ResearchAntidepressants Make Fish Easy Prey: Loss of Natural Reaction to Stress

Press Release No. 68/2020
19 August 2020

Interdisciplinary project on effects of drugs in the aquatic environment

Residues of pharmaceuticals in surface waters − in this case an antidiabetic and two antidepressants − also have effects on fish. Drugs for the treatment of depression have particularly strong effects, with fish losing their natural reaction to stress when substance concentration in the water is above a certain level. This is shown by studies carried out as part of the interdisciplinary project “Effect Network in Water Research” (Effect-Net). Scientists from Heidelberg University, the University of Tübingen and the Karlsruhe Institute of Technology (KIT) are collaborating in Effect-Net to investigate the effects of drugs and food additives in aquatic ecosystems. In addition, political scientists are investigating consumer behaviour and the question of whether consumers would perhaps switch to drugs that are more expensive but less harmful to the environment.


As documented by behavioural studies with zebrafish and brown trout, antidepressants have a particularly strong effect. The scientists worked with two selected drugs (fluoxetine, citalopram) used in large quantities for the treatment of depression. These neuroactive substances led to behavioural changes even at extremely low environmentally relevant concentrations. For example, embryos of zebrafish normally react very sensitively to changes in light intensity. They frantically swim back and forth in search for a place to hide. This behaviour changed under the influence of antidepressants. The higher the concentration of the two drugs in the water, the less the fish showed this protective reflex following changes in the lighting of their environment. The fish had lost their natural reaction to stress.

The researchers also confirmed this finding in their studies with brown trout. “Comparable to the decrease in states of depression observed in humans, the fish seem to shed their natural anxiety behaviour as the concentration of active ingredients in the water increases. This makes them easy prey for predatory fish,” explains Prof. Dr Thomas Braunbeck, biologist at the Centre for Organismal Studies (COS) at Heidelberg University and coordinator of Effect-Net.

The scientists were also able to document effects in the case of an antidiabetic drug frequently administered for type 2 diabetes. Long-term experiments showed that the active substance in environmentally relevant concentrations − i.e. quantities also detectable in rivers – led to brown trout storing more carbohydrates in the liver and losing body weight. In addition, the bacterial composition in the intestines of the fish changed significantly. The scientists found, for example, an increase in the number of bacterial species that were potentially harmful for the brown trout. “In the long term, this might lead to changes in development and overall performance,” said Prof. Dr Rita Triebskorn from the Institute of Physiological Ecology of the University of Tübingen. “In addition, we were able to show that the anti-diabetic strongly activates disease-causing genes in the bacteria of the intestinal flora. These can also negatively impact the health of the fish,” says Prof. Dr Thomas Schwartz of the Institute of Functional Interfaces at KIT.

Another aspect of the interdisciplinary research project is consumer behaviour. Many people are aware of the fact that drugs can be released into bodies of water and may harm organisms. This resulted from a representative survey carried out by Prof. Dr Jale Tosun from the Institute of Political Science at Heidelberg University in the context of Effect-Net. More than 2,000 people were interviewed, e.g. on responsibility for pollution, their attitudes towards regulatory measures and changes in personal consumer behaviour. Less widespread than knowledge about drug residues in bodies of water is the awareness of personal responsibility. “The majority of respondents consider agriculture and industry to be mainly responsible,” the Heidelberg political scientist emphasises.

The survey showed that, in principle, consumers would be willing to use more environmentally friendly products but they often lack the specific information about less environmentally harmful alternatives necessary to change their behaviour. Significantly fewer people, however, are prepared to pay more for these more environmentally friendly drugs. Prof. Tosun explains that, faced with the choice of whether there should be state regulation in the future, such as a ban on products, or financial measures, such as a tax on environmentally harmful medicines, consumers with strong environmental awareness prefer government intervention.

The “Effect Network in Water Research” project is being funded by the state of Baden-Württemberg over a period of five years as part of the “Baden-Württemberg Water Research” network. On the basis of the scientific findings, social and political scientists are also working on strategies for regulating consumer behaviour and on proposals for environmental legislation. A number of publications have already appeared. The studies started in 2016. The final results of the project are expected by the end of 2021.