Structure and features of the downstream migration of juvenile fish in the lower reaches of the Oka river

Downstream migrations of early juvenile fish are typical for most species of ichthyofauna and are an adaptive mechanism of settlements. The purpose of this work was to study the features of the downstream migration of early juvenile fish of the lower reaches of the Oka River, to assess the quantitative indicators and species structure of migrants. The materials were obtained during the work carried out in the period from May to June in 2022–2024. Ichthyoplankton cone nets with a mouth area of 0.2–0.25 m2 were used as tools. Trapping was carried out at three deep levels and was timed to coincide with the main phases of the day, depending on the degree of illumination. According to the results of the study, the slope of early juvenile fish is confined to the surface horizon, the maximum concentrations of ichthyoplankton are noted in the morning, the minimum in the evening. A number of specific features of migration activity and vertical distribution of individual species in the conditions of a riverbed stream in the lower reaches of the Oka River are highlighted. In the course of observations of the downstream migration, it was found that the substitution of dominants is observed in the structure of ichthyoplankton. Species composition of catches of sloping juvenile fish of the lower reaches of the river The Oka is quantitatively inferior to the survey results of sections of the middle and upper reaches. The lower reaches of the river are characterized by a low proportion of rheophiles in the catches of ichthyoplankton traps and a relatively high proportion of limnophiles. The concentration of ichthyoplankton in the riverbed zone of the lower reaches of the Oka River in 2022–2024 years was higher than in the upstream sections of the river, which is explained by the wide variety of topical conditions. Coastal biotopes are more productive than riverbed zones in terms of spawning and determine a high concentration of phytophilic species in the riverbed zone.