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Biological Invasions by Marine Jellyfish 241
Engraulis encrasiclolus fishery. In 1990, total harvest of this species was
66,000 t, which reflected a ~78 % reduction from 295,000 t harvested in 1988
(GFCM 1993, as referenced in Kideys 1994). It was hypothesized that M. leidyi
triggered the massive decline in the harvest of E. encrasicolus both by direct
feeding on eggs and larvae, and by competition with adults and larval
anchovies for zooplankton food (Vinogradov and Shushkina 1992; Kideys
1994). In the Caspian Sea, reductions in zooplankton biomass and diversity
followed Mnemiopsis blooms, to the extent that only the calanoid copepod
Acartia could be found in measurable concentrations (Stone 2005). Similarly
to harvest reductions in the Black Sea, catches of the zooplanktivorous fish
Clupeonella sp. dropped precipitously in the Caspian Sea (Kideys 2001a,
2001b, as referenced in Kideys 2002 and Stone 2005).
It was previously assumed that invasions into both the Black and Caspian
seas were mediated by ballast water discharge (Vinogradov et al. 1989; Ivanov
et al. 2000). Although it has been both insinuated (Kideys 2002; Shiganova et
al. 2003) and explicitly illustrated (http://www.zin.ru/projects/invasions/gaas/
mnelei_d.htm) that the invaded M. leidyi populations originated from the
Atlantic seaboard of the United States, we emphasize that there has been no
definitive evidence to support this. A recent genetic study indicated a source
region in the Southern US Atlantic (south of Cape Hatteras) or Gulf of Mexico,
and could rule out South America but not the Caribbean Sea as source of
the Black/Caspian Sea invaders (Bayha 2005). Given the widespread natural
range of this ctenophore, as well as its subtle population genetic structure, the
source region remains unresolved.
14.2.2 Beroë ovata
The invasion by Mnemiopsis leidyi was followed in 1997 by that of Beroë ovata
(Vinogradov et al. 2000), a ctenophore that feeds almost exclusively on other
ctenophores, especially Mnemiopsis (Kremer and Nixon 1976). With the invasion
of Beroë into the Black Sea, Mnemiopsis concentrations decreased
(Shiganova et al. 2001, 2003) and zooplankton concentrations, including fish
eggs and larvae, increased (reviewed by Kideys 2002; Gordina et al. 2005).
Given the perceived success the Beroë invasion has had in the Black Sea, there
have been calls for the purposeful introduction of this ctenophore predator
into the Caspian Sea. However, with the checkered past history of purposeful
animal introductions (Chap. 23), many have advised caution for the Caspian
Sea, until Beroë’s survival (Volovik and Korpakova 2004; Kideys et al. 2004a)
and trophic ecology (Kideys et al. 2004b) are studied further.