May 2023

A juvenile of brown trout. Species that thrive in cold water, like the brown trout (Salmo trutta), are facing local population extinctions at lower latitudes, when the water reaches high temperatures and the water flow is low. Our study shows that brown trout has declined in many rivers in the warmer regions of Sweden over the last thirty years, particularly in the smaller streams. Trout abundance in colder regions has instead increased in many places. Declines were also stronger for migrating populations, as compared to stream resident ones. With increasing frequency of summer heat waves and droughts, amplified by land degradation, our study highlights the need for measures that reduce the risk of reaching high water temperatures, maintain a continuous water flow, and guarantee free passage of organisms in the rivers. Photo by Joacim Näslund.

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