Opportunistic Settlers and the Problem of the Ctenophore Mnemiopsis Leidyi Invasion in the Black Sea. GESAMP Reports and Studies No. 58

This report examines the invasion of opportunistic marine species, focusing on the ctenophore Mnemiopsis leidyi in the Black Sea, within the broader context of global biological invasions and human-mediated transport such as ballast water discharge. It analyses ecological conditions that enabled the invasion, including eutrophication, environmental degradation, and low biodiversity, and assesses the species’ distribution, reproduction, and feeding behaviour. The document evaluates impacts on zooplankton, fish populations, and fisheries, highlighting severe declines in pelagic stocks and ecosystem imbalance. It outlines monitoring approaches and reviews control strategies, particularly biological control, alongside the need for coordinated international research, management, and prevention measures to limit further spread and ecological disruption.