Status of alien species in the Mediterranean and Black Sea

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Mediterranean sea – level temperature, then tropical invasive species would gain a distinct advantage
over the native fauna.
The majority of the marine flora and fauna of the Mediterranean Sea belong to the Atlanto-
Mediterranean Province. In recent period, however, numerous Indo-Pacific originated species attribute
to this sea certain tropical and subtropical characteristics. This peculiarity is clearly seen particularly
in the Levantine Basin. Some of the alien species may thrive in their new environment by displacing
the native species. The number of alien species entering the Mediterranean will probably increase in
the future and the gene pool of the recipient regions will also change. However, extinction of native
marine species due to the influence of exotic invaders has not been identified yet in both basins.
4.6 Intentionally and unintentionally introduced alien species in the mediterranean sea and
their impacts
Aquaculture is one of the important reasons for intentional introduction of alien species. In the
Mediterranean Sea, aquaculture is one of the growing sectors because of the high demand of seafood
and it is expected that this demand will increase in future. However, in bad weather conditions, some
of the seabass and seabream cages broke and a lot of fish escaped from the cages in winter in Greece,
Israel and Cyprus (UNEP/MAP/MEDPOL, 2004). Also in Turkey, mostly in the Aegean Sea, several
fish cages broke and several tonnes of fish escaped from the cages. Genetic impacts of these escaped
or released fish is a concern mostly for the genetic hybridization.
In the Mediterranean Sea, only two commercial invertebrate species, namely Crasostrea gigas and
Ruditapes philippinarum, have been introduced for aquaculture purpose between the 1960s and 1970s.
Catching of R. philippinarum in the Venice Lagoon has become a problem due to its disturbance to the
benthic community. Some problems of disease occur with Japanese oyster, C. gigas, which was
introduced to France. Intentional introduction may cause some problems and become a risk for human
health and marine biodiversity. Some species like pearl oyster, Pinctada radiata, were introduced
intentionally to Greece for aquaculture purposes (Serbetis, 1963). Japanese Oyster, C. gigas, is native
of the Northwest Pacific. It is introduced gradually in the Medterranean through aquaculture. It was
introduced in Tunisia in Lake Ichkeul for aquaculture purposes (Madhioub Zaouali, 1988). Alpbaz et
al. (1991) reported successful Crassostrea gigas aquaculture in İzmir, Turkey. Munoz –Izquierdo
(2009) reported that ten alien tunicates have become established and colonized some sectors of the
Mediterranean since 1958. They entered the Mediterranean Sea through the Strait of Gibraltar and
Suez Canal or introduced by shellfish culture.
Some accidental introduction has been observed with some species like Caulerpa taxifolia which
escaped from aquaria (Jousson et al., 1998). This species was introduced in 1984. It was observed for
the first time in Monaco and dispersed later in Spain, France, Italy and Croatia, to be reported finally
in Turkey, even different strain in the Gulf of Iskenderun (Çevik et al., 2007). Impacts of C. taxifolia
are the impoverishment of the Mediterranean algal communities which may reach 25–55percent; most
of autochthonous algae tend to disappear dramatically (Verlaque and Fritayre, 1994). The number of
polychaeta and especially amphipod species decreased in C. taxifolia meadows and, on contrary, the
species diversity of molluscs increased. The population of sea urchins, fish, amphipods and
polychaetes are also affected (Boudouresque et al., 1995). Francour et al. (1995) reported that
C. taxifolia meadows seem to be a favorable environment for the recruitment of some species of
Labriadae (Coris julis, Symphodus ocellatus), Sparidae (Diplodus annularis) and Serranidae (Serranus
caprilla) in fall. Meinesz and Boudouresque (1996) indicated that C. taxifolia is a major threat for the
coastal ecosystem of the Mediterranean Sea. Later, Meinesz (1997) reported that C. taxifolia forming a
dense cover of more then 1 ha in 5 years (1984–89) and since 1987, it has spread by vegetative
reproduction due to anchors and fishing activities (nets, trawls). He reported also that thirteen years
after its first observation in the Mediterranean Sea, 3 000 ha has been more or less affected.