Alien or allochthonous species are not naturally found in a certain area, but instead came there by human activity, either intentionally or unintentionally.
When an alien species causes changes in a certain area and endangers the local biological diversity, it becomes invasive. Invasive species can quickly take over territory and outcompete the local species for food sources, interbreed with local species, and introduce new diseases and pests. However, it is also possible that the alien species is exterminated by a local predator.
In Croatia, there are about 30 alien freshwater fish species, some of which are considered invasive. These have been introduced from Asia, the Americas, other European countries, and Afrika. Moreover, many of the domestic fish species have been transported from waters where they appear naturally, to those where they could never have come without human intervention, and in these new waters, they are alien. In most cases, these translocations have been species from the Danube Basin into the rivers of the Adriatic Basin.
Invasive fish species are one of the major causes of extinction of local, native species, in addition to watercourse regulation, construction of dams, and amelioration works. There are different ways to assess the risk from introduced species, and the most accurate are those made for a specific ecosystem. There are still many unknowns, and the research in this still young science needs to develop.