Cryoanesthesia in tambaqui Colossoma macropomum: Behavioral and electrocardiographic responses

Abstract

Cryoanesthesia is a low-cost procedure that has advantages for use in fish, however, there are only a few studies dealing with the electrophysiological evaluation and its safety-related aspects. The relationship between behavior and electrocardiographic activity allows for the monitoring of functional changes in cardiac activity in fish submitted to low temperatures. This study aimed to describe the behavior and cardiac effects of cryoanesthesia in juvenile tambaqui (Colossoma macropomum) through electrocardiographic assessment. For this purpose, 27 juveniles of tambaqui (28.9 ± 4.2 g) were used and essayed into two experiments. Experiment 1: behavioral evaluation (n = 9) and Experiment 2: electrocardiographic recordings with control group (n = 9), and fish submitted to water at 5 °C in baths followed by subsequent recovery at a temperature of 26 ± 2 °C (n = 9). Each cardiac activity recording lasted 1200 s, after which fish were immediately transferred to a bath in water at 26 ± 2 °C, for 600 s. Heart rate, QRS amplitude, QRS complex duration and RR, PQ and QT intervals were analyzed. The results showed that four tracing patterns emerged which characterized the induction phases of cryoanesthesia throughout exposure to water at 5 °C. In phase 1, there was a decrease in the amplitude of the QRS complex. In phase 2, there was an increase in the amplitude of the QRS complex and maintenance of sinus rhythm. Phase 3 showed a gradual loss of cardiac function with the onset of an atrioventricular blockade. In phase 4, a cardiac arrest occurred, albeit reversible. Heart rate decreased as exposure time was longer at low temperatures. The fish recovered completely from cryoanesthesia after the increase in temperature, showing normal heartbeats, and QRS complex duration, QT, RR, and HR intervals being restored with maintenance of sinus rhythm, indicating recovery of fish hemodynamics. Overall, tambaqui juveniles at 5 °C showed a time-dependent and gradual reduction of motor activity with loss of postural reflex and immobility. Cardiac response was a function of temperature and progressed to a reversible cardiac arrest. During recovery, stages similar to those of induction were observed, but in the opposite direction, until recovery of the postural reflex. Cryoanesthesia can be used as an alternative method for the immobilization of C. macropomum.