Dams trigger temperature-driven disease in iconic salmonid fish

Researchers from the Estonian University of Life Sciences and the Swedish University of Agricultural Sciences analysed the relationship between water temperature and proliferative kidney disease (PKD), caused by the parasite Tetracapsuloides bryosalmonae, in fourteen Estonian rivers. The resultslink opens in new page were clear: reservoirs significantly increase summer water temperatures by 4–5 degrees Celsius, turning river stretches downstream of dams into hotspots for this harmful parasite.

The researchers found that brown trout living downstream of dams were infected more frequently, harboured higher parasite loads, and exhibited more severe disease symptoms than conspecifics living upstream. “Warm water causes stress in trout as a cold-water species and increases their oxygen demand, while proliferative kidney disease leads to anaemia and reduces the fish’s ability to absorb oxygen. The combined effect of these factors can, in extreme cases, result in the mortality of juvenile fish,” explained the study’s lead author, Professor Anti Vasemägi, Professor of Fish Genetics and Genomics at the Estonian University of Life Sciences.

The study highlights that decisions regarding river restoration, management, and dam removal must also consider the risks posed by temperature-dependent diseases. “Even small reservoirs can render downstream river sections unsuitable for cold-adapted species, particularly during the warmest and most water-scarce periods of summer. Therefore, when restoring human-impacted rivers, it is important not only to remove dams but also to enhance riparian shading in order to reduce water warming,” said the first author of the study, Magnus Lauringson, a researcher at the Department of Aquaculture at the Estonian University of Life Sciences. “Given that there are more than one million dams and barriers across Europe, our findings are of considerable international relevance for river restoration and water management decisions,” Vasemägi added.

There are nearly 1,000 dams in Estonia, many of which are currently no longer in use. According to the Environmental Board, approximately 75% of these structures are impassable for fish. In recent decades, substantial efforts have been made in Estonia to remove obsolete dams. One of the most notable successes has been the removal of the Sindi dam, which restored access to large parts of the Pärnu River system for migratory fish species such as sea trout, vimba bream, and river lamprey. Removing migration barriers gives valuable fish species a renewed opportunity to recolonise their historical habitats and spawning grounds.