Status of alien species in the Mediterranean and Black Sea

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respectively. The former two assessments belonged to the initial commercial exploitation of this
ground, the latter to the period of the intensive fisheries. Reduction in the sea snail stocks from 1 500–
2 800 tonnes (virgin population) to 1 300 tonnes (exploited population) is the evidence of the impact
made by dredge fisheries. The use of knife-edge dredges adversely affected the bottom biocenoses
(Shlyakhov and Daskalov, 2008).
In 1994, the sea snail stocks were assessed along the southern and western coasts of Crimea from Cape
Ilya to Cape Evpatoriisky at 14 000 tonnes. The limit for its harvesting in the waters of Ukraine was
established at 3 000 tonnes (Shlyakov and Daskalov, 2008). Prodanova and Konsulova (1995);
Prodanov et al. (1995) presented the stock assessment and growth rate of Rapana in the Bulgarian part
of the Black Sea. They found out that in ten fishing regions the biomass of the commercial stock
(individuals with flesh weight above 60 g) and the total allowable catch of Rapana along the Bulgarian
coast during 1993 were 7 482.6 and 3 217.6 tonnes, respectively.
1.3.2 Alien invertebrate species and impacts on biodiversity
According to non-indigenous plankton species from the Black Sea, the prymnesiophyte Phaeocystis
pouchetii, was extremly abundant and clogging the gills of fish in the Bulgarian coast of the Black
Sea. There is also a list of the non-indigenous species proposed by Moncheva and Kamburska (2002),
although Gomez and Boicenco (2004) did not fully agree to the list.
The bivalve Mya arenaria, a native of the Northern Atlantic, was first detected in 1966 and became
very abundant in a short period of time in the northwestern and western part of the Black Sea, reaching
its peak abundance in 1972. Even though it was affected later adversely by regular hypoxia-anoxia
crisis that destroyed the entire benthos in the 1980s, it still retains considerable abundance in the
western coastal waters. This species is a competitor for habitats with small local bivalve, Lentidium
mediterraneum, which avoids sandy bottoms siltated by M. arenaria. Large amounts of washed
molluscs on the beach attract their consumers, such as gulls and crows.
Another bivalve of Indo-Pacific fauna, Anadara inaequivalvis, was found in the Black Sea in 1968,
and has spread to the whole basin. This species has a potential for commercial harvesting in Turkey
(Sahin et al., 2006).
In 2001, two new non-native bivalvia species were found in the Odessa Bay: edible Mytilus edulis and
Mytilus trossulus (Alexandrov, 2004). M. edulis was brought probably with ballast waters from the
Mediterranean, where it is cultured off the Spanish and Italian shores. A Pacific species M. trossulus
was brought probably with ships from Far East Russian coasts, where it is a main cultivated species
(Suprunovich and Makarov, 1990). Shipworm, Teredo navalis, one of the oldest alien bivalve species
found in the Black Sea, is in small numbers and its impacts are not significant at present because wood
is replaced by concrete or metallic underwater construction and also because ship worms become rare.
Other alien species such as ivory barnacle, Balanus eburneus, and acorn barnacle, Balanus improvises,
are typical organisms of fouling communities, which may have an adverse effects on the net cages of
sea bass aquaculture in Trabzon on the Turkish Black Sea coast.
A Mediterranean jellyfish has become a threat for humans in the Marmara and Black Sea. Compass
jellyfish Chrysaora hysoscella (Linnaeus, 1767), a temperate planktophagous species, was firstly
reported from the Sea of Marmara by Inanmaz et al. (2002). This species made a large bloom in the
Marmara Sea, Istanbul Strait and Turkish part of the Black Sea in July 2009 (Öztürk and Topaloglu,
2009). Most of beach bathers used fishing nets to protect themselves from this stinging jellyfish.
While this species is venoumous, this needs to be monitored in the Black Sea in terms of the impact on
human health and interrelation with the entire Black Sea biota. It seems that C. hysoscella could
establish its population in the Black Sea shortly.