• gillrakers;
  • morphology;
  • niche;
  • postglacial lake;
  • prey size;
  • resource segregation

Three field-identified whitefish Coregonus lavaretus forms in Lake Muddusjärvi, Finland, were compared in morphology, diet and prey size. First, these forms were studied with univariate and multivariate analysis to assess morphological divergence at a higher resolution level than in the field. Second, stomach contents were analysed to estimate diet-overlap among forms. Finally, the relationship between prey size and morphology was examined. The whitefish were assigned to the initial field-classification with 99·2% and 98·8% accuracy for morphologic and meristic traits, respectively. The small sparsely-rakered form (SSR) had the shortest rakers and largest gillraker space, followed by the large sparsely-rakered form (LSR) with intermediate gillraker length and gillraker space, while densely-rakered whitefish (DR) had the longest rakers and smallest gillraker space. The two sparsely-rakered whitefish forms (LSR and SSR), consumed mainly benthic macroinvertebrates, while densely-rakered whitefish (DR), utilized pelagic food items. Average diet-overlaps between whitefish forms were low in June-September (Schoener's α = 0·02 − 0·23). Gillraker number and length were negatively correlated to prey length in the diet (r = −0·73, and r = −0·60), while gillraker space was positively correlated with prey length (r = 0·81). The fact that these whitefish forms were morphologically and ecologically segregated, and that gillraker traits probably have a functional value in food selection, further suggests that natural selection has been important in structuring life-history trajectories into divergent niche use.