Fisheries management as an additional lake restoration measure: biomanipulation scaling-up problems

Restoration of the highly eutrophic Reeuwijk lakes (ca. 700 ha) started in 1986 by reducing the external phosphorus loading. As an additional measure to improve the quality of the lake water, the structure of the fish population in Lake Klein Vogelenzang (18 ha) was altered in 1989 by the removal of ca. 100 kg ha^–1 bream from the lake in April and December. This constituted about 50% of the total bream biomass in the lake.The fish-stock reduction in April, 1989, was initially followed by high phosphorus concentrations, probably the result of considerable phosphorus release from the sediments. The resulting heavy algal blooms that occurred reduced the transparency to very low values. During the summer the zooplankton population increased markedly in numbers coinciding with reductions in total suspended matter including (blue-green) algae. A great improvement in Secchi-disc transparency was observed and by the end of December, 1989, the bottom of the lake (1.5–2.0 m) was visible. After heavy storms in January and February 1990, transparency dropped to < 1 m as a result of resuspension of high concentrations of suspended matter from the bottom sediments. Although transparency over the rest of 1990 was higher than in 1988, i.e. the year preceding the removal of fish (biomanipulation), it was lower than expected, based on the results of 1989. The study shows that technical and biological factors can cause serious management problems for the implementation of biomanipulation in larger water bodies.