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Hazy prospects for rainbow trout: Researchers uncover hidden dangers of water 'browning'

Scientists from the University of South Bohemia and the Research Institute of Farm Animal Biology (FBN) have discovered that the increasing browning of our waters caused by organic substances poses a previously underestimated threat to rainbow trout. Fish react with a clear stress response at the molecular level.

Browning is a phenomenon that has received little attention and refers to the brown discoloration of water. It is caused by the release of fulvic acids and humic substances from the decomposition of plant material, facilitated by increasing inputs of organic matter and rising temperatures due to climate change. The effects on aquatic life have now been comprehensively analyzed for the first time—with surprising results.

An international team of researchers from the University of South Bohemia in Budweis, the FBN in Dummerstorf and the University of Veterinary Medicine in Hanover, Germany, has investigated how rainbow trout respond to increased concentrations of fulvic acid. The of the study have recently been published in the Journal of Hazardous Materials.

Molecular defense reactions detected in fish

Young rainbow trout, an important and popular food fish worldwide, were exposed in the laboratory for four weeks to two different concentrations of fulvic acid—comparable to those found in some natural waters today. While the low concentration of 5 mg carbon per liter showed little change, the higher concentration of 50 mg carbon per liter showed clear effects on the trout: Although there was still no visible external or histological damage to the gills, at a molecular level the researchers sounded the alarm.

"A total of 34 genes in the gill tissue of the fish reacted to the increased fulvic acid load," explains Dr. Alexander Rebl of the Fish Genetics Working Group at the FBN. These genes play a central role in detoxification, oxidative stress and immune defense. The activation of the aryl-hydrocarbon receptor signaling pathway, which plays an important role in the recognition and processing of harmful substances, was particularly striking," continues Rebl.

Chemical stress: What browning means for fish

The results show that browning is not just an aesthetic change to our waters. Rather, it triggers a stress response in fish that indicates an active defense against chemical stress. The consequences could be far-reaching: Constant stress costs the animals energy, weakens their health in the long term and could make them more susceptible to disease.

"We need to take the browning of water bodies seriously as an environmental problem. Even if we don't see any external damage yet, our results clearly show that fish already react to low levels of pollution," says Dr. Thora Lieke, a researcher at the University of South Bohemia. "Now we need to look more closely at the long-term effects on fish populations and the whole aquatic ecosystem."

Important basis for environmental protection measures

The results of the study provide an important basis for better assessing the condition of our freshwater systems and developing timely protection measures. Future studies must increasingly aim to better understand the long-term effects of browning on aquatic life and thus protect aquatic ecosystems in the long term.

A comprehensive understanding of the effects of climate change and human impacts on our water bodies is essential. This will not only benefit animals and the environment, but also consumers, as healthy waters are the basis for a sustainable and safe food supply. Measures to combat browning therefore also ensure the long-term quality of popular food fish such as rainbow trout.