Residue depletion and temperature- and dose-dependent withdrawal period of Oxolinic acid in Japanese eel Anguilla Japonica following bath administration under laboratory and field conditions.

Oxolinic acid (OA) is a quinolone class antimicrobial agent widely used in aquaculture, yet its off-label application via bath administration in Japanese eel (Anguilla japonica) has raised concerns regarding tissue residue persistence and appropriate withdrawal periods (WTs). This study aimed to investigate the residue depletion kinetics and calculate the WTs of OA in A. japonica following 12 h bath treatment under different water temperatures (20 °C and 28 °C) and exposure concentrations (10 and 20 mg/L). Laboratory trials were conducted under controlled conditions using recirculating systems, and additional field validation was performed at a commercial aquaculture farm. Residue concentrations in muscle plus skin were quantified using validated LC-MS/MS, and depletion data were modeled using first-order exponential regression. The calculated depletion half-lives ranged from 5.46 to 18.24 days, showing temperature- and dose-dependent variation. Corresponding WTs, derived from 95/95 statistical tolerance limits, were 78.8 and 115.7 days at 20 °C, and 27.4 and 55.9 days at 28 °C for the 10 and 20 mg/L groups, respectively. The WT estimated under farm conditions (47.3 days) closely matched the laboratory-derived value, demonstrating that laboratory-based models can reasonably predict field outcomes under similar conditions. All calculated WTs markedly exceeded the currently approved 25 days WT for oral administration of OA in eels, suggesting that extended WTs may be warranted following immersion exposure. These findings support the establishment of temperature- and concentration-dependent WTs under the conditions studied, which may help improve regulatory compliance and consumer protection in eel aquaculture.