Long-range changes in oxytetracycline concentration and bacterial resistance towards oxytetracycline in a fish farm sediment after medication

Following 10 days medication with oxytetracycline, marine sediment was sampled beneath three selected cages (cages 1, 2 and 3) at a fish farm over a period of 18 mnd., in order to detect any change in the sediment oxytetracycline concentration, bacterial number and bacterial resistance towards the drug. The bulk of oxytetracycline disappeared during the first weeks, but it persisted in the sediment at lower concentrations for quite some time after the medication. Half-life ( f 1 2 ) of oxytetracycline in the sediment was measured as: 125, 144 and 87 days under cages 1, 2 and 3, respectively. At the end of the medication, all three sediments had > 100% oxytetracycline-resistant bacteria. This value dropped to 20% after 72 days and stabilised at levels of between 10 and 50%. The change in bacterial numbers, described as total and plate counts, was due to seasonal variations rather than to the medication.