DISTRIBUTION OF THE NEW INVADER MNEMIOPSIS SP AND THE RESIDENT AURELIA-AURITA AND PLEUROBRACHIA-PILEUS POPULATIONS IN THE BLACK-SEA IN THE YEARS 1991-1993

Since the outburst of Mnemiopsis sp. in 1988-1990 in the Black Sea, tw o international cruises in June 1991 and in July 1992, and a subsequen t survey in August 1993, have determined the distributions of both the invading Mnemiopsis sp. and the resident Aurelia aurita and Pleurobra chia pileus. Aurelia aurita and Pleurobrachia pileus displayed the low est and the highest population densities of 3-14 individuals m(-2) and 172-523 individuals m(-2), respectively. The abundances of Mnemiopsis sp. varied from 12-45 individuals m(-2). The lower ends of the ranges were found in June 1991, whilst the highest abundances occurred in Ju ly 1992 and August 1993. Despite the differences in abundance, the ave rage biomass of all three species was about 200 g m(-2) wet weight in July 1992 and August 1993, though in June 1991 biomass values were app roximately half those found in succeeding years. The abundances and bi omass of Mnemiopsis sp. were more than three times higher in the easte rn than in the western Black Sea during 1992 and 1993. The biomass of Mnemiopsis sp. and Aurelia aurita decreased in 1992 and 1993 compared to its value in the 1980s, but the biomass of the deep-dwelling Pleuro brachia pileus doubled between 1990/1991 and 1993.