Electrophysiological insights into short-term cryoanaesthesia in tambaqui, Colossoma macropomum for improved procedural outcomes

Cryoanaesthesia is a technique that uses low temperatures to immobilise or anaesthetise fish without chemical agents, but its effects on tambaqui ( Colossoma macropomum) have not yet been evaluated using electroencephalographic (EEG) and electromyographic (EMG) recordings, which are essential methods for proving the effectiveness of the procedure. In order to evaluate the EEG and EMG signals of C. macropomum (32 ± 4.1 g) during exposure to 10 °C, 5 °C and 0 °C for 10 min, and to determine the most suitable temperatures for short-term cryoanaesthesia, the electrophysiological responses were analysed both during the induction process (cooling) and during recovery (at 27 °C). The fish were essayed into 4 groups ( n = 9), and according to the water temperature they were exposed to: group 1 (control): 27 °C; group 2: 10 °C; group 3: 5 °C; and group 4: 0 °C. Overall, during induction, the fish showed two phases: the first, characterised by increased neuronal excitability and muscle contraction; and the second, marked by a reduction in the amplitude of EEG and EMG signals (relaxation). Lower temperatures accelerated anaesthetic induction, but prolonged recovery time, indicating a direct dependence on temperature. All the animals showed physiological reversibility, and the temperatures tested were effective for short-term cryoanaesthesia in C. macropomum . Notably, temperatures of 0 °C and 5 °C were more suitable, promoting faster induction, lower initial excitability (as evidenced by EEG), and longer muscle relaxation, with gradual recovery. The controlled use of short-term cryoanaesthesia is a viable alternative for rapid handling procedures in C. macropomum .