CONCENTRATION AND PERSISTENCE OF OXYTETRACYCLINE IN SEDIMENTS UNDER AMARINE SALMON FARM

The concentration of oxytetracycline in the sediment under two adjacen t cage blocks in a marine salmon farm was determined following the the rapeutic use of the drug. The sediment cores were grey, indicating som e build-up of organic material. Infaunal polychaetes were present as w ere mobile fauna including crabs, starfish and flat fish. There were n o significant accumulations of undigested feed pellets under the cages . At one block where 186 kg oxytetracycline was used over 10 days the oxytetracycline concentrations were determined under a single cage tha t received 8.65 kg oxytetracycline during the treatment. Peak concentr ations in the top 2 cm of the sediment were 9.9 +/- 2.9 mug.g-1. This declined at an exponential rate (r2=0.99) with a half-life of 16 days. At the second block the oxytetracycline concentration was measured wi th a sampling programme designed to determine the horizontal and verti cal distribution of oxytetracycline under the whole cage block during and after a treatment where 175 kg of oxytetracycline were used over 1 2 days. Peak concentrations, in the top 2 cm of the sediments under th e cage block, were 10.9 +/- 6.5 mug.g-1 and this declined at an expone ntial rate (r2=0.99) with a half-life of 13 days. Nineteen days after the end of the therapy oxytetracycline was detected at depths of up to 8 cm in the sediment, but the concentration of the antibacterial agen t had decreased at all levels in the sediment 14 days later. At the en d of the treatment oxytetracycline was detected in an area of the sedi ment less than twice the area of the cages themselves. Data on current flow and sedimentation rate were used to generate a predictive model of the area of sediment that would be subject to the deposition of bot h pelleted fish feed and fish faeces. Oxytetracycline was confined to the area of sediment predicted to be subject to feed deposition that w as directly under and slightly to the west of the cage block. Oxytetra cycline was not detected in the area predicted to be subject to faecal deposition only.