Impacts of invasive ctenophores on the fisheries of the Black Sea and Caspian Sea

Gelatinous zooplankton are known to have a substantial effect on pelagic food webs by exerting topdown control on their ecosystems (Purcell and Decker, 2005), but their impact could be even greater when they invade other regions. The effects of zooplankton invasions have been well documented for the Black Sea and very recently for the Caspian Sea. Until the 1980s, three species of gelatinous macrozooplankton were present in the Black Sea: two scyphozoan medusae (Rhizostoma pulmo and Aurelia aurita) and only a single ctenophore species (Pleurobrachia pileus) (Kideys and Romanova, 2001). Then, in the early 1980s, the ctenophore Mnemiopsis leidyi (Figure 1a), already known as a voracious zooplankton feeder (Reeve et al., 1978), was transported (likely via ballast waters) to the Black Sea from its native waters of the western Atlantic Ocean. Achievement of peak biomass levels of this ctenophore at the end of the 1980s caused the onset of striking adverse ecological events in the Black Sea, such as abnormal oscillations in plankton dynamics (Kideys et al., 2000) and, most notably, a sharp decrease in the pelagic fi shery (Kideys, 1994). By the end of the 1990s, the accidental introduction of another ctenophore to the Black Sea, Beroe ovata (already known to feed exclusively on other ctenophores) (Figure 1b, 1c, 1d), coincided with another set of striking events, but this time it was benefi cial for the ecosystem and the fi shery.