It is widely known that in 1989 a tremendous biomass of Mnemiopsis leidyi was reported in the Black Sea, At the same time, drastic declines in the pelagic fish stocks were reported. Several authors, overlooking the rapid development of the fisheries industry in the Black Sea, pointed out that the new invader was the major factor responsible for the fisheries collapse in the Black Sea. This study examines the development of the Black Sea fisheries industry, along with the ecological changes that were taking place, and evaluates its effect on the ecosystem. A set of balanced steady-state models, corresponding to the periods of the 1960s to 1970s, the 1980s, before the outburst of the Mnemiopsis leidyi, and the 1990s, which reflect the present state of the ecosystem, are used to study the successful establishment of the gelatinous organisms in the Black Sea. Using these models, a series of experiments are performed in order to explore the role of each major ecological group within the Black Sea ecosystem at different stages in time over the last 30 years. The budget calculations suggest a minimal role of gelatinous species on the decline of the fish stocks, contrary to the general belief. More interestingly, the model results indicate that the decline in the fish stocks was as a consequence of overfishing and that ever-increasing plankton productivity associated with eutrophication during the 1980s led to the outburst of gelatinous organisms.