Abstract: Hydroacoustical monitoring of vendace stocks in lake Pluszne is performed regularly since 90-ties. However, in 2009 the lack of oxygen below the thermocline prevented fish to occupy the hypolimnion, which is its natural habitat. This led to a mixture of vendace and other fish species above the thermocline. The trawl catches accompanying hydroacoustical studies have contained exclusively vendace and stickleback. Investigation of TS have shown two-pick distributions, one corresponding to the size of vendace and one smaller. The two fish species were separated by thresholding. The maps of fish spatial distributions confirmed that vendace was present only in the deepest part of the lake, which is typical for this part of a year, while the other fish were distributed over the whole lake area. The worsening of environmental conditions in Lake Pluszne (increase of eutrophication) leads to declining vendace population.