| Summary: | SYNOPSIS OF RESULTS Ontogenetic niche shifts Papers I, II, III, IV, VIII, and IX show that both vendace, whitefish and Arctic charr perform ontogenetic niche shifts. All species start their lives in the benthic zone and turn to utilizing the pelagic zone at a later age. In vendace, the whole cohort performs a niche shift to the pelagic zone during the first summer (Paper II), and continue living in this zone feeding mainly on zooplankton for the rest of the life span (Paper IV and VII). Even in the littoral zone, however, the diet of age-0 vendace is dominated by crustacean zooplankton (Paper II). Thus, the niche of vendace is narrow, and the specialist nature of the species is reflected in its pelagic zooplanktivore life in a variety of localities (Paper IV, V, VII). The observed fluctuating or regularly oscillating year class strengths in vendace populations (Paper VI) are probably due to the narrow niche of the species, restricted both in the diet and habitat dimension, creating strong intra-specific interactions. The generalist nature of Arctic charr and whitefish is indicated by their variable diet and habitat use within and between lakes. The two species resemble each other in their patterns of ontogenetic niche shifts. Populations of both species maintain a basis in the epibenthic habitat, where all size groups of the population usually are present (Paper III, VIII, IX). Among certain size groups, some fish perform a seasonal habitat shift to the pelagic zone when zooplankton abundance is high. In Mjøsa whitefish, the habitat shift occurs just prior to sexual maturity at a size of 25 cm (Paper III and IV). In Femund whitefish, the habitat shift occurs at a size of between 20 - 25 cm (Paper VIII), and is mainly performed by the most slow-growing morph, which matures sexually from a body length of 25 cm onwards. In both lakes the habitat shift involves few fish larger than 35 cm, and only a part of the 25 - 35 cm length group. The change in habitat may therefore be termed niche expansion rather than niche ...
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