Bathypelagic percid fry,a strongly predominating fry community in a deep European reservoir

The distribution, species composition, abundance and shoaling behaviour of young-of-year fish were studied in the canyon-shaped Vír Reservoir (Czech Republic) in mid-June 2010. Using the SIMRAD EK60 echosounder (frequency 120 kHz), fry were acoustically sampled along the longitudinal profile of the reservoir. A framed trawl was used simultaneously to collect fry in the open water. Apparent differences were found in the density of fry between the epipelagic, littoral and bathypelagic zones. Bathypelagic fry strongly predominated in the total fry community creating 95.3% while epipelagic and littoral fry contributed only 4.7%. The bathypelagic fry were represented by perch Perca fluviatilis (84.8%), zander Sander lucioperca (14.9%) and ruffe Gymnocephalus cernua (0.15%) and were observed all along the longitudinal profile of the reservoir creating a distinct layer on the echogram. The layer, composed of shoaling and non-shoaling fry individuals, reached its greatest depth in the Bay part of the reservoir (7.5–14.5 m) and it was rising in the water column towards the tributary. The situation that the bathypelagic percid fry predominate in the reservoir is absolutely unique and it is completely different from published observations from other canyon-shaped reservoirs where epipelagic percid fry prevailed.