Abstract
A white spot regarding the fish fauna of the upper and middle Olteṭ River was covered and the significant changes appeared in this respect and in the lower course were identified. In general the passing of the river from one type of geographical/relief type unit to another, induce the changing of the ichthyological zone and the increasing of the fish species number. There where this natural situation did not appear is because different human impact causes create unnatural variations in fish communities’ compositions and individuals’ number. The presence of a various human impact especially downstream the mountain area makes to exist here a relative unbalanced fish fauna structure, in comparison with the mountainous sector. Overall, the geographical and human impact elements, and a result of them, the self cleaning capacity of river, are main driving forces which induce the fish fauna composition variability.