First confirmed record of an established population of green swordtail (Xiphophorus hellerii Heckel, 1848) in Europe

The green swordtail (Xiphophorus hellerii) is a freshwater and brackish water fish native to North and Central America. It experiences wide human-traced introduction around the world, mostly due to aquarium release. Individuals of the green swordtail population from the warm spring with artificial origin located in Asparuhovo district, Bulgaria, were collected. The invasiveness screening tool for non-native freshwater fishes (FISK v2) calibrated for the Balkan region was used to assess potential invasion risk of the green swordtail to the coastal freshwater bodies of the Eastern Balkans. This study is the first confirmation on an existing self-sustained population of X. hellerii in Bulgaria and respectively in Europe. Climate change and the trend of warmer winters are prerequisites for the acclimatization of this species to the temperature out of the studied thermal spring. Therefore, the potential of the green swordtail to colonise new habitats is increasing with such facilitated acclimatization. We conclude that the green swordtail poses a high risk to become the next new invasive species in Southern Europe.