Brain cooling marginally increases acute thermal tolerance in Atlantic cod

The physiological mechanisms determining thermal limits in fishes are debated but remain elusive. It has been hypothesised that loss of motor function observed as a loss of equilibrium during an acute thermal challenge is due to direct thermal effects on brain neuronal function. To test this hypothesis, we mounted cooling plates on the head of Atlantic cod (Gadus morhua) and quantified whether local cooling of the brain increased whole-organism upper thermal tolerance (CT_max). Brain cooling reduced brain temperature by 2–6°C and increased CT_max by 0.5–0.7°C relative to instrumented and uninstrumented controls, suggesting that direct thermal effects on brain neurons might contribute to setting upper thermal limits in fish. However, the improvement in CT_max with brain cooling was small relative to the difference in brain temperature, demonstrating that other mechanisms (e.g. failure of spinal and peripheral neurons, or muscle) may also contribute to controlling acute thermal tolerance in fishes.