Oxytetracycline residues in channel catfish: a feeding trail

Channel catfish were fed a sinking diet containing 3% oxytetracycline (OTC) once-a-day (SID) or twice-a-day (BIO) for 2 and 3 weeks and liver and muscle residues were determined. HPLC analysis for muscle indicated mean OTC levels of 0.052 ppm (0-0.15) for control, 0.242 ppm (0-2.95) for SID, and 0.979 ppm (0-11.0) for BID-dosed fish. Liver was 0.003 ppm (0-0.08) for control, 0.188 ppm (0-0.52) for SID, and 1.022 ppm (0-9.3) for BID-dosed fish. The Charm 11 assay detected OTC (control point set at 100 ppb) in 6.9% of muscle and 10.0% of liver in controls, 47.4% of muscle and 56.0% of liver in the SID group, and 78.0% of muscle and 80.0% of liver in the BID group. An interfering peak on HPLC was detected in 24.0% of the muscle samples of controls, but not livers, which affects this assessment. Liver false negative rate was 29.0% and false positive rate was 13.0%, and setting the optimal control point changed the false positive rate to 12.9%. A positive correlation (0.946) for muscle and liver OTC levels was calculated, indicating liver may be a suitable alternative to muscle for screening OTC in channel catfish. The CITE PROBE Tetracycline test kit failed to reliably detect OTC in catfish muscle homogenates.