Are Diel Vertical Migrations of Young-of-the-Year Perch Perca fluviatilis Under Direct Control of Light Intensity? Evidence from the Large Field Experiment

In reservoirs, perch fry create simultaneously three different communities – epipelagic, bathypelagic and littoral. Bathypelagic perch fry perform regular diel vertical migrations (DVM) staying in deep water during daylight hours and spent the night close to water surface. Cech et al. (2005) stressed the question whether the DVM of perch fry is definitely under direct control of light intensity or whether the twilight is only a cue and all the behaviour is genetically coded. To test this hypothesis directly in the field, large black foil (50 x 50 m) was used to prevent light penetrate the water column. The behaviour of perch fry was continually monitored for 5 days using two SIMRAD EK60 echosounders working with frequency of 120 and 400 KHz. One stationary transducer (circular – ES120-7G) was deployed as up-looking and one semi-mobile transducer (circular – ES400-7C) was deployed as down-looking. With water surface exposed to light, perch fry performed DVM with amplitude of 12 m being after sunrise and before sunset attacked by adult perch in upper 5 m of water column. With water surface covered by black foil when no light penetrated throughout the water column the DVM vanished and perch fry stayed unattacked close to water surface.