The “whitefin” gudgeon Romanogobio cf. belingi was recorded in the Nida River, a large lowland tributary of the upper Vistula (Southeastern Poland), for the first time in 2009. Since then, it has been caught during the periodical (three times per year) monitoring only sporadically. Conversely, in October and November 2012 R. cf. belingi was recorded frequently along an ∼60-km lowermost stretch of the Nida River. The abundance of this fish gradually increased downstream. This paper provides details of that phenomenon and discusses it in the context of the currently known distribution of this species.
