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Summary

This study is based on a 23-year sampling of fish and compares raw population abundance and biomass obtained from three to six consecutive electrofishing passes in a small lowland stream in Poland, with values of calculated density and a biomass from the Zippin model. Three density estimates and four biomass estimates obtained with three passes were higher than those estimates based on six catches. This discrepancy occurred because fewer fish were collected in the first pass than in the second pass. However, when the six consecutive passes were considered, the higher raw abundance and biomass in the second pass did not produce any significant error in the density calculation. A catchability decline resulted from the low number of juvenile fish captured when there was a high number of fish that had successfully spawned in a given year. Nevertheless, fish density assessment on the basis of three runs in lowland streams is recommended for quantitative sampling, and the problems that sometimes emerge will be explained in this study.