Impact of flood on distribution of bathypelagic perch fry layer along the longitudinal profile of large canyon-shaped reservoir

Previous studies have shown that under normal spring conditions, the layer of bathypelagic perch Perca fluviatilis fry (BPF) is equally distributed along most of the longitudinal profile of reservoirs. In the first half of June 2004, local flooding entering the large canyon‐shaped Orlík Reservoir (Czech Republic) completely flushed out the existing fry community from the 31 km long riverine part of the reservoir. The pelagic zone in this reach was then recolonized by cyprinid (mainly roach Rutilus rutilus and bream Abramis brama) fry being of either littoral or riverine origin. Subsequently, the BPF layer was recorded only in the 22 km long lacustrine part of the reservoir. In the upper reach of this part where, due to the sudden increase in volume, the water current slowed down rapidly and fry originating from both riverine and central parts of the reservoir gathered in high numbers, two distinct BPF layers were observed. During mid‐day, the upper BPF layer, created predominantly by shoaling fishes (abundance >126 000 individuals ha^?1), occurred between 6 and 10 m. A second, lower BPF layer, created by non‐shoaling fishes exclusively (30 000 individuals ha^?1), was recorded between 12 and 17 m depth. Both upper and lower BPF layer were composed of perch (69·6 and 66·8% in abundance respectively) and zander Sander lucioperca (29·8 and 28·6% in abundance respectively). In the lower BPF layer, ruffe Gymnocephalus cernuus also contributed considerably to the fry assemblage (4·0% in abundance). Perch from the upper BPF layer (mean 25·1 mm total length, L_T) did not differ in size from perch from the lower BPF layer (mean 25·0 mm L_T). Similarly, zander from the upper BPF layer (mean 27·2 mm L_T) were almost the same size as those from the lower BPF layer (mean 26·9 mm L_T). Perch from both BPF layers, however, were noticeably smaller than zander. The results from acoustic survey and complementary net catches suggest that no epipelagic perch fry were found in the reservoir where thermal stratification had been destroyed by flooding and windy weather.