Planktivory and diet-overlap of densely rakered whitefish (Coregonus lavaretus (L.)) in a subarctic lake

Three sympatric whitefish (Coregonus lavaretus (L.)) forms, one being pelagic and two benthic, segregate available habitat and food resources in subarctic Lake Muddusjarvi, northern Finland. Zooplankton availability in the lake, food composition, diet-overlap and growth of densely rakered (DR) whitefish were examined during June to September to explore the reasons for the small individual size of the pelagic form. DR whitefish used zooplankton as main food item and prey selection followed zooplankton species density proportions in the lake. Zooplankton density and water temperature was highest in July-August. The average lengths of Bosmina spp., Daphnia spp., Calanoida and Cyclopoida in DR whitefish stomach were higher than in zooplankton sample during June-September, except Calanoida in June. Diet-overlap between DR whitefish age groups was high at all months indicating intercohort resource competition. DR whitefish reached sexual maturity at 3 years of age and at the length of 12 cm, after which somatic growth almost ceased. Reason for the small average size and slow growth of DR whitefish were connected to high diet-overlap between age groups and early sexual maturation.