Status of alien species in the Mediterranean and Black Sea

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Moreover, in November 2009, a few individuals of a jellyfish from the Red Sea, Cassiopea
andromeda, were found on the shore of Kilyos, just off the Istanbul Strait.
1.3.3 Impacts of alien species on pelagic fisheries in the Black Sea
Alien species in the Black Sea have been examined in several studies. There is, however, no accurate
data to explain their impacts on the fisheries of some commercial pelagic fishes, such as bluefish,
mackerel, and bonito, and commercial demersel fishes, such as whiting, turbot, red mullet, and striped
mullet. On the other hand, the impacts of M. leidyi and B.ovata on small pelagic fish, such as anchovy
and sprat, have been studied intensively as described below.
Sprat
Sprat, Sprattus sprattus, is one of the most abundant and commercially important pelagic fish species
in the Black Sea, and it serves also as an important food source for larger fish as well (Ivanov and
Beverton, 1985). Sprat reaches maturity at one year and reproduces during the whole year, but its peak
spawning takes place between November and March.
Its spawning during winter and spring in deeper layers was relatively unaffected by M. leidyi because
of its low biomass in those deep layers. Thus, there is little competition for prey and predation on sprat
eggs and larvae (Shylakov and Daskalov, 2008).
In summer, the juvenile and adult sprats leave the upper warmed layer, avoiding severe competition
for food with other plankton-consumers including M. leidyi. During this period, their preferred food
consists mainly of the cold-water Calanus and Pseudocalanus copepod species living below the cold
intermediate layer of the water column. It should be noted that these preys are also available to M.
leidyi as they migrate to the thermocline at night for their daily feeding where they can be consumed
by the ctenophore. This can partly explain the reduction of the sprat stock during the Mnemiopsis
population outburst in the early 1990s. Like the other commercial stocks, heavy overfishing took also
place before and during the M. leidyi outbreak, aggravating the stock depletion (Prodanov et al.,
1997).
Black Sea anchovy
The Black Sea anchovy, Engraulis encrasicolus, is the most important commercial pelagic fish species
distributed over the whole Black Sea. In October-November, it migrates to the wintering grounds
along the Anatolian and Caucasian coasts and forms dense wintering concentrations until March,
becoming subject to intensive commercial fishery.
Anchovy competes for food with M. leidyi (Grishin et al., 1994) and this competition probably further
affected the anchovy population growth (Oğuz et al., 2008). The initial outbreak of M. leidyi was
reported in 1988–89 in the Black and Azov Seas. It appears that the catastrophic reduction of the
Black Sea anchovy stocks in the late 1980s was due to the combined effect of two factors: the
excessive fishing and M. leidyi outburst (Grishin et al., 2007). It is noteworthy that the sharp decline in
anchovy catch happened after the outburst of M.leidyi at the end of 1980s and the early 1990s
(Niermann, 2004) (Figure 7). In addition, Kideys (2002) mentioned that as Mnemiopsis feeds on the
eggs and larvae of the anchovy, it was responsible for the collapse of the anchovy fisheries.