TISSUE DISTRIBUTION AND ELIMINATION OF OXYTETRACYCLINE IN SEAWATER CHINOOK AND COHO SALMON FOLLOWING MEDICATED-FEED TREATMENT

The time course of oxytetracycline (OTC) tissue residues were studied in two different species of the pacific salmon: the chinook salmon (On corhynchus tshawytscha) and the coho salmon (Oncorhynchus kisutch). Th e chinook salmon were treated with OTC-medicated feed at the rate of 7 5 mg OTC kg(-1) body weight per day for 21 days in 9 degrees C or 15 d egrees C seawater. The coho salmon were treated at the rate of 100 mg OTC kg(-1) body weight per day for 42 days in 10 degrees C seawater. N on-medicated diet was offered to the salmon at the conclusion of the O TC treatment period. The salmon were medicated by OTC for a prolonged period to establish the maximal OTC withdrawal time. At different time s during and after OTC medication, five or six salmon were randomly se lected from the tank and killed. The tissues of the salmon were analyz ed for OTC by high-performance liquid chromatography (HPLC). The tissu e distribution profiles of OTC in both species of the pacific salmon w ere found to be very similar, OTC tissue concentration decreased in th e order liver > bone > kidney = skin > muscle at the conclusion of the OTC treatment period. However, OTC tissue concentrations in the chino ok salmon were about twice as high as those of the coho salmon. OTC el imination from different tissues of salmon treated under similar exper imental conditions could be described by a single temperature-dependen t elimination rate constant. Our results also show that OTC concentrat ions in the muscle of chinook salmon fell below the HPLC detection lim it of 0.05 mu g g(-1) at Day 41 and Day 65 post-dosing when the acclim ation temperatures were 15 degrees C and 9 degrees C, respectively. Th ese results confirm the current Canadian guidelines on OTC withdrawal time for farmed salmon.