Abstract

From November 1995 to August 1996, restocked noble crayfish Astacus astacus (L.) were tracked in a 1.63 km long stretch of running water of the small River Sempt, which was inhabited by a unique stock of this native crayfish species until 1978. Because of the failure of intensive restocking activities since 1985, the aim of the study was to supply detailed information about individual migratory movements in quantity and quality, habitat and structure preference as well as the nocturnal activity of the tracked crayfish. Using mini aquatic transmitters, fixed externally on the carapace with tissue adhesive, a total of 13 male adult noble crayfish were tracked every second day during three investigation periods of 8 to 9 weeks each. Results show individual migratory behaviour. Apart from small activity ranges in the order of decameters, movements over some hundred meters per night, upstream as well as downstream, could be observed. Despite a considerable variety of shelter, a preference for the roots of willow and alder tree was documented through the frequency and duration of occupation. In addition 4 crayfish each were tracked over 24 hours in two observations. The crayfish showed almost exclusively nocturnal activity, though the starting and ending point of activity as well as the duration of activity varied. In the observation of 9 crayfish during the summer months, a common migratory pattern could be recognised by subdividing the individual motions of the crayfish into three periods. After stocking, all crayfish showed the same high motion activity through large-scale movements and a high frequency in changing shelters. Three to 9 days after stocking, most of the crayfish became sedentary. At this time, small-scale movements and long occupation of a suitable shelter were typical. Fifteen days after stocking, once again high motion activity and extraordinary large-scale movements down-stream were observed for the majority of the crayfish. As a result, 6 of the 9 crayfish left the investigated stretch of running water within a few days by passing a hydropower station downstream. Due to the lack of further movement signals it must be assumed that the crayfish did not survive. At the end of 55 days of investigation, only three crayfish became sedentary, two in a little pond connected with the river and only one remained in the initial research area. No impact of environmental changes to explain the downstream migration could be determined.

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