Abstract <p>The combined effect of water level and temperature regimes on the efficiency of natural reproduction of the phytophilic group of fishes has been assessed for the first time for the lower reaches of the Oka River in Nizhny Novgorod oblast. Changes in the areas of spawning areas of the Oka River at different water levels have been studied for the first time using space images. The boundaries of 45 spawning sites are linked to a geographical network of coordinates based on Landsat satellite imagery, and 20 spawning areas have been identified at different water levels. It is revealed that the area of spawning grounds can vary 12 times depending on the water level. A formula has been obtained for the dependence of the spawning area on the water level (applicable in the range of levels 67.00–74.52 m). Data on the timing of fish spawning in the Oka River are presented. It has been established that the average annual median of the relative abundance of fingerlings of phytophilic fish species in the Oka (within Nizhny Novgorod oblast) is 4900 specimens/ha, which is 2.9 and 1.6 times more than in Gorky and Cheboksary reservoir, respectively. Coefficients of multiple correlation between indicators of the level regime during the spawning period and the number of fingerlings of various fish species are obtained.</p>