Electroencephalographic Response in Juvenile Tambaqui, Colossoma macropomum, Exposed to Short-Term Anaesthetic Baths with Geraniol and Citronellol

SIMPLE SUMMARY: Anaesthetics have been increasingly used for live fish manipulation to facilitate management, reduce stress and for welfare purposes. Plant derived natural compounds have been recommended as alternatives to synthetic drugs. Herein, juveniles of tambaqui fish, Colossoma macropomum, were exposed to geraniol and citronellol, which have been proposed as promising natural products for fish anaesthesia. The electroencephalographic response was characterised upon short-term exposure of fish to both compounds in anaesthetic concentrations, and throughout recovery in clean water. While geraniol-exposed fish showed an adequate anaesthetic effect, with depression of the central nervous system and a gradual recovery of the brain electrical activity, fish exposed to citronellol had an altered electroencephalographic response during induction, somewhat incompatible with an appropriate anaesthetic effect and smooth recovery. ABSTRACT: The aim of this study was to evaluate the level of neuronal depression in juvenile tambaqui, Colossoma macropomum, exposed to geraniol (GRL) and citronellol (CTL) in immersion baths. A total of 36 juveniles weighing 35.2 ± 9.4 g were used, organised into six experimental groups: I—control (clean water); II—ethanol (water containing the highest volume of ethanol used in the anaesthetic pre-dilution); III—GRL induction (70 µL·L(−1)); IV—CTL induction (90 µL·L(−1)); V—GRL recovery; VI—CTL recovery. Electroencephalographic (EEG) recordings were performed for 300 s in each group. EEG tracings of the control and ethanol groups showed regular and similar activity. Upon exposure to the anaesthetics, irregularities were observed in the tracings showing neuronal excitability and increased amplitudes, mainly in the case of CTL. Overall, GRL-exposed fish showed depression of the central nervous system with low and regular tracings throughout induction, presenting a gradual recovery and stable tracings, which were consistent with an adequate general anaesthetic effect. On the other hand, fish exposed to CTL showed altered EEG activity during induction, that could be considered incompatible with an appropriate anaesthetic effect and smooth recovery, presenting high and irregular EEG tracing amplitudes.