Ecology of sympatric whitefish (Coregonus lavaretus (L.)) forms in a subarctic lake

This thesis concerns the ecology of sympatric whitefish (Coregonus lavaretus (L.)) forms in a subarctic lake, northern Finland. Despite of rather common occurrence of sympatric whitefish forms in the northern hemisphere, limited amount of information exist about their ecology and morphological divergence. Furthermore, whitefish form data is usually gathered in various different lakes and collective studies of the various ecological aspects in a single lake has been scarce. Thus, this thesis focuses on the ecology of sympatric whitefish forms in Lake Muddusjärvi. The main objectives were to examine the level of morphological divergence, morphometry-feeding environment relationship and niche segregation of sympatric whitefish forms. In addition, the impacts of predation on sympatric whitefish forms were evaluated. Morphometric and meristic analyses indicated divergence of whitefish population into three distinct forms. The most pronounced differences between sympatric whitefish forms were found in gillrakers, head and pectoral fin traits, which were correlated with their feeding environment. Deep water (>10 m) dwelling benthivore, small sparsely rakered whitefish (SSR) had the lowest number of short and extremely widely spaced gillrakers. Shallow water (<10 m) dwelling benthivore, large sparsely rakered whitefish (LSR) had intermediate number of short and widely spaced gillrakers. Planktivorous densely rakered whitefish (DR) used partly pelagic habitats having the largest number of densely spaced and the longest gillrakers. Distinct specialization to benthic and pelagic niches was supported also by low food and habitat overlap between sympatric whitefish forms. The food resources available for different whitefish forms influenced to their growth. LSR, which used littoral bottoms providing the greatest benthic food resources, had the fastest growth. SSR utilized scanty benthic food resources in profundal bottoms had the slowest growth. The growth of DR consuming mainly pelagic zooplankton was also slow. Whitefish was the main prey for piscivores in Lake Muddusjärvi. However, the importance of whitefish forms in predator’s diet differed distinctly. DR was the main prey for all piscivores. Especially, salmonids brown trout (Salmo trutta L.) and Arctic charr (Salvelinus alpinus (L.)) used almost exclusively DR, whereas burbot (Lota lota (L.)) and pike (Esox lucius L.) used also other species and benthic whitefish forms. LSR was able to reach size refuge from predation earliest due to the fastest growth. SSR used profundal habitat, where feeding efficiency of visually chasing predators is low, and was the least preyed whitefish form. Risk of predation was high for pelagic DR, which avoided predation by habitat selection. DR dwelled in vicinity of bottom during continuous daylight in mid summer. Towards autumn DR performed diel vertical migrations ascending to midwater and surface during dusk and descending to bottom at dawn. Vertical migrations of DR were related to risk of predation induced by brown trout, which dwelled in pelagic habitat. Benthic habitat offered refuge for LSR and SSR, which did not perform diel vertical migrations.