Hydrochemical studies of the trout farming area in Kondopoga Bay of Lake Onego were carried out in the winter of 2022. Surveys at the deep-water site near the cages revealed dissolved oxygen starvation (0.8 mg/l) and a high content of ammonium (0.410 mg N/l) and mineral phosphorus (787 μg P/l, or 97% of total phosphorus) in the near bottom level, which is comparable to wastewater from the Kondopoga pulp-and-paper mill. At other sites located in the trout farming area and at K50 located in the zone affected by Kondopoga PPM wastewater, which spreads over the bottom of the bay in winter, the dissolved oxygen content varied from 6.8 to 13.1 mg/l, the concentrations of NH₄⁺ (up to 0.068 mg N/l) and P_min (up to 51 μg P/l) were significantly lower than near the cages. Our data indicate the trout farms have a significant effect on the water chemical composition, confirmed by a previously detected long-term trend for an increase in TP content and intensive growth of phytoplankton in the central part of Kondopoga Bay.