Influence of Illumination on the Locomotor Activity in Smolts of European River Lamprey Lampetra fluviatilis (L.)

Behavior of smolts of European lamprey and changes in their locomotor activity have been studied at varying illumination regimes. The maximal level of the locomotor activity of the smolts in aquaria and in an Open Field experimental device is recorded at night at the lowest studied illumination (0.9 lx). This maximum coincides in time with the appearance of migrating smolts in the river channel flow. In daytime, at illumination of 900lx, the locomotor activity of the smolts is lower, which corresponds to the absence of daytime downstream migration. The change in the locomotor activity over 24 h is one of the mechanisms by which the downstream migration of lamprey smolts is achieved. The increase in the locomotor activity starts soon after evening decrease in illumination below 0.1 lx and falls on the first half of night. The period of increased locomotor activity ends before morning twilight. The presence of the circadian rhythms of locomotor activity in smolts of the European river lamprey is suggested.