The article presents annual data (1999-2016) of ecological surveillance on the influence of newish for the Caspian Sea invader Mnemiopsis leidyi (A. Agassis, 1865) on marine and anadromous fish species. There has been given the analysis of unfavourable effect of invader Mnemiopsis leidyi (A. Agassis, 1865) on the abundance of marine, anadromous, semi-anadromous fishes and Caspian seal. There have been considered mechanisms, sequence and degree of impact of ctenophoran on ichthyophauna and marine mammals via a nutritive base. It has been stated that mnemiopsis distribution corresponds to seasonal distribution of zooplankton and meroplankton, whereas ctenoforan being an organism with aggressive "r-strategy" of development influences ichthyophauna both directly (eating pelagic roe and larvae of sea fish) and indirectly (via nutritive base). Drastic reduction of catches of the Caspian sprat, in particular anchovy sprat and big-eyed sprat, is stipulated by nutritional competence between these fish species and ctenoforan Mnemiopsis leidyi. In turn, decrease in number of the Caspian sprat resulted in change of the nutrient budget of the sturgeon (Russian sturgeon, stellate sturgeon, beluga) and in decrease of their number and commercial stocks. There have been offered the solutions to the ecological problem and measures to restore sturgeon stocks. Results of the research can be used in forecasting commercial fish species stocks in the Caspian Sea and in determining the damage from annual invasions of Mnemiopsis ( Memiopsis leidyi (A. Agassis, 1865)) in the northern part of the Caspian Sea.