As the pace of globalization has accelerated and transportation has become easier in recent years, the movements and mobility of human beings, plants and animals gained momentum. Thus, accidental or deliberate transportation of alien species have also become easier and increasingly widespread.

Today, invasive alien species are one of the largest threats to global biodiversity in addition to economic, social and environmental problems they create in both aquatic and terrestrial environments. These species cause damage of more than $1.4 trillion a year to the world economy, which corresponds to 5% of the global economy. They cost Europe €12 billion Euro a year and this amount continuously increases.

In several countries, hundreds of species including aquatic-terrestrial plants, mammals, birds, reptiles, microorganisms, amphibians and fish species are defined as “alien, exotic or invasive species”. Invasive alien species refer to species which are introduced accidentally or deliberately into an ecosystem where they are not normally found and dominate the ecosystem by affecting biodiversity directly or indirectly, within a specific period of time.

The Convention on Biological Biodiversity defines invasive species as alien species which threaten ecosystems, habitats or other species by causing economic or environmental damages via its establishment and invasion.  Invasive species cause several problems in five continents. In Europe, there are over 12,000 alien species, 15% of which are invasive. It is found that almost 21% of alien species identified in European inland waters can be attributed to the aquarium fish species. In Europe, alien specifies cost almost 12,5 billion Euros including expenditures made for keeping invasive species under control, conducting researches on this issue, combating and monitoring these species in addition to their damage to agriculture, fishery, forestry and human health.

Turkey is home to 14 of the worst alien species, which are listed in the 100 of the World’s Worst Invasive Alien Species, published by the International Union for Conservation of Nature.

14 of the worst alien species found in Turkey are as follows : Eastern American mosquito fish (Gambusia holbrooki), Warty Comb Jelly or Sea Walnut  (Mnemiopsis leidyi), Veined Rapa Whelk (Rapana venosa), The Prussian carp or silver Prussian   Carp (Carassius gibelio), Zebra Mussel (Dreissena polymorpha), Water Hyacinth (Eichornia crassipes), Killer Algae (Caulerpa taxifolia), Rainbow Trout (Oncorhyncihus mykiss), The Crucian Carp (Carassius Carassius), Mozambique tilapia (Oreochromis mossambicus), Nutria (Myocastor coypus), Red-eared Slider (Trachemysscripta elegans), Ship Rat (Rattus Rattus) and African Sharptooth Catfish (Clarias gariepinus).

The opening of Suez Canal, effects of climate change and lack of biodiversity in the recipient environment have facilitated the establishment of new species. Alien plant and animal species have rapidly spread in the Mediterranean causing experts to have concerns about their growth pace. If these species continue to grow in such pace, the number of alien aquatic plant species may exceed the number of local species found in the Mediterranean Sea in future.  It is inevitable that commercial fishery will also face some challenges and damages since the Mediterranean species will disappear due to global warming, they will be replaced with the Red Sea species and the Black Sea will also become more like the Mediterranean Sea. Thus, fish stocks will be damaged even further. 790 new fish species arrived at the Mediterranean Sea after the opening of Suez Canal. The number of invasive alien species in the Eastern Mediterranean has reached 450.  

While the issue of invasive alien species has been heavily debated in the last decades, climate change has also rapidly increased the spread of these species.  While almost 350 fish species are found in our inland waters, 25 new fish species are introduced to our inland waters.  The number of fresh water fish species under threat is 49.

Since it is very difficult and costly to remove or eliminate these species from our inland waters once they are introduced, it is of great importance to be more careful in the process of fish releasing.   Turkey’s inland waters are under threat due to fish species released uncontrollably.  In Turkey, several lakes and rivers are under invasion of the alien species including mosquito fish, zander, the Prussian carp, silver Prussian carp, pumpkinseed, sunfish, stone moroko and zebra mussel, which are not native to the fauna of Turkey. These species, which invade fresh waters, feed on organisms and living creatures in the environment and compete with the native species for food while they drive many endemic and native species extinct by infecting the environment with diseases and parasites they carry.  They also eat fish eggs and fish larvae and pose a great threat to biodiversity.