Abstract: The study was conducted in Lake Lansk (surface area - 1042 ha, maximum depth - 53 m), in which the former dominant was vendace. Commercial catch results from the past decade indicate that there has been a seventy-two-fold decreased in vendace stocks. Hydroacoustic estimates indicate that there are 7.9 million fish in the pelagic zone. Approximately 93% of these inhabit the epilimnion (6.8 thousand fish·ha^-1), which is 7.6 times larger than that in the hypolimnion. There were 2.5 times fewer fish in a similarly sized southern region of the lake in 2004 than in 2001. Using the numbers, species structure, and individual weight, the biomass of the fish inhabiting the pelagic zone was calculated at 194.7 kg·ha^-1. The largest share was of vendace at 45.7% (89.0 kg·ha^-1), followed by roach at 18% (35.0 kg·ha^-1) and bleak at 13.1% (25.5 kg·ha^-1). The share of 0⁺ age group vendace was determined to be 70%. This might explain the decrease in commercial vendace catches caused by poor environmental conditions and overfishing. It also forecasts an improvement in resources of this species in the coming years.