Residual depletion of eugenol as an anesthetic in growing phase pacu (Piaractus mesopotamicus)

The aim of this study was to evaluate the pharmacokinetics of eugenol in pacu (Piaractus mesopotamicus) juveniles following exposure to an immersion bath. Pacu juveniles (n = 42) with an average weight of 546.40 ± 74.51 g and total length of 25.43 ± 1.52 cm were subjected to a bath with 100 mg L^-1 eugenol previously dissolved in 95% ethanol (1:10). After a 10-min immersion, the fish were transferred to anesthetic-free aquaria for recovery. Blood, kidney, muscle, spleen, gills, and brain samples were collected at intervals 0, 0.5, 1.0, 3.0, 6.0, 12.0, and 24.0 h post-anesthesia. Pharmacokinetic parameters were calculated using non-compartmental methods. The highest eugenol concentrations were recorded immediately after anesthesia (0 h) in all tissues, in descending order: plasma > kidney > muscle > spleen > gills > brain. The plasma and brain half-lives of eugenol were 1.12 h and 0.73 h, respectively, with elimination constants of 0.62 and 0.95 per hour. In muscle and kidney tissues, the half-lives were 0.14 h and 0.09 h, with elimination constants of 4.97 and 7.44 per hour, respectively. In the spleen and gills, the half-lives were 0.21 h and 0.23 h, with elimination constants of 3.26 and 3.07 per hour. In conclusion, eugenol is rapidly absorbed and eliminated in growing phase pacu, reaching peak concentrations soon after anesthesia and showing a significant reduction up to 24 h.