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

As an effect of climate change, heat waves 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
(CTmax) protocol. Our results show that normal brain function is preserved
at moderate to high temperatures, however, at CTmax, 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 CTmax. Interestingly,
ventilation rate remained high even at extreme temperatures and at CTmax,
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 investigations.