| Summary: | To improve water quality in a heavily eutrophicated lake (Köyliönjärvi, SW Finland), mass removal of fish was performed in 1992–1998. The fish stock declined from an estimated 170–250 kg ha–1 in 1991–1992 to 40–90 kg ha–1 in 1996–1998 or to 12%–25% of the initial biomass. The biomass of the larger cladocerans slightly increased in 1991–1996 but decreased again in 1997, and chlorophyll a levels varied inversely with the cladoceran biomass. Cyanobacteria initially declined, but altogether the water quality effects of fish removal remained small up to the summer 1997. The roles of phosphorus, submerged macrophytes (Elodea) and fish were further explored in a factorial enclosure experiment. Significant treatment effects were only observed in the earliest phase of the experiment, when the presence of macrophytes decreased and that of fish increased phytoplankton chlorophyll a; later fish treatments were lost. Phosphorus additions had no effect on water quality, but at the end of the experiment phytoplankton chlorophyll a was negatively correlated with the biomass of large cladocerans and positively correlated with total phosphorus concentration. The enclosure effect was strong, all enclosures having much lower nutrient and chlorophyll levels than the surrounding lake. The experiment suggests that it is possible to improve water quality through removal fishing even in hypertrophic lakes, but the fish stock, including the young-of-the-year fish, must be decimated to a very low level.
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