Background. The topmouth gudgeon, Pseudorasbora parva (Temminck et Schlegel, 1846) (known also as stone moroko), is an alien species of the fish fauna of many European freshwater ecosystems. In large quantities, its may affect negatively the fish fauna and the functioning of ecosystems. It competes for food with native fish species and they occupy their habitat. The knowledge about the invasion dynamics may help to find suitable control measures. The aim of this study was to find and observe the population of P. parva in a small river. Materials and methods. Inventory fishing on three sampling sites at the Ciemięga River was carried out from 2003 to 2007. The abundance and density of P. parva in the river were calculated. The size distribution of P. parva and its length–weight relation were determined. Abundance, density, length, and mass of the fish from all study sites were analyzed statistically. Results. P. parva was first recorded in the Ciemięga River in the autumn of 2005. The abundance of P. parva in relation to the structure of the local ichthyofauna was the smallest in the spring and the highest in autumn. The total length of P. parva ranged from 15 to 104 mm, and the mass from 0.2 to 10.4 g. Taking into account the number of the specimens, P. parva constituted between 0.9% and 57.2% of the local ichthyofauna, while its mass ranged from 0.0% to 35.1% depending on the study site and the season. The density of P. parva ranged from 72.9 to 6.5 CPUEn, while the mean value for the river was 30.6 CPUEn. Conclusion. In Polish waters P. parva is an alien species and there is still little information about its occurrence in the flowing waters. Since 2005, this species was present almost in all inventory catches in the Ciemięga River. Its presence was dependent on the kind of the habitat and the presence of predatory fish species (eg brown trout). P. parva occurring numerous may be a lot of competition for native fish species, and therefore an important problem is to understand his habitat preferences and interactions with native fish species.
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