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Most fish species undergo ontogenetic niche shifts from feeding on pelagic zooplankton, to larger benthic invertebrates and in some cases also to fish. These ontogenetic niche shifts have strong impact on the interactions within and between species, with effects on individual growth, population abundance and food web dynamics. The productivity of northern lakes is mainly controlled by light-limited primary production in benthic habitats, highlighting the importance of lake bathymetry for the abundance of benthic algae feeding macroinvertebrates, which is an important resource for fish. Theory predicts that variation in fish size structure and biomass can arise due to size-dependent differences in competitive abilities between juvenile and adults in each of their niches and by variation in niche- and habitat-specific resource production i.e. pelagic zooplankton and benthic macroinvertebrates.

In this thesis, using gradient studies in mountain lakes, I studied how habitat-specific production and lake bathymetry variation affect growth, size structure and biomass in Arctic char and brown trout populations. Results showed that lake bathymetry determine the benthic contribution to whole lake primary production and the degree of ontogenetic niche shift from zooplankton to macroinvertebrates. In correspondence with theory, production of Arctic char and brown trout were related to stage- and habitat-specific gross primary production (GPP) as an increased benthic contribution to whole lake GPP in general increased individual size, population production and biomasses. Lake bathymetry also influenced the niche shift to piscivory in brown trout as reliance on piscivory were higher in relatively deep lakes more dominated by Arctic char. Finally, in a model approach, responses to different size selective harvest regulations showed that the size structure of Arctic char were more sensitive to fishing in shallow than in deep lakes. Size regulations protecting both smaller and the largest adults were shown to best preserve size structure, especially in shallower lakes. Collectively, these results contribute to the understanding of how variation in productivity and availability of stage- and habitat-specific resources and the presence of ontogenetic niche shifts affect the growth, size structure and biomass of fish. Specifically, the results highlighted the importance of shallow benthic habitats for individual size and biomass of salmonids in mountain lakes and suggests that management strategies based on relationships between lake bathymetry and population size structure and biomass could be a simple approach for sustainable management of lake salmonid population.