Research Article Bottom-up control of whitefish populations in ultra-oligotrophic

Abstract. Lake Brienz, an oligotrophic pre-alpine Swiss lake, went through a mesotrophic period between around 1960 and 1990. The lake is moder-ately turbid caused by fine suspended solids from glaciers. In 1999, yield of the economically important whitefish collapsed to about 10 % of preceding years. Age and growth analysis of the two whitefish types examined – small and large type – revealed an almost complete halt of growth from 1999 until June 2000, paralleled by poor condition. Zooplankton data showed that cladocerans, the preferred food of white-fish, were rare from January 1999 until June 2000. In order to elucidate the trophic relationships between zooplankton and fish, the «Wisconsin Bioenergetics Model 3.0 » was applied. The analysis showed that poor growth and condition of whitefish in 1999 and 2000 were caused by the scarcity of primary food organisms. The relatively small and slender fish could not be caught by legal gillnets, which resulted in poor fishing yield. Evidence is presented that cladoceran biomass governs food consumption by the fish («bot-tom-up effect»), while the effect of fish predation on cladocerans was found to be negligible, most likely also during the period of poor growth. Turbidity did not appear to significantly interferewith the feeding of whitefish. Growth, condition and commercial yield of whitefish partly increased again after 2000, but due to the very low productivity of Lake Brienz, fishing yield will remain low. Food chains in such oligotrophic systems are fragile. It is likely that a future collapse of the cladoceran population and, thus, the whitefish fishery will happen again. Key words. Coregonids; fishing yield; food chain; bottom-up effects; bioenergetics model; oligotrophication