As a result of eutrophication caused by increased nutrient input via major rivers during the last few decades, the Black Sea ecosystem has been subject to extreme changes in recent years. These changes first became evident in the 1980's, with abnormal phytoplankton blooms and a large increase in medusae (Aurelia aurita) biomass. Then, the introduction of a new species (a lobate ctenophore, Mnemiopsis sp.) into the Black Sea radically affected the whole ecosystem. This species competes with anchovy for the edible zooplankton as well as possibly consuming anchovy eggs and larvae in the Black Sea. The mass occurence of Mnemiopsis appears to be one of the most important reasons for the sharp decrease of anchovy and other pelagic fish stocks in the Black Sea. Although the future of the Black Sea ecosystem seems rather bleak, it is suggested that in addition to reducing anthropogenic impact, systematic studies are essential if the Black Sea fisheries are to recovery.