Distribution of investigated sites in the Drwęca River system. Colour circles: sites where the crayfish species were detected (numeration as in Tab. 1, small circles: <20 specimens, big circles: >20 specimens); empty circles: sites where crayfish were not detected; diamonds: sites of historical release of P. leniusculus. Stream sections of the nature reserve in the Drwęca River system are marked in blue.