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®, 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 μg kg⁻¹, but not for a market with an MRL of 10 μg 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.

本研究的目的是估计土霉素 (OTC) 的停药时间 (WT)，将土霉素 (OTC) 腹膜内给药于两种鲑鱼，虹鳟鱼 ( Oncorhynchus mykiss ) 和大西洋鲑鱼 ( Salmo salar )。这是在鲑鱼养殖场在海水育肥阶段在田间条件下完成的。包括来自不同海水养殖场的七个网箱，每个网箱作为一个实验单元，用腹膜内剂量的 OTC（范围为 20 至 35 mg kg ^-1）。从每个海网箱中随机抽取10条鱼在处理后的每个采样点进行实验室分析，根据每个网箱的收获情况而变化。所有选定的鱼都用单一剂量的两种不同的商业 OTC 制剂处理；配方 A (OTC 30%) 和配方 B (OTC 20%)，均为额外标签​​。研究是根据 VICH GL 57 指南进行的。样品保持低温直至实验室分析。使用三重四极杆质谱仪 (Waters®, Xevo TQ-S micro) 分析均质样品。该分析方法针对实验室使用进行了优化，并按照欧盟委员会决议 2002/657/EC 对线性、回收率和精密度进行了内部验证程序。检测范围，包括检测限（LOD）和定量限（LOQ），

根据从残留消耗研究中获得的天数和上次采样观察到的天数，大多数采样的笼子的浓度允许兽药适合市场，最大残留限量（MRL）值为 200 μg kg ⁻¹，但不适用于 MRL 为 10 μg kg ^{−1 的市场}，正如预期的那样，无论使用的剂量和商业兽药如何。根据考虑的 MRL，虹鳟鱼和大西洋鲑鱼的停药时间分别为 5390 至 1676 度和 4061 至 1641 度。这些从现场样品中获得的估计值可以提供关于必须考虑的 WT 的参考，以避免大西洋鲑鱼和虹鳟鱼片中存在 OTC + 4-epi-OTC 残留，从而保护消费者健康。需要进一步研究以评估其他因素对不同鲑鱼物种野外条件下 WT 的影响。