An Investigation on the abundance and distribution of Mnemiopsis leidyi in the southwestern the Caspian Sea (Lisar, Anzali and Sefidroud)

The alien ctenophore Mnemiopsis leidyi which was transported from the Black Sea into the Caspian Sea by the end 1990s has been affecting the ecosystem in this new environment. In this study ,spatial and temporal distribution of M.leidyi was studied during 2001 to 2006 in three transects (Lisar, Bandar Anzali & Sefidroud) in the south of e Caspian Sea. Each transect had four stations located at 5, 10, 20 and 50 m bottom depth contours. Mnemiopsis leidyi achived maximum biomass (1411 ± 805 g/m2 ) in summer 2006. Minimum biomass (3.3±2.6 g/ m2 ) of the ctenophore was measured in winter 2001. The highest biomass was at stations with 20 m depth (1355±1072 g/m2 ) and lowest biomass (20-50 m layer 19.5±16.9 g/m2 ) was obtained at that 50 m depth. Small specimens (<5 mm) contributed about 90 % total abundance of the population. The maximum length was 70 mm. The greatest average biomass 461±354 g/m2 was measured in Sefidroud region and the smallest biomass (245±205 g/m2) observed in Anzali region. The variations of temperature and salinity were important parameters in fluctuation of abundance, biomass and size groups in different seasons and depth in the southwestern Caspian Sea