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The data of long-term ecological studies (1999–2014) are used for assessing the impact of a new Caspian invader Mnemiopsis leidyi A. Agassiz, 1865 on marine and anadromous fish species. The introduction of the ctenophore and its massive development in the Caspian Sea have mainly affected planktophagous fish, especially the most abundant species anchovy sprat (Clupeonella engrauliformis Borodin, 1904) and sturgeons (Family Acipenseridae). Mnemiopsis leidyi is one of the main negative factors preventing the restoration of the populations of Caspian fishes, especially anchovy tyulka. The critical state of the resources of the spawning parts of sturgeon populations (Family Acipenseridae) has been reinforced as a result of the massive development of M. leidyi.

The main focus of review: To determine the Status of invasive species with special attention to the most aggressive invader Mnemiopsis leidyi in the Caspian Sea.

Objectives:

1. To assess vectors, pathways of invasive species introduction, their composition and their role in the Caspian ecosystem.

2. To analyze seasonal and interannual dynamics of the Mnemiopsis leidyi population in the all areas of the Caspian Sea; its impacts on the Caspian ecosystem including food recourses (zooplankton) and fish stocks along with other emerging environmental threats such as increased nutrient load from the coast, structural and quantitative changes in phyto-and-zooplankton communities, the appearance of harmful algal bloom (HAB) in the Southern Caspian.

3. To prepare recommendations for follow-up actions on invasive species management: - for the protection and control of the Caspian Sea from any sources of accidental invasive species introductions - for possibility of biological control of invader Mnemiopsis leidyi.

The primary source of this Review has been national reports from every of the five Caspian littoral countries, which included results of national observations of ecosystem state and biodiversity of the Caspian Sea. But due to fragmental data of the national observations on invasive species