Recirculating aquaculture system (RAS) is a novel alternative to classical aquaculture that allows for maximum control of the growing conditions as well as reducing the water utilized. However, the increased recirculation of the water leads to a potential accumulation of emergent contaminants in the system. A possible solution has been tried with the addition in the system of peracetic acid, a strong oxidant used as a disinfectant. In this study, the occurrence of 34 emerging contaminants (ECs), belonging to 5 different classes (benzophenones, bisphenols, Parabens, benzotriazoles, and benzothiazoles) were investigated in an Atlantic salmon post-smolt (Salmon Salar) farming RAS. ECs were analyzed to evaluate their occurrence and evolution in the different matrices of the system which included water from the tanks, produced sludge, fish food, and liver extracted from Atlantic salmons. The target analytes were extracted using ultrasound-assisted solid-liquid extraction coupled to Solid-phase Extraction (SPE) and liquid-liquid extraction (LLE) and subsequently analyzed in ultra-high performance liquid chromatography coupled to tandem mass spectrometry (UHPLC-MS/MS). Satisfactory extraction efficiency was achieved for the ECs in all the matrices, although the best precision was accomplished in the aqueous matrix. 28 out of 34 ECs were found and their occurrence profiles showed a high concentration of parabens chemicals, their derivatives, and 1-H-BTR in fish food as well as in all the matrices of the system. Moreover, a high occurrence of 2-OH-BTH was found in the system although it is was not found either in food or make-up water added into the system. The evaluation of the occurrence of ECs over time under exposure to peracetic acid showed an increase in oxidated species such as 2-OH-BTH and 3,4-DHB where 1-H-BTR exhibited a slight diminution. These observations were translated into salmon’s liver in the increase of 2-OH-BTH and high occurrence of 1-H-BTR (range of 385-653 ng g-1) whereas no parabens were detected in the animal’s tissue.This thesis provides the first approach to the occurrence of ECs in RAS. Further research on the transformation that occurred under peracetic acid is needed to understand the distribution of these pollutants in the system as well as further studies in the post-smolt salmon, including other tissues.