Abstract.Through ultrasonic tagging, fish living in running water and in slow‐flowing water in man‐made weir basins were tracked over a period of 3–6 days. Results showed that the fish living in the weir basins changed position more frequently and moved over larger areas than fish living in running water. As a consequence of the different behavioural patterns of fish in rapids and in weirbasins, densities of salmon are lower in the weir basins than in the rapids. Therefore, weirs creating large areas of slow‐flowing water should not be established in rivers where an optimal production of Atlantic salmon is t
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