The articles provides a comparative analysis of the fish population of Lake Pyaozero (White Sea basin), one of the largest water bodies in northern Karelia. In 1959–1965, Kumskoye Reservoir, one of North Europe’s biggest storage reservoirs, was formed on the River Kovda involving lakes Pyaozero and Topozero. It maintains a long-term water level regulation regime. Data obtained before the rise in the lake water level and after the regulation of the River Kovda are reported. During the formation of the reservoir, fish of the salmonid and coregonid families became much less abundant, while non-native species, such as pike-perch and smelt, became naturalized. In the past few decades, commercial fisheries on Lake Pyaozero have declined, whereas amateur fishing is on the rise. The fish population is dominated by freshwater arctic species, which require a certain percentage of oxygen in the water. The lake is now inhabited by 17 fish species of 10 families. Not only valuable commercial (vendace, whitefish, pike-perch) but also the most common (perch, roach) species have become less abundant in catches. The results provide new insights into the pre- and post impoundment abundance dynamics of species differing in life cycles. To change the situation in the reservoir and to augment its productivity, measures should be taken to restore the abundance of salmonid and coregonid fishes.