Food preference of adult whitefish in eutrophic Lake Hallwil (Switzerland), and the question of cannibalism

The aim of this study was to investigate the role of food uptake and prefer ence of adult Lake Hallwil whitefish (Coregonus suidteri FATIO) on year cla ss strength, with special emphasis on the situation in late winter and spri ng, the time when whitefish larvae hatch and may be vulnerable to cannibali sm and food competition. Whitefish is the dominant and commercially most im portant fish species in eutrophic Lake Hallwil. Yield started to increase i n 1977 thanks to stocking with prefed larvae since 1970, but has not been s table due to strong fluctuations in year class strength (YCS). Because food is one of several key parameters influencing population dynamics, food pre ference of adult whitefish was studied. In spring, 20-35 percent of the whi tefish exclusively preyed on lake bottom fauna when zooplankton density was low due to low water temperature. 50-65 percent of the stomachs contained zooplankton exclusively during spring. In summer and fall, only I percent o f the stomachs contained exclusively benthic prey while 80 percent of the w hitefish preyed exclusively on zooplankton. The percentage of whitefish eat ing both planktonic and benthic organisms was about 15-20 percent over the whole year. Artificial hypolimnetic oxygenation since 1985 has substantiall y increased living space of benthic organisms. This has resulted in an impr oved benthic food base for Lake Hallwil whitefish. In spring, as soon as th e zooplankton density had risen, whitefish switched from benthic prey to pl anktonic crustaceans. The question of potential food competition between ad ult whitefish and larvae was studied by comparing our data on feeding habit s and feeding depth of adults with the respective data for larvae from the literature. Cannibalism of adult whitefish on larvae was suspected to influ ence population dynamics of Lake Hallwil whitefish, but no larvae were foun d in the stomachs studied. We conclude that the different feeding depth and food preference of whitefish larvae and adults in spring make intraspecifi c competition and cannibalism unlikely.