Spatial distribution of oxytetracycline and elevated frequencies of oxytetracycline resistance in sediments beneath a marine salmon farm following oxytetracycline therapy

The concentrations of oxytetracycline and the frequencies of oxytetracycline resistant microorganisms were determined in 11 samples taken from the sediments in the vicinity of a block of fish cages at a marine salmon farm. The cage block contained 10 tonnes of Atlantic salmon smells and a total of 20 kg of oxytetracycline were administered during the 12 day treatment. Samples cores were collected by divers 5 days after the end of the period of therapy and the top 2 cm of each core was analysed. HPLC analysis was able to quantify the oxytetracycline concentrations in three of the six samples taken directly under the cage block. The mean concentration under the cage was between 0.65 and 1.2 mu g g(-1) (n = 6) depending on the values attributed to samples where the concentrations were below the level of quantitation (1.2 mu g g(-1)). In the five samples taken from locations not directly under the cage block oxytetracycline was only detected in the sample taken adjacent to, and down current from, the cage block. This sample was collected 10 m to the west of the cage block and contained 4.2 mu g g(-1) oxytetracycline. These data indicate that oxytetracycline was confined to an area of the sediment which was smaller in extent than the area of the cage block itself. The frequencies of resistance to oxytetracycline in the microflora cultured from the samples were determined by differential plating on 2216V media, containing 25 and 100 mu g ml(-1) oxytetracycline. Analysis of eighty three samples from sites free of overt human influence demonstrated that the background levels of resistance at these two selection concentrations were 1.3 +/- 1.3% and 0.4 +/- 0.6%, respectively. Elevated frequencies of resistance were detected in samples from a wider area than the cage block. The median frequency of resistance in the samples (n = 6) taken from directly under the cage block was 1.4% at 100 mu g ml(-1) and 5.3% at 25 mu g ml(-1). In the samples (n = 5) taken from outside the cage block the frequencies were 5.3% at 100 mu g ml(-1) and 8.5% at 25 mu g ml(-1). There was no correlation between the concentration of oxytetracycline in a sample and the frequency of resistance that was determined in the culturable microflora in that sample.