The "Dnipro-Donbas" Channel is an important link among the Ukrainian reservoirs, it was created for irrigation of the steppe zone of Ukraine. To maintain the state of the channel at a high appropriate ecological level and to prevent water quality reduction the measures with use of fish biomeliorants (silver carp and bighead carp, grass carp, common carp) are applied. In May and August 2017 in the "Dnipro-Donbas" hydrotechnical channel, due to unknown reasons, there was a local mass death of the silver carp (Hipophthalmichthys molitrix). As a result of complex hydrobiological and ichthyopathological studies it was discovered that in the spring the feed base of fish-microfitophages is poor, the base of biomass of phytoplankton in the channel is multicellular blue-green algae and Diatoms. A small-sized protococcal algaе, valuable for silver carp, is not found in the plankton (except of one species-Pediastrum duplex). As a result, there are cases of starvation of Hipophthalmichthys molitrix during the spring season. It is noted that prolonged starvation of the silver carp causes not only the disorder of metabolic processes connected with gastrointestinal digestion, but also leads to a disturbance of the respiratory process of fishes, which also serves as an indirect cause of the fish death, even with sufficient oxygen content in water. In the summer the local mass death of Hipophthalmichthys molitrix in the channel occurs due to the complex action of negative factors, including the instantaneous intake of blue-green algae from inlet chamber, the excessive development of bacterial flora and synergistic enhancement of the toxic effect on the fish organism. Arrangements are proposed to prevent the mass death of fish-microfitophages (silver carp Hipophthalmichthys molitrix) in the "Dnipro-Donbas" channel in the spring and summer during periodic pumping of water along the route of the canal.