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Numerous anthropogenic stressors, including river regulation, excess loadings of nutrients and sediment, channelisation, as well as thermal and hydrological stressors driven by climate change impact riverine ecosystems worldwide. In a time when freshwater degradation and the rate of global warming are faster than ever, understanding the potential interactive effects of local and catchment‐scale stressors with large‐scale climatic conditions is essential to enhance our ability to plan effective conservation, restoration, and mitigation measures.In this study we analysed a dataset spanning the whole of Sweden using a space‐for‐time approach to investigate interactive effects of land use, river regulation, and climate on brown trout (Salmo trutta) abundance in streams.We found that in warmer regions trout populations were negatively affected in catchments with more intense river regulation by hydropower dams (i.e. ≥10 m3/km2 total reservoir storage volume). In such catchments, a 7°C warmer mean summer air temperature was associated with an average between 44% and 83% decline in trout abundance. In catchments with less intense river regulation, trout abundance instead increased moderately with increasing temperature.We also found that brown trout abundance declined with increasing areal extent of urban areas when found in combination with ≥20% agricultural land use. When agricultural land use reached maximum values (84%), brown trout abundance decreased from an average of 13 individuals per 100 m2 in catchments with no urban areas to values ≤1 in catchments with ≥5% urban land use. Also, brown trout abundance declined with increasing agricultural land use in catchments with ≥3% urban land use.Our study brings innovative empirical evidence of interactive effects between river regulation, land use and climate on brown trout populations. From a management perspective our findings suggest that: (1) restoring natural flows (e.g. through dam removal) and riparian vegetation could mitigate adverse effects of climate change; and (2) restoration measures that minimise the effects of agriculture and urban land use (e.g. reduction of nutrient levels and restored riparian buffer zones) could help rehabilitate brown trout in catchments with high anthropogenic land use change. However, given the large observed variation between streams, we advise for bespoke management actions stemming from sound knowledge of local habitat conditions and target populations, whenever possible, using an ecosystem management‐based approach. [ABSTRACT FROM AUTHOR]

• Evaluating a new species for contaminant monitoring. • Stable isotopes to deduce breeding strategy. • Isotope methods and ecological knowledge improve mercury contaminant interpretation. Contaminant monitoring in biota is important for determining environmental status and to detect or prioritize action on hazardous substances. Predators higher up a food chain are often used for monitoring of contaminants that bioaccumulate. However, it is not always possible to find higher predators that are both abundant and have a wide distribution for national or international contaminant monitoring. Great cormorants (Phalocrocorax carbo) are a widespread and increasingly common top predator of fish in fresh, brackish and salt water. We evaluate the suitability of great cormorant eggs as a matrix for contaminant monitoring by using stable isotopes of carbon, nitrogen and sulfur. Despite the fact that cormorants are migratory, egg isotope values showed a significant separation between five breeding colonies in Sweden (1 fresh water lake, 3 Baltic sites and 1 marine site). This high degree of separation indicates that eggs are primarily produced using local resources (not stored body resources) and that contaminants (mercury concentrations in this study) measured in eggs likely reflect levels in fish prey caught close to the breeding area. Compound specific stable isotope analysis was used to estimate cormorant trophic position (TP) and concentrations of mercury in eggs were positively related to TP. The results show that a multi-isotope approach, combined with good ecological diet knowledge allow for meaningful and comparative interpretation of mercury concentrations in biota and that great cormorant eggs appear a suitable matrix to measure locally derived and maternally transferred contaminants. [ABSTRACT FROM AUTHOR]