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Anthropogenic climate warming is affecting ecosystems on a worldwide scale. Global air temperatures are continually rising. Correspondingly, lake water temperatures will increase, considerably changing the habitats and ecosystems of an immense number of species. Many of these species are ectotherms like fish, which are especially vulnerable. This thesis investigates the impact of climate warming on the ecological responses, such as changes in growth, abundance, and biomass, of the brown trout (Salmo trutta L.) populations in the high–alpine lakes of Tronntjärnarna, Jämtland County, Sweden. These lakes have low biodiversity due to a cold and barren environment, with brown trout as the sole fish species. The study tests if observed changes in temperature may have had an impact on the measured ecological responses between each instance of fish sampling, done in 1992, 2005, 2020, and 2023.

The results confirm that the estimated water temperature has increased over the study years. Moreover, expected positive effects on individual growth with increasing temperatures were, unexpectedly, absent. Conversely, a tendency that the growth rate was higher in the colder years of 1992 and 2005 instead of the warmer years of 2020 and 2023 could be observed, contrary to expectations. Also, at the population level, neither abundance nor biomass changed over time. It is discussed that the results may indicate the presence of dynamics outside of those analyzed in this study, such as generational adaptation or increased resource limitation relative to metabolic demand at higher temperatures. However, it is apparent that the studied ecological responses seem to be generally unaffected, despite an observed increase in temperature.

Therefore, this thesis highlights the potential resilience of the brown trout populations in Tronntjärnarna, and possibly, other allopatric cold–water brown trout populations, when faced with the effects of climate warming. Additionally, it emphasizes the importance of further studies to give better insights into the long–term impacts of climate warming on these high–alpine freshwater fish populations.