Abstract
Immersion anaesthesia is widely used in fish handling and research, yet optimization of protocols requires not only effective immobilization but also consideration of internal physiological responses. This study evaluated the effects of immersion‐administered lidocaine (60 and 80 mg/L) as an adjuvant to clove essential oil (CEO; 100 μL/L) on anaesthesia induction and recovery in juvenile Oreochromis niloticus under tightly controlled temperature (27.5°C ± 0.3°C). Behavioural endpoints included loss of equilibrium (LOE), time to an operational anaesthetic plane and recovery time; opercular respiratory rate was assessed as an internal physiological indicator. In the main experiment, adding lidocaine to CEO did not significantly alter LOE, time to anaesthetic plane or recovery time relative to CEO alone. In contrast, respiratory‐rate trajectories differed significantly among treatments during both induction and recovery, indicating protocol‐dependent modulation of ventilatory dynamics despite similar time‐based anaesthetic endpoints. A complementary preliminary experiment showed that lidocaine alone (60 or 80 mg/L) did not induce LOE or anaesthetic plane within a predefined 5‐min exposure window, although it produced clear dose‐dependent changes in respiratory rate relative to water and ethanol controls. Together with qualitative observations of reduced behavioural activation, these findings are compatible with a possible calming or sedation‐like effect of lidocaine, although this was not formally measured and should be considered a hypothesis. Overall, lidocaine did not improve the operational speed of a CEO‐based immersion anaesthetic protocol in Nile tilapia, but it did modulate physiological responses relevant to protocol refinement. These results highlight the importance of integrating both behavioural and internal physiological indicators, together with strict thermal control, when evaluating immersion anaesthesia protocols in fish.