Recent human-induced changes in both marine and freshwater environments are likely to alter adaptative landscapes in migratory species that utilise both habitats. Anadromous salmonids, like the brown trout (Salmo trutta), have recently experienced considerable changes in the marine environment due to a large increase in aquaculture-induced pathogens and parasites. In addition, the freshwater habitats that these fish use for spawning, nursery, and over-wintering are impacted by hydropower and habitat degradation, forming habitats of variable quality. There is increasing concern that these alterations may shift fitness landscapes, leading to adaptations disfavouring anadromy, resulting in reduced recruitment due to diminished individual growth potential and survival whilst at sea. Current declines in marine individual growth coupled with an increase of marine mortality, have resulted in negative demographic consequences for brown trout populations and concerns about its conservation status. This is further impacted by harvesting pressure in both marine and freshwater environments, with larger individuals predominantly targeted. This thesis aims to improve our understanding of the impacts generated by anthropogenic disturbances upon the anadromous traits of individuals and the resulting consequences for populations. Collectively, the three papers in this thesis present evidence for altered patterns of anadromy and fragile demography in five brown trout populations located in Norway’s longest fjord, Sognefjorden. By deployment of acoustic telemetry Paper I used empirical biotelemetry data to fit multi-state mark-recapture models that were used to simulate individual migration trajectories of smolts (first-time migrants) and veteran migrant brown trout. These simulations identified alternate strategies of anadromy, from which individual estimates of growth, fecundity and survival were generated. Individuals undertaking longer-distance migrations were rewarded with increased growth and fecundity, however the main contribution to population-level fecundity was derived from middle-distance migrants, due to high mortality among reduced numbers of long-distance migrants. When compared to historical mark-recapture data collected prior to aquaculture installations and hydropower development, contemporary anadromy has diminished, indicating that selection for extended feeding migrations is under pressure in Sognefjorden. Paper II combined the simulated migration trajectories (generated in Paper I) with openly available models of salmon lice distribution and infestation, to estimate the consequences of accumulated salmon lice exposure during sea-sojourn. The findings revealed that despite equal exposure to salmon lice within the fjord, the degree of infestation varied strongly among populations, with long-distance migrants most impacted. When combined with the findings from Paper I this work implies that brown trout are under selective pressure to reduce their seaward migration extent to avoid direct mortality, due to infestation of salmon lice. Paper III identified individual traits of dispersal among four study populations, by integration of genetic and biotelemetry data. High rates of directional dispersal were observed, suggesting that within the extensive landscape of Sognefjorden, connectivity among populations is significant. However, low rates of effective dispersal indicated that most dispersal is either performed without the intention to spawn (e.g. for overwintering purposes), or that dispersal results in ineffective spawning events. When effective dispersal did commence, this generated directional gene flow which resulted in multiple sink populations being supplemented by dispersing individuals, primarily from a single source population. The excess of available recruits in this source population was attributed to high survival within the fjord. Thus, dispersing individuals from this population potentially provide genetic variance pertinent to advantageous fjord-use behaviour. Thereby likely increasing overall resilience of the metapopulation to variation and shifts in contemporary conditions within the fjord.