CONCENTRATION AND PERSISTENCE OF OXYTETRACYCLINE IN SEDIMENTS UNDER A MARINE SALMON FARM

The concentration of oxytetracycline in the sediment under two adjacent cage blocks in a marine salmon farm was determined following the therapeutic use of the drug. The sediment cores were grey, indicating some build-up of organic material. Infaunal polychaetes were present as were mobile fauna including crabs, starfish and flat fish. There were no 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 that received 8.65 kg oxytetracycline during the treatment. Peak concentrations 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 with a sampling programme designed to determine the horizontal and vertical distribution of oxytetracycline under the whole cage block during and after a treatment where 175 kg of oxytetracycline were used over 12 days. Peak concentrations, in the top 2 cm of the sediments under the cage block, were 10.9 +/- 6.5 mug.g-1 and this declined at an exponential 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 agent had decreased at all levels in the sediment 14 days later. At the end of the treatment oxytetracycline was detected in an area of the sediment 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 both pelleted fish feed and fish faeces. Oxytetracycline was confined to the area of sediment predicted to be subject to feed deposition that was directly under and slightly to the west of the cage block. Oxytetracycline was not detected in the area predicted to be subject to faecal deposition only.