This specimen of fish refers to the Longnose sucker Catostomus catostomus (Forster, 1773). In the Soviet Union there were several attempts to acclimatize this species to various water systems of the Baltic Sea basin. Perhaps in Poland this species was unintentionally acclimatized, since several times there were deliveries to the Polish rivers of different Siberian fish (Baikal grayling, Pelyad, Siberian sturgeon and others.

I agree with the identification as Catostomus catostomus.

It doesn't look like a Catostomus to me; the mouth appears to be terminal rather than sub-terminal. I would guess a pikeminnow Ptychocheilus spp.

 Dear Craig: Ptychocheilus spp. is solely North American fish genus. 

I think that the mouth is not terminal.  When I look at the photo in the greatest size what I see I interpret as the top of the snout.  The mouth is not visible at all because it is inferior.

It looks like Catostomus sp