THE CONSEQUENCES OF A DRASTIC FISH STOCK REDUCTION IN THE LARGE AND SHALLOW LAKE WOLDERWIJD, THE NETHERLANDS - CAN WE UNDERSTAND WHAT HAPPENED

In 1990 an experiment started in the large and shallow lake Wolderwijd (2700 ha, mean depth 1.5 m) to improve the water quality. About 75 % of the fish stock was removed (425 000 kg fish). The fish was mainly c omposed of bream and roach. In May 600000 young pikes (3-4 cm) were in troduced. In May 1991 the water became very clear (Secchi depth 1.8 m) during a spring bloom of large Daphnia. Then the grazing by zooplankt on was eight times higher than the primary production of algae and the total suspended matter concentration became very low. Compared to the situation before the fish reduction, the grazing had increased only s lightly, while the primary production had decreased significantly in e arly spring. The fish stock reduction might have contributed to the re duction in primary production by a reduced internal nutrient load. The clear water period lasted six weeks. Daphnia disappeared in July due to food limitation, the algal biomass increased and the Secchi depth b ecame 50 cm. Daphnia did not recover during summer, due to predation t hat was not caused by 0 + fish but by the mysid shrimp Neomysis intege r. Neomysis could develop abundantly, because of the reduced biomass o f the predator perch. The production of young fish had been low becaus e of the cold spring weather. The cold weather was probably also respo nsible for the slow increase in density of macrophytes. After 1991, pe rch probably can control Neomysis. Due to lack of spawning places and shelter for 0 + pike, pike was probably not able to control the produc tion of 0 + fish. In a lake of this scale, it will not be easy to get more than 50% coverage of macrophytes, which seems necessary to keep t he algal biomass low by nutrient competition. Therefore, we expect als o in the future a decrease in transparency in the summer. Locally, esp ecially near Characeae, the water might stay clear.