Habitat Switch and Niche Overlap in Coregonid Fishes: Effects of Zooplankton Abundance

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Abstract
Vendace (Coregortus albula) in Lake Mjøsa lived in the pelagic zone and gillnet catches were correlated with water temperature (r2 = 0.457, p < 0.005). Whitefish (Coregonus lavaretus) exploited both epibenthic and pelagic areas. Whitefish with body lengths between 25 and 35 cm moved from the epibenthic to pelagic zone during summer; other length groups remained in epibenthic habitats. The pelagic fish returned to epibenthic areas in the autumn. Cladocerans were important food items for both fish species in the pelagic zone. The large copepod Limnocalanus macrurus was eaten only by vendace whereas surface insects were more important prey for whitefish. This suggests that vendace is the more specialized zooplanktivore. The pelagic gillnet catch of whitefish and the pelagic habitat overlap between whitefish and vendace increased with increasing zooplankton densities at depths between 0 and 50 m (r2 = 0.609 and 0.494, respectively, p < 0.01). During spring and autumn we observed a time lag between the change in zooplankton abundance and habitat switches of whitefish. The fish moved to the more profitable habitat several days after food availability had changed, indicating an element of learning in choice of habitat.