The distribution of the bathypelagic perch Perca fluviatilis fry (BPF) layer and its qualitative and quantitative changes were studied along the longitudinal profiles of the large canyon-shaped Slapy and Orlik Reservoirs (Czech Republic), using acoustic methods (SIMRAD EY500 echosounder, split-beam transducer, Sonar5 post-processing software). fit Slapy Reservoir (sampled in late May), the BPF layer created by the mass of non-shoaling perch larvae and juveniles (average total length, L-T. 10(.)4 mm) was recorded from the darn for 29 kin Upstream. The BPF layer only vanished in the upper third of the reservoir due to the extremely cold hypolimnetic water discharged from Orlik Reservoir, which is situated upstream ill the cascade. Both abundance and size of BPF increased significantly upstream following. in Slapy Reservoir, the trophic gradient. Ill Orlik Reservoir (sampled in late May), the BPF layer was created predominantly by shoaling perch fry individuals (average L-T 31(.)8 mm). recorded along, the whole longitudinal profile of the reservoir (> 50 km, filled with relatively warm water). Both the sizes of BPF and their shoaling activity again increased significantly From the darn towards the inflow following, in Orlik Reservoir, the troplaic gradient. Pooling the data from both reservoirs, it was evident that the tendency to flock in it dense layer and, much later, to create distinct shoals, increased continuously With the size of BPF A number of variables describing the BPF layer in Slapy and Orlik Reservoir are given. (c) 2007 The Authors Journal compilation (c) 2007 The Fisheries Society of the British Isles.