Diel vertical migrations, distribution and ontogeny of bathypelagic layer of European perch (Perca fluviatilis L.) fry in reservoirs

 We have found that in the pelagic zone of reservoirs, European perch (Perca fluviatilis L.) fry create simultaneously two different communities – epipelagic and bathypelagic. The epipelagic perch fry spent the whole 24 hours in the upper 4 m of the water column (i.e. above the thermocline) whereas the bathypelagic perch fry performed diel vertical migrations with an amplitude exceeding 10 m, being in relatively warm epilimnion during the night and migrating into the cold hypolimnetic layers during the day. At this time the bathypelagic perch fry community was present as a thick scattering layer, which could be followed by the echosounder. The epipelagic perch fry remain pelagic until June or July, and then move inshore to the littoral area. On the other hand, the bathypelagic perch fry remain in the open water up to autumn. Moreover, unlike bathypelagic perch fry no epipelagic perch fry could be found in the reservoir where thermal stratification had been destroyed by flooding and windy weather. The phenomenon of the bathypelagic perch fry was overlooked for decades and was restricted to the role of an interfering noise on the screen of commercial fisherman's echosounders. Using scientific echosounders and complementary net catches the bathypelagic perch fry layer was recently recorded along most of the longitudinal profile of several canyon-shaped reservoirs (up to 55 km long). The size, abundance and shoaling activity of bathypelagic perch fry increased significantly in direction from the dam towards the tributary following the reservoir's trophic gradient. The presence of the bathypelagic perch fry layer in the riverine part of those reservoirs is restricted by local floods or cold hypolimnetic water released from the reservoir situated upstream in the cascade.