Status of alien species in the Mediterranean and Black Sea

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alien species impacted also on marine biodiversity, mainly by habitat competition and species
displacement.
Regional cooperation is essential to minimize and reduce the impacts of alien species both in the Black
and Mediterranean Seas. In this context, regional and international organizations should establish a
common alien species database easily accessible to all stakeholders or should harmonize already
existing databases on alien species. An early warning system needs to be developed for unpredictable
alien species blooms or impacts. Countries should collect also catch statistics of most commercial
alien species for better fisheries management. Specific studies mostly on the impacts on fisheries by
alien species are highly required to develop fisheries reporting and monitoring system. Besides, for
some countries, it is highly recommended to implement capacity building programmes in terms of
species identification and data collection. Finally, key species and key habitats, for example,
Posidonia meadows, should be protected to combat alien species invasion.
INTRODUCTION
The invasion of alien or exotic species in the Black and Mediterranean Seas has been recorded for
many years. It has speeded up in recent years, with many examples of negative impacts on marine
ecosystems, on the local marine fauna and flora and on socio-economic activities, such as fisheries.
The Black and Mediterranean Seas are interconnected by the Turkish Straits System. These narrow
straits act as a biological corridor, a barrier or an acclimatization zone for marine species. Shiptransported
species, such as Rapana venosa, or introduced species to the Black Sea, such as Liza
haematocheila, dispersed to the Mediterranean Sea. Since the Suez Canal opening, the Mediterranean
Sea has been connected also to the Red Sea, thus the Indian Ocean, and allowed a massive invasion of
tropical fauna to the mostly eastern Mediterranean Sea. Several Indo-Pacific species (lessepsian
migrants) have entered and started to colonize in the western Mediterranean Sea besides the Aegean,
Marmara and Black Seas. The Black and Mediterranean Seas, due to strong temperature gradients
spatially from Gibraltar to Kerch Strait, temporally tropical in summer and temperate in winter,
represents a suitable area for the introduction of species from different origin and region.
In recent years, the fish composition and fish catch amount have changed due to the alien species.
Moreover, some poisonous fish and invertabrate species also established themselves mostly in the
eastern Mediterranen Sea and start causing problems for human health and biodiversity. As it
happened in the Black Sea, some intentionally- or unintentionally-introduced species are found in the
Mediterranean Sea. It is foreseen that alien species might affect more seriously the fisheries of the
Black and Mediterranean basin in the coming years.
In this review, the Black and Mediterranean Seas are examined separately to be more concise,
although they are interconnected each other both physically and ecologically.
1. STATUS OF ALIEN SPECIES IN THE BLACK SEA
1.1 Main characteristics of the Black Sea
The Black Sea is one of the world’s most isolated seas from the major ocean and the largest anoxic
body of water on planet (87 percent of its volume is anoxic). The total surface area of the Black Sea is
423 000 km 2 and its catchment area is over 2 million m 2 . It is surrounded by Turkey, Bulgaria,
Romania, Ukraine, the Russian Federation and Georgia (see Figure 1). On the north-eastern corner, the
Black Sea is connected to the Sea of Azov through the Kerch Strait, and on the south-western corner,
to the Sea of Marmara through the Istanbul Strait (Bosphorus). The maximum depth is
2 212 m. The most striking characteristic of the Black Sea is probably the high level of hydrogen
sulphide (H2S). The level of H2S is 150–200 m deep and has been relatively stable, although seasonal
and annual fluctuations have been observed.