The impacts of explosive jellyfish proliferations are quite well known: injured swimmers; clogged fish nets; feeding competition for fishes, especially for juvenile stages; predation of fish eggs and larvae. The possible causes of these increasing growths are somewhat controversial. One significant problem, presumably, is overfishing. Fish that feed on zooplankton are in feeding competition with the jellyfish. If the fish are absent, then the jellyfish have an abundance of available food. It is also known that jellyfish are more robust than many species of fish, especially in the sense that they can tolerate much lower oxygen concentrations. Oxygen deficiencies in the oceans, in turn, occur increasingly as a result of eutrophication and increasing concentrations of organic matter in the seas. As a result, more organic material sinks into the deep water where it is decomposed by oxygen-consuming microorganisms, a general decrease in oxygen and formatting of hypoxic events. Climate change, which causes warming of the ocean surface, can exacerbate this situation, and only small amounts of the oxygen consumed by microorganisms at greater depths are replaced. Thus, increasing jellyfish scourges could therefore be a result of combined stressors that lead to a restructuring of the marine ecosystems, which will disadvantage the classical food chain “phytoplankton – zooplankton – fish”. From these newly-created conditions of synergistic effects of overfishing and increased eutrophication, on the other hand benefits jellyfish abundance.