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Abstract
Patterns of cyprinid migration were studied in a fishway located at a lock on the Elbe River, Czech Republic, weekly from spring to fall 2003. 24 hrs observations of fish were made with a Riverwatcher Fishcounter manufactured by Vaki (Iceland). The counter records revealed distinct diel patterns of migration. During spring, fish migrated mostly during the light part of a day, in summer and fall this pattern changed. Water temperature and illumination intensity were found as the major driving forces initiating spawning migration and controlling its development and diel pattern. This hypothesis was verified by significant influence of interaction of weather and atmospheric pressure on the number of migrating fish. Because circling of fish within a pool and between adjacent pools of the fishway was a common behavioural pattern, overestimates offish migration acitivity occurred, especially in the downstream direction. These results point out that circling behaviour and the effect of weather condition on fish migration should be taken into account in every fishway monitoring effort.