Roach Rutilus rutilus (L., 1758) is a representative of boreal flat faunistic complex and is a core of current fish fauna of Rybinsk Reservoir. [12] Roach hatchling dominate the coastal groups of main biotopes: open littoral zones, protected shallow waters, coastal biotopes of rivers that fall into Rybinsk Reservoir. [23] The high roach number is due to the breeding capacity of the kind, along with the euryphagy, the undemanding to spawning gravels, and the resistance to rapid changes in temperature during the spawning period. The most important features of breeding are the structure of the spawning population, fecundity, and age (size) at which the maturation occurs [7, 10, 13, 15].
The timing of roach maturation are presented in several studies [3, 5, 8, 16, 18]. Authors focused on the age of appearing mature specimen and on the timing of a mass maturation. We found no data of age and size of late maturing roach specimens in available researches.
The number of works was dedicated to roach fecundity of Rybinsk Reservoir. Most of them were conducted in the channels of Volga [4, 5, 20, 25] and Mologa [8, 19], the Sit river, and the Main channel (centre of Rybinsk Reservoir) [24]. Fecundity was determined by the gravimetric sampling with gonad fixing in formalin solution or alcohol. In this work, we determined fecundity without gonad fixing, using wholemount of little spawn sub-samples to count the number of oocytes [17]. Previously, such a method was used by one of the authors, studying age dynamic of bream fecundity [6]. The study findings on roach fecundity, performed by Komova N.I. [25], became material for comparison.
The purpose of our work was to determine the timing of maturation and fecundity of roach of Rybinsk Reservoir.
The material was collected in 2013 - 2020 in the Volga Channel, Rybinsk Reservoir. We measured the length of fish to the end of scale covering (I, cm), the fish weight and gonads weight on electronic balance with 1 and 0.005g accuracy depending on the size of subsample. We determined the beginning of maturation by the presence in gonads the oocytes at the stage of trophoplasmic growth. For constructing ogive of maturation, we determined the mature level of 480 fish with a length from 7 to 20 cm. Scale, vertebrae and operculum determined the age of fish. Individual absolute fecundity (IAF) was determined by gravimetric sampling, based on the number of eggs in sub-samples of 100400 mg, which were taken out of fresh gonads in triplicate. We determined fecundity in 374 females roach with a length from 10.4 to 31.5 cm. Individual relative fecundity (IRF) was determined to the fish weight without gonads.