Climate-driven population responses of resident brown trout, Salmo trutta: Trends and future projections

The climate is changing at an alarming rate with consequences such as species and population extinctions, changes in species distribution and phenology. However, mechanisms underlying these global trends are not well understood, especially at a population level. Climate effects on demographic traits and population dynamics have recently received increasing attention as key importance for understanding the ecological impacts of climate change. The effects on demographic traits might vary across populations that are adapted to respective local environmental conditions. Hence, different populations within the same species might perform differently while subjected to the same climate scenario. In this thesis, I focused on how environmental variability and its effects on key life history traits of freshwater resident brown trout, Salmo trutta, influence population dynamics. In particular, I explored effects of variations in temperature, precipitation and general winter conditions together with other ecological variables to elucidate how climate induced variations in demographics characteristics might result in altered population dynamics. I explored these matters mainly through statistical models based on climate variations in spatio-temporal environmental data, studying individual differences within and between-populations. Further, I included possible adaptive responses to climate change in the population projections to assess possible ecoevolutionary rescue scenarios. I found evidence for local environmental adaptations in early growth of brown trout. I argue that higher growth rate associated with populations experiencing more precipitation and low temperatures, was an adaptation to short, harsh and unstable environments in streams during early life stages. Further, I found that density and temperature interacted in a non-additive and complex way as controlling agents of growth performance, where a general positive effect of warm temperature minimised an apparent negative effect of density. Finally, by combining demographic parameters into population models I was able to infer mechanistic effects of climate change at the population level. In general, simulations including increasing and more variable temperatures in the next 100 years resulted in negative effects on the brown trout population growth rates. The effect on the population growth was, however, dependent on the population’s spatial location and the spawning reaction norms in relation to altered individual growth rates. Simulated adaptations towards maturation at younger and smaller fish countered a negative population effect of a changing climate in some situations, thus showing possible eco-evolutionary rescue scenarios. It is clear that climate may induce phenotypic plasticity in life history traits. I hope that my thesis will contribute to illuminate why and how altered life history traits might affect an ectothermic species, such as the brown trout, at the population level in a changing climate. In addition, I have tried to explore an intersection between ecological and evolutionary responses to address how adaptive evolutionary scenarios might counteract the effects of a rapidly changing climate. I propose that the type of model framework used in this thesis should be a compelling applied tool for nature managers to infer future mitigation efforts at the population-level