Cardiorespiratory adjustments to chronic environmental warming improve hypoxia tolerance in European perch (Perca fluviatilis)

Aquatic hypoxia will become increasingly prevalent in the future as a result of eutrophication combined with climate warming. While short-term warming typically constrains fish hypoxia tolerance, many fishes cope with warming by adjusting physiological traits through thermal acclimation. Yet, little is known about how such adjustments affect tolerance to hypoxia. We examined European perch (Perca fluviatilis) from the Biotest enclosure (23°C, Biotest population), a unique ∼1 km2ecosystem artificially warmed by cooling water from a nuclear power plant, and an adjacent reference site (16–18°C, reference population). Specifically, we evaluated how acute and chronic warming affect routine oxygen consumption rate (ṀO2,routine) and cardiovascular performance in acute hypoxia, alongside assessment of the thermal acclimation of the aerobic contribution to hypoxia tolerance (critical O2tension for ṀO2,routine: Pcrit) and absolute hypoxia tolerance (O2tension at loss of equilibrium; PLOE). Chronic adjustments (possibly across lifetime or generations) alleviated energetic costs of warming in Biotest perch by depressing ṀO2,routineand cardiac output, and by increasing blood O2carrying capacity relative to reference perch acutely warmed to 23°C. These adjustments were associated with improved maintenance of cardiovascular function and ṀO2,routinein hypoxia (i.e. reduced Pcrit). However, while Pcritwas only partially thermally compensated in Biotest perch, they had superior absolute hypoxia tolerance (i.e. lowest PLOE) relative to reference perch irrespective of temperature. We show that European perch can thermally adjust physiological traits to safeguard and even improve hypoxia tolerance during chronic environmental warming. This points to cautious optimism that eurythermal fish species may be resilient to the imposition of impaired hypoxia tolerance with climate warming.