Oxytetracycline residues in a freshwater recirculating system

When oxytetracycline (OTC) medicated feed is fed to fish in a recirculating aquaculture system, antibiotic residues could accumulate in fish tissue, water, biofilter sand and sediment to a greater extent than in single pass or serial reuse aquaculture systems. In two trials, oxytetracycline-medicated feed (3 g active ingredient per pound of feed) was fed to adult rainbow trout at 1% b.w. per day for 10 days. OTC residues were assayed in fish muscle (with skin attached), water, sediment (e.g., fish feces, uneaten feed) and biofilter sand. For both trials, oxytetracycline was detected during the 10 days of treatment in all matrices assayed. In trout muscle, OTC concentrations increased to an average of 1.8 etag/g by day 10 of treatment and then declined to <0.2 μg/g by 21 days posttreatment. For water entering and exiting the biofilter, OTC concentrations increased to 0.5 μg/ml by day 10 and was not detectable (<0.001 mug/ml) by 21 days post-treatment. For biofilter sand, OTC concentration was approximately 14 mug/g by day 10 and decreased to < 2 μg/g by 21 days posttreatment. In sediment samples, OTC concentrations increased to 1900 μg/g by day 10 and declined to <2 mug/g by 21 days post-treatment. In this system, OTC concentrations in trout muscle were well below 2 mug/g by 21 days after withdrawal of the drug. After input of medicated feed to the system was stopped, OTC concentrations in water, sediment and the biofilter declined and did not increase during the post-treatment period. (C) 2002 Elsevier Science B.V. All rights reserved.