Water temperature differentially affects muscle residue depletion of doxycycline and flumequine in olive flounder (Paralichthys olivaceus).

Water temperature can affect antimicrobial residue depletion in ectothermic fish, but responses may differ among compounds. This study evaluated muscle residue depletion of doxycycline and flumequine in olive flounder (Paralichthys olivaceus) at laboratory temperatures of 10, 19, and 28 ℃ and under commercial field conditions at approximately 19 ℃. In the laboratory experiment, doxycycline was administered orally at 50 mg/kg once daily for 3 days, and flumequine at 20 mg/kg once daily for 7 days. A total of 300 fish were assigned to six temperature-compound groups (50 fish/group), and muscle samples were collected from 10 fish per group on days 3, 5, 10, 20, and 30 after the final administration. Residues were quantified using a validated LC-MS/MS method. Doxycycline residues showed significant temperature-dependent differences (P < 0.05) and were consistently highest at 10 ℃. On day 3, mean doxycycline residues were 65.7% lower at 19 ℃ and 72.0% lower at 28 ℃ than at 10 ℃; temperature-effect coefficients were negative at all sampling points (-0.119 to -0.072), with 95% confidence intervals below zero. Flumequine showed a different pattern: the highest measured concentration within the sampling schedule occurred at 28 ℃ on day 3 (P < 0.05), and this concentration was 5.1- and 3.8-fold higher than those at 10 and 19 ℃, respectively, before decreasing by 91.4% by day 5. The temperature-effect coefficient for flumequine was positive on day 3 and negative from days 5 to 30, with 95% confidence intervals above zero on day 3 and below zero thereafter. Because sampling began on day 3, this value should not be interpreted as the true peak concentration. Field residues were lower than corresponding laboratory residues. These findings indicate that water temperature does not simply accelerate residue depletion uniformly, but reshapes observed residue profiles in a compound- and sampling-phase-dependent manner.