Evaluation of the reliability of indicators of consciousness during CO2 stunning of rainbow trout and the effects of temperature

A two‐part experiment was conducted to determine whether visual indicators of consciousness such as equilibrium, eye‐roll reflex and ventilation are reliable for evaluating whether CO2 stunning of rainbow trout (Oncorhynchus mykiss) is humane. In part 1, the time taken until the loss of visual indicators in rainbow trout during CO2 stunning was monitored under field conditions at 14, 8 and 2°C. Here, we clearly demonstrate that it takes longer for visual indicators to disappear as temperature decreases, with significant differences in the time taken until the loss of equilibrium between 2 and 14°C, and significant differences between all temperatures in the time taken until the loss of eye‐roll reflex and ventilation. In part 2, rainbow trout were equipped with external non‐invasive electrodes for recording EEG prior to, and following, CO2 stunning to assess the presence or absence of visually evoked responses (VERs), which are indicative of brain function and sensibility. The resulting EEG recordings during CO2 stunning at 10°C demonstrated a poor relationship between visual indicators of consciousness and loss of sensibility, as VERs were present up to 3.5 min after ventilation was lost and up to 6.5 min after the fish lost equilibrium. Collectively, these results show that cold‐water temperatures prolong the time taken until loss of consciousness and that visual indicators are insufficient for determining when sensibility is lost in rainbow trout during CO2 stunning.