Diel and seasonal habitat and food segregation of three sympatric Coregonus lavaretus forms in a subarctic lake

Habitat use of three sympatric whitefish Coregonus lavaretus forms was determined using hydroacoustics, pelagic trawling and epibenthic gillnetting in the subarctic Lake Muddusjärvi during the day and night in June, August and September. Whitefish constituted 97% of the numerical catches and whitefish with high number of gill rakers (DR) were the most abundant whitefish form. Forms with low numbers of gill rakers used only epibenthic habitats during both the day and night in all study periods: large whitefish with low numbers of gill rakers (LSR) dwelled mainly at depths 0–10 m, whereas small whitefish with low numbers of gill rakers (SSR) used deeper (>10 m) habitats. LSR and SSR whitefish consumed mainly benthic macroinvertebrates during all study occasions. The planktivorous DR whitefish used both epibenthic and pelagic habitats, but vertical habitat selection varied both over time of day and season. In June, when light intensity was continuously high, DR whitefish did not perform diel vertical migrations. In August and September, when dark nights were distinguishable, DR whitefish ascended from the bottom to the pelagic at dusk to feed on zooplankton, and descended at dawn. DR whitefish used pelagic habitats only at the lowest light intensities during the night, which was probably related to the high predation risk from brown trout Salmo trutta.