The total fish abundances of two gravel-pit lakes connected with the River Rhine have been observed by a hydro-acoustic method (echo-sounder: Lowrance X16) for about one year. Diel (24-hour-period) as well as seasonal changes in fish distribution and aggregation behaviour have been established. Concerning the assessment of the fish stocks, these changes in fish behaviour have to be considered. Some clear fluctuations of fish numbers, which coincide with flood events of the river are interpreted as indications of fish migration from the river into the lentic water.

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