Abstract

The objective of this research study was to estimate the withdrawal time (WT) for oxytetracycline (OTC), administered intraperitoneally to two species of salmonids, rainbow trout (Oncorhynchus mykiss) and Atlantic salmon (Salmo salar). This was done in salmonid farms during the fattening phase in seawater, under field conditions. Seven cages from different marine farms were included, each as an experimental unit, treated with an intraperitoneal dose of OTC (in a range from 20 to 35 mg kg(-1)). From each sea cage, 10 fishes were randomly selected for laboratory analysis at each sampling point after treatment, which were variable according to the harvest of each cage. All selected fish were treated with a single dose of two different commercial OTC formulations; Formulation A (OTC 30%) and Formulation B (OTC 20%), both extra label. Study was performed according to VICH GL 57 guideline. Samples were kept cold until laboratory analysis. Homogenized samples were analyzed using a mass triple quadrupole spectrometer (Waters (R), Xevo TQ-S micro). The analytical method was optimized for use in the laboratory and an internal validation procedure was performed following European Commission Decision 2002/657/EC for linearity, recovery and precision. The detection range, including the limit of detection (LOD) and limit of quantification (LOQ), were calculated according to VICH GL49 guidance. According to the degree days obtained from the residue depletion study and those observed in the last sampling, most of the sampled cages had concentrations that allow the veterinary drug to be suitable for a market with a maximum residue limit (MRL) value of 200 mu g kg(-1), but not for a market with an MRL of 10 mu g kg(-1), as might be expected, regardless of the dose and commercial veterinary drug used. Depending on the considered MRL, withdrawal times ranged from 5390 to 1676- and 4061-1641-degree days, for rainbow trout and Atlantic salmon, respectively. These estimates, obtained from field samples, can provide references regarding the WTs that must be considered in order to avoid the presence of OTC + 4-epi-OTC residues in Atlantic salmon and rainbow trout fillets, and thus protect the consumer health. Further studies are needed to assess the impact of other factors on WT under field conditions in different salmonid species.