Oxytetracycline Residues in Tissues of Cultured Eel and Ayu and the Effect of Cooking Procedures on the Residues

The tissues levels of oxytetracycline were determined after oral administration (50mg/kg) in cultured eel (Anguilla japonica) and ayu (Plecoglossus altivelis). The water temperature was adjusted to 28°C for eel and 18°C for ayu. Oxytetracycline levels were determined by HPLC. Concentrations of oxytetracycline were in the order of bone>liver>skin>muscle>serum for eel and liver>bone>skin>muscle>serum for ayu. The elimination half-lives of serum, muscle and liver were 4, 6 and 11 days for eel, and 5, 7 and 6 days for ayu, respectively. The elimination times of serum, muscle and liver were calculated to be 4, 5 and 25 days for eel, and 10, 14 and 24 days for ayu, respectively. However, the elimination half-lives and elimination time of skin and bone could not be calculated because oxytetracycline residues persisted in skin and bone in both fish species, and no elimination phase could be recognized up to 30 days. Residual oxytetracycline was reduced by 70-80% in muscle and liver by the usual methods of cooking (boiling, baking and frying), whereas the reduction in bone was only 30-50%.