## 3 Results and Discussion
### Assessment of the PCB Concentration in the Water of Lake Baikal
#### 3.3.4 Toxicity Equivalence for Dioxin-like PCBs  
In Baikal water, we did not identify congeners of polychlorinated dibenzo-p-dioxins and dibenzofurans exceeding the detection limit of 0.01 pg/L [11]. Only one dl-PCB congener, 118, was identified during the monitoring of PCBs between 2015 and 2021. Its concentrations ranged from \(\leq\)10 to 110 pg/L. In seasons with the extreme concentrations of PCBs, its concentration in the water increased to 350 pg/L. The toxicity of this congener was assessed by the dioxin toxicity equivalents (TEQs) for dl-PCB within the WHO-TEQ (2005) pg/L range from 3 \(\times\) 10\({}^{-4}\) to 3.3 \(\times\) 10\({}^{-3}\). In seasons with the extreme concentrations, the WHO-TEQ (2005) pg/L value reached 0.01 in May 2015 in the southern basin and 7 \(\times\) 10\({}^{-3}\) in September 2019 in the northern basin of Lake Baikal. In the muscle tissues of _C. migratorius_, 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, and for congener 126, the value was two orders of magnitude higher, up to 4.7. 3.5 Comparative Analysis of the PCB Concentrations in the Surface Water of the Background Sites and in Bioindicator Fish  
Currently, there is no common criterion for assessing the pollution of water by PCBs. According to the interim environmental quality standard (EQS), the PCB concentration in the surface waters of the European Union should not exceed 1 ng/L [71]. According to the standards established in Russia, the PCB concentration in water bodies for drinking and domestic water use should not exceed 1000 ng/L, but the PCB concentrations are not allowed in the water bodies for fishery purposes [72]. According to the standards of the US Environmental Protection Agency [73], the PCB concentration in water less than 14 ng/L does not pose a danger to aquatic organisms and human health. The level of PCBs in Baikal water was higher than in the water from the Antarctic and Arctic regions but comparable with or much lower than the concentrations of PCBs in waters of the continental lakes such as Como (Italy), the Caspian Sea (northern part, Russia), and Lake Baiyangdian (China) (Table 1). The use of fish to assess the pollution of different water bodies by PCB pollutants based on the level of their accumulation in fish was a rather difficult task due to the discrepancy between the characteristics of fish (age and the number of lipids) as well as to the methods used for the detection of PCBs (number of congeners). We compared the accumulation level of \(\sum\)?PCBs in the muscle tissue of _C. migratorius_ with the accumulation of \(\sum\)?PCBs in _S. trutta fario_ inhabiting alpine lakes of Southern and Central Europe. The species _S. trutta fario_ was the closest to _C. migratorius_ in terms of taxonomic characters. Individuals corresponding in age and lipid content were taken for comparison. Comparable levels of \(\sum\)?PCBs in C. _migratorius_ and _Salmo trutta_ indicated the same concentration level of bioavailable PCBs in Baikal water and alpine lakes [30; 31; 32; 33]. In the tissues of _Salmo trutta_ from Lake Iseo and Lake Endine situated in the industrial regions of Northern Italy [22], the \(\sum\)?PCB value was three to seven times higher than in _C. migratorius_: 2600, 1100, and 330 ng/g (lw), respectively.