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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

Sampling was conducted from the vessel Svetlomor 2 by personnel from AmC, the Ministry of Ecology and Natural Resources (MENR) and SOCAR. Sediment samples for physical and chemical analysis consisted of 2 replicates, while 3 replicates were taken for biological analyses. Analyses were carried out at AmC Caspian Environmental Laboratory.

The sediment characteristics and metal concentrations around Chirag were found to be highly variable within and between stations, with generally little change being observed from 2008. Despite the detection footprint remaining unchanged from previous years, LAO concentrations continue to reduce. THC and PAH concentrations were generally low, with the highest concentrations being recorded at stations directly to the northeast of the platform.

Ba concentrations were highest within the LAO contamination footprint, suggesting the higher concentrations to be the result of historical drilling discharges. As observed on previous years the concentration of Cd was also highest within this area. The LAO and Ba footprint, indicating the extent of contamination from platform drilling discharges was found to extend 1250m NE, 500m N, 750m E, 1000m W and 250m S of the Chirag platform position.

The 2010 macrobenthic community was numerically dominated by polychaetes and amphipods.

Abundance and species richness were highest at stations on the periphery of the survey area and decreased towards the platform. The lowest abundance an taxonomic richness were found at stations within the LAO and Ba footprint, with the sparsest communities being present at stations directly to the northeast of the platform. Despite a continual reduction in hydrocarbon concentrations from 2006, a general reduction in abundance and taxonomic richness has been observed at the Chirag survey area from 2000 to 2008. This negative trend has reversed in 2010, with a survey wide increase in annelid abundance and amphipod abundance and taxonomic richness being observed. No distinct change in the physicochemical characteristics has been observed between 2008 and 2010 that can be attributed to the abrupt reversal of these trends.

No additional physicochemical impacts from operational activities have been observed between 2008 and 2010 and in general contamination levels continue to reduce