Tissue distribution and elimination of oxytetracycline in seawater chinook and coho salmon following medicated-feed treatment

Abstract

The time course of oxytetracycline (OTC) tissue residues were studied in two different species of the pacific salmon: the chinook salmon ( Oncorhynchus tshawytscha) and the coho salmon ( Oncorhynchus kisutch). The chinook salmon were treated with OTC-medicated feed at the rate of 75 mg OTC kg −1 body weight per day for 21 days in 9 °C or 15 °C seawater. The coho salmon were treated at the rate of 100 mg OTC kg −1 body weight per day for 42 day s in 10 °C seawater. Non-medicated diet was offered to the salmon at the conclusion of the OTC treatment period. The salmon were medicated by OTC for a prolonged period to establish the maximal OTC withdrawal time. At different times during and after OTC medication, five or six salmon were randomly selected from the tank and killed. The tissues of the salmon were analyzed for OTC by high-performance liquid chromatography (HPLC). The tissue distribution profiles of OTC in both species of the pacific salmon were found to be very similar; OTC tissue concentration decreased in the order liver > bone > kidney = skin > muscle at the conclusion of the OTC treatment period. However, OTC tissue concentrations in the chinook salmon were about twice as high as those of the coho salmon. OTC elimination from different tissues of salmon treated under similar experimental conditions could be described by a single temperature-dependent elimination rate constant. Our results also show that OTC concentrations in the muscle of chinook salmon fell below the HPLC detection limit of 0.05 μg g −1 at Day 41 and Day 65 post-dosing when the acclimation temperatures were 15 °C and 9 °C, respectively. These results confirm the current Canadian guidelines on OTC withdrawal time for farmed salmon.