LONG-TERM RESPONSES TO FISH-STOCK REDUCTION IN SMALL SHALLOW LAKES - INTERPRETATION OF 5-YEAR RESULTS OF 4 BIOMANIPULATION CASES IN THE NETHERLANDS AND DENMARK

The effects of fish stock reduction have been studied in 3 Dutch lakes (Lake Zwemlust, Lake Bleiswijkse Zoom and Lake Noorddiep) and 1 Danish lake (Lake Vaeng) during 4-5 years. A general response is described. The fish stock reduction led in general to a low fish stock, low chlorophyll-a, high Secchidisc transparency and high abundance of macrophytes. Large Daphnia became abundant, but their density decreased, due to food limitation and predation by fish. The total nitrogen concentration became low due to N-uptake by macrophytes and enhanced denitrification. In Lake Bleiswijkse Zoom the water transparency deteriorated and the clear water state was not stable. The fish stock increased and the production of young fish in summer was high. Clear water occurred only in spring. Large daphnids were absent in summer and the macrophytes decreased. In Lake Zwemlust, Lake Vaeng and Lake Noorddiep the water remained clear during the first five years. In summer of the sixth year (1992) transparency decreased in Lake Zwemlust (with high P-concentration of 1.0 mg P l-1). Also in Lake Vaeng (with a low nutrient concentration of 0.15 mg P l-1) a short term turbid stage (1.5 month) occurred in summer 1992 after a sudden collapse of the macrophytes. Deterioration of the water quality seems to start in summer and seems related to a collapse in macrophytes. At a low planktivorous fishstock (e.g. Lake Vaeng) the duration of the turbid state is shorter than in presence of a high planktivorous fish biomass (e.g. Lake Zwemlust, and later years of Lake Bleiswijkse Zoom).