Diel Vertical Movements: Warm Water Does Not Prevent Vendace (coregonus Albula (L.)) from Tracking Zooplankton

We observed small schools of vendace (Coregonus albula (L.)) which occasionally crossed the thermocline and migrated into the exceptionally warm (22 to 23 degrees C) epilimnion of a large Finnish lake in August 2010. This is against previous observations regarding diel vertical migration (DVM) in this species. The ascending schools were detected for a short period of time about an hour after sunset. The findings were based on data gathered with a 120 kHz echo sounder, an Acoustic Doppler Current Profiler, a Laser Optical Plankton Counter, a Multi Net plankton sampler and a mid-water trawl for the experimental sampling of pelagic fish. The study emphasises the importance of examining DVM with different kinds of samplers simultaneously. We are unable to state the ultimate reason why some vendace migrated from cool to warm water, but we can state that there was a lot of food for them in the epilimnion around sunset. It would have been impossible to detect the phenomenon using only mean or maximum abundances of fish, and we had to use very fine scales to detect the phenomenon on echograms. The ascension of macrozooplankton, mostly Chaoborus flavicans (Meigen), followed a rhythm described by a third order polynomial function. Vendace were feeding in the evening near sunset within the ascending anti-predation window in which there was enough light for detecting zooplankton and shelter against predators, while during daytime the majority of stomachs of vendace (63%) and smelt (Osmerus erperlanus (L.)) (50%) were empty in mid-water depth layers of the lake where the main pelagic fish populations were detected. Based on our observations on fish movements during thermal stratification in August, we conclude that vendace has a potential to tolerate by behavioural means warm water up to 23 degrees C.