On the downstream migration of ichthyoplankton along the Bulgarian shore of the Danube river [Cyprinidae, Gobiidae, Percidae]

alevines animal migration biological rhythms bulgaria bulgarie cours d'eau cursos de agua cyprinoidei danube danube river fish larvae freshwater fishes ichthyology ichtyologie ictiologia larve de poisson migration animale peces de agua dulce percoidei poisson d'eau douce rivers rythme biologique