Overlooked threat: dams trigger temperature-driven disease in iconic salmonid fish

A new study published in Communications Biology reveals a critical, yet previously overlooked, environmental consequence of man-made dams constructed across rivers and streams. By investigating a key indicator species of ecosystem health, the brown trout (Salmo trutta), researchers from the Estonian University of Life Sciences and the Swedish University of Agricultural Sciences demonstrated that small river impoundments significantly elevate water temperatures and drastically increase the pathogenic impact of Proliferative Kidney Disease (PKD).

The Hidden Hazard

Dams provide essential services for humans but profoundly alter river ecology. While issues like habitat fragmentation are well-documented, the new findings focus on the complex interaction between dams, thermal regimes, and disease. The international research team examined juvenile brown trout and measured water temperatures both upstream and downstream of 14 small reservoirs and artificial impoundments.

The results were compelling: reservoirs releasing surface water substantially raise downstream temperatures (by up to 4–5 °C) during the summer. These warmed sections turn into hotspots for the myxozoan parasite Tetracapsuloides bryosalmonae, which causes a deadly disease among salmonids. Brown trout in these sections exhibited consistently higher infection prevalence, greater parasite loads, and more severe signs of PKD compared to their upstream counterparts.

Implications for River Management and Climate Resilience

"The reservoirs are not just warming the water; they are creating the ideal conditions for an epidemic," explains Prof. Anti Vasemägi, lead researcher. "Below dams releasing surface water, salmonids are caught in a thermal disease trap: higher temperatures spike their oxygen needs just as the water's oxygen-carrying capacity drops. Warmer water also increases parasite proliferation and accelerates disease progression, making salmonids anemic and less capable of extracting sufficient oxygen from the water. Ultimately, this deadly synergy leaves cold-water trout defenseless against heatwaves, pushing them from the frying pan into the fire."

“Dams are creating micro-climates that fast-forward the effects of global warming,” says first author Dr. Magnus Lauringson. “Small dams are everywhere, and our findings show they are amplifying the threat of climate change by simultaneously creating severe thermal stress and an increased disease burden.”

The authors urge a shift in the current paradigm for assessing dam removal and river restoration. To prevent widespread fish mortality, mitigation measures, such as nutrient load reduction, dam removal and improving riparian shading to cool the water, are necessary to safeguard vulnerable cold-water salmonid populations globally.

The study highlights that river management and dam removal decisions must adopt a more holistic approach, formally including the risk of temperature-driven disease in environmental impact assessments. Given that surface-release dams are ubiquitous across global river networks, the study's implications are widespread and highly relevant to global river restoration efforts.