Individual and population-level food consumption by cyprinids and percids in a mesotrophic lake

Mesotrophic Lake Pohjalampi in eastern Finland was fished intensively from 1992 to 1996. In 1996, the biomasses of the dominant fish species, roach an d bream, were reduced to 14% and 6%, respectively, of that in 1992. The tot al biomass of percids was, however, unchanged or slightly increased. The pr oportion of benthic invertebrates in the total food of the fish decreased f rom 57% to 36% during the study period, whereas the proportion of zooplankt on increased from 31% to 56%. At the same time the number of young planktiv orous fish increased. Thus, the main object of fish predation turned from b enthic invertebrates to zooplankton due to change in fish population struct ure. At the beginning of the study the amount of detritus eaten by cyprinid s was about 620 kg . ha(-1) . year(-1), but this decreased by 82% due to th e intensive fishing. Thus, by recycling nutrients from the sediment, the de trivorous-benthivorous feeding of cyprinids is an important factor in the n utrient balance of the lake.