Date: 04.05.2018

Converting day into night

Converting day into night in an attempt to block worldwide present behavioral phenomenon

1. Diel vertical migrations (DVMs) belong among the most pronounced movements in both marine and freshwater ecosystems. A general pattern of DVMs has been well-described for many aquatic taxa from phytoplankton and zooplankton to cartilaginous and bony fishes. The DVMs were described even for various life stages of European perch (Perca fluviatilis), but whether the migrations are directly controlled by light and what is the ultimate cause of the diel vertical shifts, has remained poorly understood.

2. Undertaking a large-scale field experiment in a thermally stratified, canyon-shaped reservoir, we demonstrated for the first time that DVMs of a bathypelagic early juveniles community, dominated by European perch larvae and juveniles prior the metamorphosis, were under direct control of the light intensity, i.e. they did not operate as a genetically fixed behaviour.

3. Prior to the experiment, the depth distribution of the bathypelagic perch early juveniles was strongly correlated with the light intensity on the water surface. The community underwent regular DVMs between the epilimnion and hypolimnion reaching a maximum amplitude of 13 m.

4. Hydroacoustic recordings by the echosounder SIMRAD EK 60 (frequency 120 and 400 kHz) showed that during the experiment, when the surface was covered with a large black non-transparent foil (2500 m2; simulated conditions of complete and constant darkness), the regular vertical movement of the bathypelagic perch early juveniles was interrupted and the community occupied the epilimnion constantly for 24 hours.

5. Immediately after the foil was removed at midday, the bathypelagic perch early juveniles were exposed to a steep increase of light intensity (from <1 LUX to >100 × 103 LUX) and they escaped into the hypolimnion where they were safe from visual predation which took place in the bright surface layers (epilimnion particularly). Our findings imply that occupying a deep, dark refuge in the daytime is essential for survival of perch in their early life stage.

More information about the unique large field experiment and its impact on fish behavior (both prey and predator) can be found in a study of Sajdlová Z., Frouzová J., Draštík V., Jůza T., Peterka J., Prchalová M., Říha M., Vašek M., Kubečka J., Čech M. (2018). Are diel vertical migrations of European perch (Perca fluviatilis L.) early juveniles under direct control of light intensity? Evidence from a large field experiment. Freshwater Biology 63/5: 473-482; DOI: 10.1111/fwb.13085 (OPEN ACCESS).

A sequence of the up-looking echograms made by the echosounder SIMRAD EK 60 (120 kHz) showing a) a common pattern of diel vertical migration of the bathypelagic early juveniles community and b) early juveniles behaviour, when the water surface was covered with the black foil (identical periods of the day). Red rectangles define the position of the migrating bathypelagic early juveniles community during the open water surface conditions (cf. a) and b)). The dashed line indicates the upper border of the thermocline.

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