Oxytetracycline (OTC) is a broad spectrum antibacterial agent approved for 
treating certain bacterial diseases in salmonids cultured in water temperatures 
greater than or equal to 9 °C. A prominent disease of juvenile salmonids is 
coldwater disease. Although coldwater disease may be effectively treated with 
OTC, this disease typically occurs at water temperatures below 9 °C. The study 
was conducted to provide the information necessary to expand the OTC label to 
include the treatment of diseased salmonids cultured at water temperatures below 
9 °C. The study objectives were to determine the time after treatment (feeding 
fish OTC medicated feed) when the OTC concentration in the skin-on fillet tissue 
of juvenile coho salmon (Oncorhynchus kisutch) treated with OTC decreased to a 
concentration below the tolerance limit of 2000 ng/g and to determine the 
depletion rate for OTC in the skin-on fillet of juvenile coho salmon treated 
with OTC. To fulfill these objectives, juvenile coho salmon were cultured and 
fed OTC medicated feed at a rate of 88.2 mg OTC per kg body weight per day at 
water temperatures ranging from 4.1 to 8.5 °C on the Quilcene National Fish 
Hatchery (a U.S. Fish and Wildlife Service hatchery), located in Quilcene, WA. 
This rate of treatment was slightly greater than the maximum rate of 82.7 
mg/kg/day designated on the label. Pairs of skin-on fillets were taken from 
individual fish sampled on days 4 and 10 during the treatment period, one day 
after the last treatment, then four, eight, 14, and 19 days after the last 
treatment. The OTC concentration in skin-on fillet samples were determined with 
a high performance liquid chromatography analytical method developed at the 
Upper Mississippi Science Center. The maximum mean OTC concentration was 932 
ng/g one day after the last treatment day and the minimum mean OTC concentration 
was 32 ng/g observed 19 days after the last treatment day. The loss of OTC from 
the fillet tissue was biphasic with a terminal phase half-life of 4.9 days.