The vertical distribution of maraena whitefish (Coregonus maraena) early juveniles in different times of day in a newly created oligotrophic lake

Diel vertical distribution of strictly pelagic juvenile (23–47mm total length) maraena whitefish Coregonus
maraena (Bloch, 1779) was repeatedly investigated in spring primarily using hydroacoustics in the artificial postmining
Most Lake in the Czech Republic. At the same time, an ichthyoplankton trawl was used to identify
acoustical targets. During the day, fish performed extensive shoaling behaviour in depths between 2 and approximately
40m and were not accessible for trawling. By evening, with decreasing light intensity, shoals started
to disintegrate and at night fish were relatively homogeneously distributed in the water column from the surface
down to a depth of 40 m. Juvenile maraena whitefish could be caught by trawl as the only fish species at night.
Shoaling behaviour started again approximately 1.5 h before sunrise. The data showed steep decreases in fish
density between the two surveys in spring which indicates significant mortality of early juvenile coregonids as a
result of poor availability of zooplankton in a highly oligotrophic post-mining lake