Fish predation and reduction in body size in a Cladoceran population: palaeoecological evidence

1 We studied the recent history (1852‐1982) of Lake Pyhä‐järvi, south‐west Finland, using both cladoceran microfossils and independent historical data. The objective of the study was to assess the impact of introduced planktivorous whitefish Coregonus lavaretus s.1. on zooplankton, especially on the main prey species Bosmina coregoni Baird s.str. 2. A size‐frequency analysis of carapace remains preserved in the sediments clearly shows a size shift in a Bosmina coregoni population. The carapace length of B. coregoni decreased by 11.0% after the introduction of the size‐selective planktivorous whitefish, indicating a parallel body‐size reduction. 3.During the study period no changes could be found in the carapace length of Chydorus sphaericus O. F. Müller, which was not preyed upon.