Caspian Sea: State of the Environment 2019

5.5.3. Invasive species

In the Caspian Sea (Southern, Middle and the south-western part of the Northern Caspian), Mnemiopsis affects all levels of the ecosystem in one way or another, completely restructuring it. The numbers and diversity of zooplankton (the main food source for Mnemiopsis leidyi) are de- clining significantly. In 2015, Mnemiopsis biomass and numbers on the western coast of the Southern and Cen- tral Caspian were higher than they had been throughout the 15-year period between 2001 and 2016. This is having an adverse impact on the formation of food resources for fish which feed on zooplankton and zoobenthos. Recent studies have established that the basis of the Mnemiopsis habitat in the Caspian Sea was formed in the Southern Caspian, where it oc- curs all year round, unlike in other areas. As the weather gets warmer in spring, the Mnemiopsis population increases due to the growth of indi- viduals and the commencement of reproduction. The population spreads first to the southern part of the Middle Caspian in April and May, then to the northern part of the Middle Caspian in June and July, reaching the Northern Caspian in late July or early August. Monitoring conducted by Azerbaijan over a pe- riod of 13 years (2001–2013) identified the prin- ciples behind its distribution along the western coast of the Southern and Middle Caspian. About 60 per cent of Mnemiopsis is concentrated south of where the Kura River flows into the sea. In gen- eral, higher concentrations of Mnemiopsis are ob- served in the water column closer to the surface. At depth, 50 per cent of Mnemiopsis are concen- trated at 10 m and 87 per cent at up to 75 m. Juve- nile Mnemiopsis, in the 0–5 and 6–10 mm rang- es, make up 95 per cent of the population in the Middle Caspian and 91 per cent in the Southern Caspian. The maximum Mnemiopsis size in the Middle Caspian is 36–40 mm and in the Southern Caspian – 51–55 mm (Zarbaliyeva et al. 2016). Recent studies have produced important out- comes, such as the use of molecular genetic tech- niques to study invasive corridors and invasion

impact and in invasion control, the use of infor- mation technology and modelling of invasive spe- cies population trends, the course of the invasive process in the Black and Caspian Seas, data on the accumulation of chemical elements by the inva- sive species and their impact on water quality. These studies demonstrated that, as the popula- tion of the ctenophore Beroe (Beroe abyssicola) grows, the number of Mnemiopsis may decrease, and the influence of Beroe on other elements of the food chain will be negligible. Special mea- sures to combat the accidental introduction of Beroe into the Caspian Sea are not, therefore, recommended. The Scientific Council of the Russian Federation Interdepartmental Ichthyological Commission (March 2015) issued a summary of the outcomes of the scientific study on the impact of Mnemi- opsis on fish stocks (“On the impact of alien spe- cies on the state of sturgeon stocks in the Caspian Sea”) and confirmed its conclusions.

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