Oxytetracycline depletion from skin-on fillet tissue of coho salmon fed oxytetracycline medicated feed in freshwater at temperatures less than 9 degrees C

Oxytetracycline (OTC) is a broad spectrum antibacterial agent approved in t he USA for treating certain bacterial diseases in salmonids cultured in fre shwater at temperatures greater than or equal to 9 degreesC. This study was conducted to provide the information necessary to expand the OTC label to include treatment of diseased salmonids cultured in freshwater at temperatu res below 9 degreesC. The study was designed to treat juvenile coho salmon (Oncorhynchus kisutch) with OTC-medicated feed and determine the depletion of OTC from the skin-on fillet tissue. Oxytetracycline depletion was evalua ted in juvenile coho salmon (weight range, 13-62 g) fed OTC-medicated feed at a rate of 88.2 mg OTC/kg body weight/day for 10 days. Pairs of skin-on f illets were taken from individual fish on days 4 and 10 during the treatmen t phase and on days 1, 4, 8, 14, and 19 during the depletion phase. Water t emperatures during the study period ranged from 4.1 degreesC to 8.5 degrees C. The OTC concentrations in medicated feed and skin-on fillets were determ ined with high-performance liquid chromatography methods. The maximum mean OTC concentration in fillet tissue was 932 ng/g, I day after the last treat ment and decreased to 32 ng/g 19 days after the last treatment. The log-lin ear loss of OTC from the fillet tissue was biphasic with a terminal phase h alf-life of 4.9 days. Published by Elsevier Science B.V.