Northern Mediterranean region is characterised by an exceptional richness of the freshwater ichthyofauna. Many fish species of this region are endemic to a single or a few river basins. This is also the case of the Neretva river basin (Adriatic Sea slope), where 17 out of 34 native species are endemic solely to this river basin. However, these unique Mediterranean freshwater ecosystems are fragile and are susceptible to human-induced changes, including introduction of alien fish species. We report here a finding of the 32 nd alien fish species in the Neretva river basin, spirlin Alburnoides sp., which was found at two localities in Bosnia and Herzegovina. The spirlin species was identified by molecular means as so far unnamed species with the native range in the Sava river basin (Danube river basin, Black Sea slope). Based on the comparison of cytochrome b sequences, the introduced population originated most probably from nearby rivers of the Danube basin in Bosnia and Herzegovina (Vrbas and Bosna river basins). Such a high number of alien fish species reported in a single river basin is alarming and pointing to a necessity of raising public awareness, especially among local fishermen.

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