Biological: Eradication may be impossible in practice. A variety of predators (including medusae and fish) consume M. leidyi in its native regions. Reduction of M. leidyi populations in the Black Sea occurred after one of its predators, the ctenophore Beroe ovata, was introduced to the region (Costello, 2001).
One of the factors that provoked high level of population development of M. leidyi in the Black Sea but was not observed within its natural range-estuarial waters of North America was the absence of a predator feeding on M. leidyi and controlling its population size (Purcell et al., 2001). In 1997, another invader, the ctenophore Beroe ovata Mayer 1912, was found in the northeastern Black Sea. It is a predator feeding on planktivorous comb jellies - especially M. leidyi (Konsulov and Kamburskaya, 1998). As with its predecessor, B. ovata arrived with ballast waters from the same coastal waters of North America (Seravin et al., 2002). Development of B. ovata considerably decreased the population of M. leidyi that had deformed the Black Sea ecosystem for over a decade. The reduction of the M. leidyi population limited its influence on the ecosystem and consequently we observed a recovery of the main components of the Black Sea pelagic ecosystem – zooplankton (including meroplankton), phytoplankton, dolphins and fish as well as their eggs and larvae (Shiganova et al.,2000a,b; 2001 c).
Conscious of this, and bearing in mind the devastating impact of M. leidyi on the fisheries in the Black and Azov Seas in the 1990s, we began a number of initiatives in 2001 with a view to take stock of the situation, review and assess remedial measures and take concrete actions. After deliberation, we proposed the introduction of a potential predator of M. leidyi as the only truly viable option. As shown by the example of the Black Sea, the best – and so far only - candidate for this is another ctenophore species, Beroe ovata. After the accidental introduction of Beroe ovata to the Black Sea, the abundance of M. leidyi here immediately dropped to levels so low that no further damage was inflicted. In fact, the ecosystem almost immediately began to recover. It is anticipated that the results of a Beroe ovata introduction in the Caspian will be similar. Summer 2003 is now the target date for the implementation of this plan (Dumont and Shiganova, unpublished).