Decoding thermal resilience in fish: acute warming tolerance is associated with neural failure in rainbow trout

As an effect of climate change, heatwaves pose an increasingly more frequent and severe threat to fish populations. Yet, the physiological mechanisms underlying thermal tolerance in fish remain unclear. One hypothesis is that thermal tolerance may be limited by neural failure at high temperatures. Here, we used an electrophysiological approach to test this by assessing the relationship between brain function, determined via recordings of visually evoked responses (VERs) on the electroencephalogram (EEG), and cardioventilatory performance, determined via recordings of ventilatory electromyography (EMG) and electrocardiogram (ECG), in adult rainbow trout (Oncorhynchus mykiss) exposed to a critical thermal maximum (CT_max) protocol. Our results show that normal brain function is preserved at moderate to high temperatures; however, at CT_max, the fish exhibited loss of VERs, indicating brain dysfunction associated with insensibility. This suggests a strong link between neural failure and upper thermal tolerance in fish. Although heart and ventilatory rates increased with warming, heart rate significantly declined at CT_max. Interestingly, ventilation rate remained high even at extreme temperatures and at CT_max, indicating that neural ventilatory drive was maintained across thermal extremes. The factors underlying thermally induced neural failure and its implications for fish in a warming world require further investigation.