## 4 Conclusions  
The results of assessing concentrations of PCBs in the water of Lake Baikal obtained within the framework of conventional monitoring and biomonitoring confirmed the background level of PCB concentrations. The average concentration and statistically significant range of the detected \(\Sigma_{7}\)PCB concentrations in Baikal water were 0.30 and 0.26-0.34 ng/L, respectively. This concentration level was higher than in the waters from the Arctic and Antarctic areas but is comparable with or much lower than the PCB concentrations in the waters of continental lakes. Biomonitoring of PCBs using _C. migratorius_ as a bioindicator in the lake water revealed five dl-congeners that were present in the water below LOQ of the method used. The most toxic congener 126 (the toxicity equivalence factor, TEF\({}_{2005}\) is 0.1) was found in single individuals of the older age group. The total concentrations of dl-congeners in _C. migratorius_ ranged from 1.3 to 1.7 ng/g (ww). The total toxicity equivalent of the detected dl-congeners (except for congener 126) was within the WHO-TEQ\({}_{(2005)}\) pg/g (ww) range from 0.03 to 0.06. This level of dl-PCBs corresponded to their concentrations in fish inhabiting European alpine lakes but was much lower than in fish from the Baltic Sea. In the course of choosing a system for monitoring PCBs in Baikal water, the preliminary stage of the study included water sampling at 21 stations (water surface area 31,700 km\({}^{2}\)). Based on the analysis of the monitoring results, five reference stations were selected for water sampling: 3, 6, 11, 16, and 19. The reduction in the number of stations did not distort the characteristics for the assessment of the PCB concentrations in the lake water. The control of PCBs in the surface water was possible by determining the concentrations of two of the seven indicator congeners, 52 and 101, because the concentrations of these congeners were highly correlated with each other, all indicator congeners, and \(\Sigma_{7}\)PCBs. They also have the maximum level of concentrations in water samples, and homogenous peaks of PCBs were recorded in chromatograms. According to the data on the correlation analysis, the congeners identified in _C. migratorius_ were divided into three groups, and we proposed indicator congeners for these groups: congener 101 (correlation group from congeners 118-dl, 153, and 180), 138 (correlation group of congeners 105-dl, 123-dl, and 156-dl) and 52 (correlation group of congeners 28, 114-dl, and 126-dl). The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app12042145/s1](https://www.mdpi.com/article/10.3390/app12042145/s1)]([https://www.mdpi.com/article/10.3390/app12042145/s1](https://www.mdpi.com/article/10.3390/app12042145/s1)), Table S1: Sampling stations, Table S2: Characteristics of the _C. migratorius_ individuals caught in the course of biomonitoring, Table S3: Concentrations of indicator congeners in the water, ng/L, Table S4: Concentrations of indicator congeners in _C. migratorius_, ng/g (ww), Table S5: Concentrations of dl-congeners in _C. migratorius_. ng/g (ww), Table S6: Results of the analysis of the data from conventional monitoring by the PERMANOVA method, Table S7: Results of the analysis of the data from biomonitoring by the PERMANOVA method, Table S8: Relative error statistics for repeated measurements of PCB concentrations in water and _C. migratorius_, Table S9: Statistical parameters of the average values of PCB concentrations in water and _C. migratorius_. A.G.G. : concept of the study, methodology, writing, and preparation of the original draft; O.V.K. : water and fish sampling, chromatography-mass spectrometric analysis; Y.S.B. : statistical analysis. All authors have read and agreed to the published version of the manuscript. This study was carried out within the framework of the State Task, project No. 0279-2021-0005 (121032300224-8). Not applicable. All primary data are available in tables Tables S3-S5.