Graded concentrations of lidocaine hydrochloride in the modulation of behavioral, cardiac, and muscular responses of the Amazon freshwater fish tambaqui (Colossoma macropomum)

The present study was designed to characterize the behavior and evaluate the electromyographic and electrocardiographic responses of tambaqui (C. macropomum) exposed to different concentrations of lidocaine hydrochloride in immersion baths and after transfer to anesthetic-free water. For this purpose, fish juveniles (25.38 ± 6.5 g) were acclimated to aquariums for 15 days under controlled environmental and water quality conditions. Four concentrations of lidocaine hydrochloride (LA) (150, 175, 200, and 225 mg/L LA) were used in immersion baths to evaluate behavioral, electromyographic, and electrocardiographic changes during short-term anesthetic induction (5-min exposure) and in a washout. Upon exposure to LA, and irrespective of concentration, fish showed loss of the righting reflex, along with significant myorelaxation, which was partially achieved at concentrations of 150 and 175 mg/L LA and fully attained at 200 and 225 mg/L LA. Notably, a decreased heart rate and alteration of the T wave tracing occurred at higher concentrations. Notwithstanding, although protocols using LA at 225 mg/L or above could require greater care in monitoring, the prompt reversibility of such responses in recovering fish indicates that LA can be safely used in immersion baths for tambaqui C. macropomum as an alternative to other anesthetics, especially those in which important side effects have been documented.