THE DOWNSTREAM MIGRATION OF YOUNG FISHES IN RIVERS - MECHANISMS AND DISTRIBUTION

The present communication reviews the results of our numerous investig ations, obtained during 1964 and 1989. We distinguish three forms of d ownstream fish movement (passive, active, and active-passive), and thr ee types of mechanisms of their realization. The mechanisms of the fir st order create preconditions for downstream migration and promote you ng fishes to enter into water currents. Mechanism of the second order bring about these preconditions through rheoreaction ''neutralization' ', and the mechanism of the third order only correct the distribution of fishes already moving downstream. The daily and seasonal dynamics o f fish downstream movement, their vertical and horizontal distribution are considered in the text. The drift dynamics of young fishes is con nected with the variability of diverse abiotic and biotic factors. The variety of behavioural mechanism that lead to downstream movement pro vides a basic for the adaptive of migratory fishes, opposite of the na rrow specialization of relatively resident forms.