Antibacterial residues in marine sediments and invertebrates following chemotherapy in aquaculture

Abstract

Salmon net-cage culturists in the USA use oxytetracycline, and to a lesser extent Romet^® 30 and amoxycillin, to prevent or treat bacterial disease. This study examined the environmental fate of oxytetracycline and Romet^® 30 at three farm sites and in flow-through, sediment microcosms dosed with antibacterials at rates intended to mimic farm conditions. The frequency of detection of oxytetracycline in sediments beneath the farms paralleled drug usage, with residues rarely detected beneath a farm that used very little oxytetracycline (8.5 kg active ingredient), and concentrations commonly between 0.5 and 4 μg g⁻¹ at a farm that used 186 kg in a single prophylactic treatment period. The presence of oxytetracycline residues in surficial and subsurface sediments even before that treatment indicated persistence since at least the prior summer (10 months previous) or possibly longer. The area of sediments containing measurable oxytetracycline residues was very localized, however, with residues detectable only under the cages and to a distance of 30 m, but absent from a 100 m site. In laboratory microcosms, one-fourth to one-half of the oxytetracycline remained in microcosm sediments after 60 days, and the one treatment in which loss of the drug approximated an exponential curve indicated a 36-day half-life. Sulfadimethoxine and ormetoprim, the active ingredients in Romet^® 30, appeared to be very short-lived in marine sediments, based on preliminary data. Residues of both drugs at concentrations slightly above the analytical level of detection were found in one microcosm 2 days after the cessation of treatment, but no residues were found in microcosms 22–34 days after treatment or 21–62 days after treatment at two farm sites. We also collected crabs and oysters from the area surrounding the one farm that relied upon antibacterials most heavily. No more than trace oxytetracycline residues (about 0.1 μg g⁻¹) were found in oysters (Crassostrea gigas) or Dungeness crab (Cancer magister) collected under the farm, but about half the red rock crab (Cancer productus) collected under the cages during and within 12 days of oxytetracycline treatment contained oxytetracycline in meat at concentrations of 0.8 to at least 3.8 μg g⁻¹, well in excess of the US Food and Drug Administration limit for commercially sold seafood of 0.1 μg g⁻¹.