Yearly variations of impact of natural hybrids of bream and roach (Abramis brama (L.) × Rutilus rutilus (L.)) in Rybinsk Reservoir

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The occurrence of hybrids in the reservoir may be one of the most important pieces of evidence of reproduction conditions in fish populations. The hydrological conditions (temperature and water level) dur� ing the spawning period must be considered in studying the reproduction peculiarities of potentially hybrid� ized fish species in addition to the other factors. The studies of two spawning grounds were carried out. These grounds were the mouths of Shumarovka River and Krasnyi Ruchei in the years of low (2003) and high (2004, 2008, and 2009) water levels, when significant water temperature fluctuations were observed during the spawning period (2004 and 2008), or the delay of the warming up to spawning temperatures occurred (2003 and 2009). Young hybrids were found at both spawning grounds only in 2003. In 2004, no hybrids were observed; in 2008, they were registered only in Krasnyi Ruchei; and in 2009, they were registered only in the mouth of the Shumarovka River. These observations indicated sharing of the spawning grounds by bream and roach in the years of low water level and in the years that were characterized by a delay in spring water warm� up. When the water level was sufficient and the spawning periods coincided, the grounds were mainly used by roach; thus, the occurrence of its hybrids with bream was rare and random. DOI: 10.1134/S2075111711030076