RÉSUMÉ

Swimming performance of perch and bleak larvae was investigated. It has been found that the studied species show the resistance to flow from the early life stages: the pelagic perch larvae at A₂, and bleak – at B stage, just after detachment from substrate. The critical swimming speed in perch fry were lower than in bleak of the same length. Estimated values of Reynolds number have shown that by the hydrodynamic criterion (reaching Re = 5,0·10³), perch larvae turn to nekton at approximate length of 22 mm, and bleak—of 15 mm. This transition is supported by morphological changes which reduce hydrodynamic resistance of the fish body: the fin fold disappears, scale develops, the body gets more streamlined shape.