Adaptive phenotypic divergence of sympatric morphs in a single species may have significant evolutionary consequences. In the present study, phenotypic impacts of predator on zooplankton prey populations were compared in two northern Finnish lakes; one with an allopatric whitefish, Coregonus lavaretus (L.), population and the other with three sympatric whitefish populations. First, we examined whether there were phenotypic associations with specific niches in allopatric and sympatric whitefish. Second, trait utility (i.e. number of gillrakers) of allopatric and sympatric whitefish in utilizing a pelagic resource was explored by comparing predator avoidance of prey, prey size in environment, and prey size in predator diet. The allopatric living large sparsely rakered (LSR) whitefish morph, was a generalist using both pelagic and benthic niches. In contrast, sympatric living whitefish morphs were specialized: LSR whitefish was a littoral benthivore, small sparsely rakered whitefish was a profundal benthivore and densely rakered (DR) whitefish was a pelagic planktivore. In the lake with allopatric whitefish, zooplankton prey did not migrate vertically to avoid predation whereas, in the lake with sympatric whitefish, all important prey taxa migrated significantly. Trait utility was observed as significantly smaller size of prey in environment and predator diet in the lake with DR whitefish than in the lake with only LSR whitefish.

Planktivory and diet-overlap of densely rakered whitefish (Coregonus lavaretus (L.)) in a subarctic lake Un resumen en españ ol se incluye detrás del texto principal de este artículo.

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The variations in the vertical migrations by zooplankton and food composition of fish in the shallow, clay-turbid Mustionselkä basin of Lake Hiidenvesi were studied (water depth 3 m). Daphnids showed signs of reverse migration, their density in the surface layer being highest during the day. The reverse migration was probably due to predation threat by the predatory cladoceran Leptodora kindtii. Chydorus sphaericus, which was consumed especially by the day active bleak (Alburnus alburnus), showed normal vertical migration. Copepods showed no clear vertical movements. Leptodora kindtii, as well as the large herbivore Limnosida frontosa, showed no vertical migrations, but their overall abundance increased considerably during the night, indicating horizontal movement towards the open water at night. Due to the shallowness of the water, vertical migrations may not be a good predator-avoidance strategy for the largest cladocerans, which may be able to perform horizontal migrations.