and 14 days. Elevated PCO2 independently had little impact on either physiology or behavior in juvenile rockcod, whereas warming resulted in significant changes across acclimation time. After 14 days, fH, fV and M˙O2 significantly increased with warming, but not with elevated PCO2. Increased physiological costs were accompanied by behavioral alterations including increased dark zone preference up to 14%, reduced activity by 12%, as well as reduced escape time suggesting potential trade‐offs in energetics. After 28 days, juvenile rockcod demonstrated a degree of temperature compensation as fV, M˙O2, and cellular metabolism significantly decreased following the peak at 14 days; however, temperature compensation was only evident in the absence of elevated PCO2. Sustained increases in fV and M˙O2 after 28 days exposure to elevated PCO2 indicate additive (fV) and synergistic (M˙O2) interactions occurred in combination with warming. Stressor‐induced energetic trade‐offs in physiology and behavior may be an important mechanism leading to vulnerability of Antarctic fishes to future ocean change. Although the Southern Ocean is experiencing some of the fastest rates of change, few studies have explored how Antarctic fishes may be affected by co‐occurring ocean changes, and even fewer have examined early life stages. Juvenile rockcod Trematomus bernacchii demonstrated a metabolic capacity to acclimate (at whole organism and cellular levels) to warming and CO2‐aci