Abstract: Vendace (Coregonus albula L.) is an ecologically important element in the ecosystem of low trophy deep lakes. Investigations were carried out in Pluszne lake (surface area 900 ha, maximal depth 51 m) using Simrad EY-M (70 kHz) and EY 500 split beam (120 kHz) echosounder. Fish species identification was based on pelagic trawling. Water temperature and dissolved oxygen content were measured at 1 meter depth intervals. In June oxygen saturation of the upper hypolimnion reached nearly 100% while in October it dropped to 8% and approached the sublethal level for fish. The autumnal thermal and oxygen stratification caused that 80% of the fish became trapped in 10 m water stratum. The 20-fold decrease in the living and feeding space area for coregonids in autumn was the likely cause of their weakened condition and low reproduction. This happens directly preceding spawning and is the probable cause of decreased occurrence, or even disappearance of coregonids from many lakes.