Sleeping with the fishes: contradictory results on behavioural and neurophysiological assessments of chemical narcosis in rainbow trout

Anaesthesia in fish is commonly assessed through visual inspection, yet behavioural indicators of consciousness are not always reliable. Complementary measurements of brain activity can enhance accuracy and generate evidence-based welfare guidelines. Here, we anaesthetized rainbow trout with five common anaesthetic agents and assessed presence or absence of visual evoked responses (VERs) on the electroencephalogram as a neurophysiological indicator of unconsciousness combined with the behavioural indicators of consciousness balance and mobility to determine the duration of induction-to and recovery-from narcosis. As expected, both induction and recovery times varied depending on anaesthetic compound and dose. Tricaine methanesulfonate stood out as the compound with the most rapid induction and recovery. Notably, we observed significant discrepancies between neurophysiological and behavioural indicators of consciousness. For example, metomidate induced immobility at relatively low concentrations, whereas much higher doses were required to achieve unconsciousness. Across compounds, trout typically regained motor function well before VERs. This may suggest biological prioritization of mobility during recovery from unconsciousness, or potential visual impairment caused by the anaesthetic agents. These mismatches have important implications for the interpretation of behavioural responses under anaesthesia and should be carefully considered in both research and applied settings.