INFLUENCE OF DAILY VARIATIONS OF LIGHT AND TEMPERATURE ON THE EMERGENCE RHYTHM OF GRAYLING FRY (THYMALLUS-THYMALLUS)

The emergence rhythm of grayling fry (Thymallus thymallus) was studied in four aquaria fitted with traps under four different conditions of light and temperature. Both light and temperature induced an emergence rhythm characterized by a 24-h period. Under ambient light and constant temperature, emergence peaked at the end of the night before the onset of dawn. Under constant dim light and varying temperature, emergence peaked in early morning during the coldest hours. When both light and temperature fluctuated daily the emergence pattern was equivalent to the "sum" of these previous two regimes and was similar to that described in the field in a previous report. In the absence of light and temperature variations, no emergence rhythm was observed.