RIASSUNTO
ABSTRACT
The study was conducted from January 2008 to December 2008, focused on spatial and temporal distribution of Mnemiopsis leidyi and zooplankton populations in the south-western area of the Caspian Sea. In summer and winter M. leidyi abundance and biomass values were between 1900 ± 1400 ind/m2 , 110 ± 220 ind/m2 and 370 ± 250 g/m2, 1.0 ± 1.3 g/m2 , respectively. The most individuals of a ctenophore (about 90%) were < 5 mm in length. A total of 19 zooplankton species were identified. Among them seven species were merozooplankton and 12 species were holozooplankton. The annual mean zooplankton abundance was recorded 5300 ± 1400 ind/m3 which were lower than the previous years. The mass occurrence of M. leidyi appears to be one of the most important factors that reduce the zooplankton species and abundance, hence degrade the Caspian Sea natural ecosystem.
INTRODUCTION
The Caspian Sea is the largest inland body of water in the world, sustaining large stocks of small commercially important zooplanktivorous and pelagic fish. In such a diverse ecosystem, a consistent, significant decrease in the number of grazing zooplankton is expected to decrease the pelagic fish stocks and their predators (Kideys et al., 2008). The biological diversity of the Caspian Sea and its coastal zone makes the region one of the most valuable ecosystems in the world. The Caspian Sea is fed mainly by the Volga River, which provides 82% of total reverie inflow and supports a large scale fishery composed primarily of small pelagic fish such as the kilka (Clupeonella spp.). Mnemiopsis leidyi is a highly fecund comb jellyfish that feeds extensively on zooplanktons. The main diet of ctenophore in the southwestern Caspian Sea was copepods (66%) during the summer and autumn (Bagheri and Sabkara, 2003), with a similar finding previously recorded in the Black Sea (Mutlu, 1999).