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The aim of this study was to investigate the role of food uptake and preference of adult Lake Hallwil whitefish (Coregonus suidteri FATIO) on year class strength, with special emphasis on the situation in late winter and spring, the time when whitefish larvae hatch and may be vulnerable to cannibalism and food competition. Whitefish is the dominant and commercially most important fish species in eutrophic Lake Hallwil. Yield started to increase in 1977 thanks to stocking with prefed larvae since 1970, but has not been stable due to strong fluctuations in year class strength (YCS). Because food is one of several key parameters influencing population dynamics, food preference of adult whitefish was studied. In spring, 20-35 percent of the whitefish 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 1 percent of the stomachs contained exclusively benthic prey while 80 percent of the whitefish preyed exclusively on zooplankton. The percentage of whitefish eating both planktonic and benthic organisms was about 15-20 percent over the whole year. Artificial hypolimnetic oxygenation since 1985 has substantially increased living space of benthic organisms. This has resulted in an improved benthic food base for Lake Hallwil whitefish. In spring, as soon as the zooplankton density had risen, whitefish switched from benthic prey to planktonic crustaceans. The question of potential food competition between adult whitefish and larvae was studied by comparing our data on feeding habits and feeding depth of adults with the respective data for larvae from the literature. Cannibalism of adult whitefish on larvae was suspected to influence population dynamics of Lake Hallwil whitefish, but no larvae were found in the stomachs studied. We conclude that the different feeding depth and food preference of whitefish larvae and adults in spring make intraspecific competition and cannibalism unlikely. |
