Alien fishes in some tributaries of the Maritsa river in Bulgaria.
The paper presents a study of alien fishes in the fish fauna of the Maritsa River tributaries' upper and middle zones. The study uses a large ichthyological sample, taken from four tributaries of the Maritsa River: the rivers Topolnitsa, Luda Yana, Stryama and Chepinska. The sample was collected in the period 2005-2011. Analysis identifies that out of 18 fishes, established in the studied rivers, three are invasive: the Prussian carp (Carassius gibelio), the Topmoouth gudgeon (Pseudorasbora parva), the Pumpkinseed sunfish (Lepomis gibbosus), and one is introduced: the Rainbow trout (Oncorhynchus mykiss). Alien species do not exceed 3% of the ichthyofauna composition of the studied rivers. It is identified that this pattern is partially caused by the fast water flow and absence of suitable habitats for these species. O. mykiss is only present in proximity to fish firms, wherefrom they have been accidentally released. This species comprises less than 0.2% of the total sample. C. gibelio and L. gibbosus are distributed in all studied water courses and are the most numerous (1.2% and respectively 0.7% of the sample ). P. parva is found only in the rivers Topolnitsa and Luda Yana and represents 0.5% of the sample. In general, the number of nonnative fish in the studied rivers is relatively small and this is why they are expected the impact on local fish fauna was small. However human activities can cause an increase in the population density of alien fishes in the Maritsa River tributaries.