We review three long-term research programs performed over the last four decades on the ecology and management of oligotrophic lake systems with different fish communities at 69 N in Norway. Through whole-lake perturbation experiments, intensive culling of stunted fish removed 35 tons (1984)(1985)(1986)(1987)(1988)(1989)(1990)(1991) of Arctic charr Salvelinus alpinus in Takvatn (15 km 2 ) and 153 tons (1981)(1982)(1983)(2002)(2003)(2004) of European whitefish Coregonus lavaretus in Stuorajavri (25 km 2 ). In Takvatn, the overcrowded charr population decreased to 20% of the initial abundance, whereas brown trout Salmo trutta abundance increased. Somatic growth improved strongly in both species. In charr, ontogenetic habitat shifts broke down, the diet changed to more benthos, and plankton-borne parasites decreased. High abundance of juvenile, littoral charr provided new prey for trout, creating an alternative, predator-regulated stable state. Similar density reductions, positive effects on growth and reduced parasite loads occurred in whitefish in Stuorajavri. Despite the heavy culling, however, a new stable state did not occur and the fish community returned to the pre-culling situation. In the Pasvik watercourse, vendace Coregonus albula invaded around 1990 after an upstream introduction. The population of this non-native, highly specialized planktivore increased rapidly, resulting in steep density declines in zooplankton and the native planktivorous whitefish morph, and large changes in energy flow and structure and dynamics of the lacustrine food web. These programs show that long-term research is essential for understanding the ecology of manmade disturbances and providing a scientific basis for management efforts.

Competition is assumed to shape niche widths, affecting species survival and coexistence. Expectedly, high interspecific competition will reduce population niche widths, whereas high intraspecific competition will do the opposite. Here we test in situ how intra-and interspecific competition affects trophic resource use and the individual and population niche widths of two lacustrine fish species, Arctic charr and brown trout, covering a 40 year study period with highly contrasting competitive impacts prior to and following a large-scale fish culling experiment. Initially, an overcrowded Arctic charr population dominated the study system, with brown trout being nearly absent. The culling experiment reduced the littoral Arctic charr density by 80%, whereupon brown trout gradually increased its density in the system. Thus, over the study period, the Arctic charr population went from high to low intraspecific competition, followed by increasing interspecific competition with brown trout. As hypothesized, the relaxed intraspecific competition following the experimental culling reduced individual diet specialization and compressed population niche width of Arctic charr. During the initial increase of the brown trout population, there was a large dietary overlap between the two species. Over the subsequent intensified interspecific competition from the population build-up of brown trout, their trophic niche overlap chiefly declined due to a dietary shift of Arctic charr towards enhanced zooplankton consumption. Contrary to theoretical expectations, the individual and population niche widths of Arctic charr increased with intensified interspecific competition. In contrast, the diet and niche width of brown trout remained stable over time, confirming its competitive superiority. The large-scale culling experiment and associated long-term research revealed pronounced temporal dynamics in trophic niche and resource use of the inferior competitor, substantiating that intra-and interspecific competition have large and contrasting impacts on individual and population niches.

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Reproductive traits differ between intralacustrine Arctic charr morphs. Here, we examine three sympatric lacustrine Arctic charr morphs with respect to fecundity, egg size and spawning time/site to assess reproductive investments and trade‐offs, and possible fitness consequences. The littoral omnivore morph (LO‐morph) utilizes the upper water for feeding and reproduction and spawn early in October. The large profundal piscivore morph (PP‐morph) and the small profundal benthivore morph (PB‐morph) utilize the profundal habitat for feeding and reproduction and spawn in December and November, respectively. Females from all morphs were sampled for fecundity and egg‐size analysis. There were large differences between the morphs. The PB‐morph had the lowest fecundity (mean = 45, SD = 13) and smallest egg size (mean = 3.2 mm, SD = 0.32 mm). In contrast, the PP‐morph had the highest fecundity (mean = 859.5, SD = 462) and the largest egg size (mean = 4.5 mm, SD = 0.46 mm), whereas the LO‐morph had intermediate fecundity (mean = 580, SD = 225) and egg size (mean = 4.3, SD = 0.24 mm). Fecundity increased with increasing body size within each morph. This was not the case for egg size, which was independent of body sizes within morph. Different adaptations to feeding and habitat utilization have apparently led to a difference in the trade‐off between fecundity and egg size among the three different morphs.

1. Ecosystems at high latitudes are exposed to some of the highest rates of climate warming on earth, and freshwater ecosystems in those regions are already experiencing extended ice-free seasons and warmer waters. The dominant fish species in these ecosystems are cold-water salmonids, which play a central ecological role in lake ecosystems, where they are often exposed to size-selective fisheries that truncate their size and age distributions, making them potentially vulnerable to exploitation and environmental perturbations.2. Here, we address the combined effects of climate-induced water temperature increase (using regionally downscaled climate models based on the RCP-4.5 and RCP-8.5 climate scenarios together with an air-to-water temperature model) and gillnet harvesting, over the period from 1950 to 2100, on the somatic growth, demography and vulnerability of Arctic charr Salvelinus alpinus (L.), using an eco-genetic individual-based model. The model captures successive annual lifehistory events, including the key processes of size-dependent mortality, age-and size-dependent maturation described by a probabilistic reaction norm, temperature-dependent growth, size-dependent reproduction and density-dependent recruitment.3. Our model predicts that higher water temperatures will increase the somatic growth of Arctic charr, leading to larger body size at age and increased stock biomass: for RCP-8.5, we predict an 80% increase in stock biomass in the year 2100 relative to the year 2000 in the absence of fishing. Interestingly, this potential increase in biomass in future climate scenarios will be partially masked by harvesting: for a fishing mortality of 0.3 year −1 , we predict a mere 40% increase in stock biomass in 2100 relative to 2000. Despite the predicted increase in stock biomass, yield will increase substantially only when fishing mortality is low. In addition, climate warming will accentuate the age-truncation effect of harvesting, which will target younger individuals, including immatures, thus elevating the vulnerability of the population to environmental perturbations. | 271Journal of Applied Ecology SMALÅS et AL.