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In recent decades, due to fluctuations in sea levels, environmental pollutions, and invader species invasion, the ecosystem of the Caspian Sea has been changed. In this study, assessing impact on ecosystem functiuons was conducted according to the new invader species (Mnemiopsis leidyi) by using commercial catches during three periods: 1996-2000, 2001-2006 and 2007-2011; in three phases of the invasion: establishment, expansion and adjustment, respectively. The results showed that in the three periods the average of catch of bonyfish were 15700.0±1993.2, 18430.8±3857.6 and 19176.6±1579.6 mt which was no significantly different among them (p>0.05) but the catch of kilkas (75080.0±16171.7, 25129.5±10441.5 and 21105.2±5199.2 mt, repectively) and sturgeons (1010.2±242.8, 385.5±264.3 and 51.8±19.9 mt, respectively) declined sharply and were significantly different among them (p<0.05). During three periods, ecosystem functions had a wide ranged change. The ecosystem functions were shifted E0 (No impact) in first period to E2 (moderate impact) and E3 (strong impact) in the second and third periods. Loss functions of two keystone species (anchovy and bigeye kilka) can be one of the resesons of sharp decline in upper layer of food web chain, i.e. sturgeons and Caspian seal in the Caspian Sea.  

Keywords: Mnemiopsis leydi, Ecosystem function, Caspian Sea, Iran

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