Drug-Polluted Waters Are Altering Wild Salmon Behavior | Image Source: www.vox.com
STOCKHOLM (Sweden), 10 April 2025 – A new field study published in Science magazine raises urgent environmental questions about the unintended consequences of pharmaceutical pollution on global waterways. According to researchers at the Swedish University of Agricultural Sciences, wild Atlantic salmon exposed to traces of drug clobazam were able to complete their migration to the sea faster than their drug-free counterparts. At first glance, it might look like an unexpected asshole for brass species. But scientists warn: don’t be fooled. The broader consequences could cause problems for aquatic ecosystems that already remain under the influence of multiple anthropogenic threats.
Pharmaceuticals such as benzodiazepines, antidepressants and painkillers, many of which are psychoactive, flee into freshwater systems around the world. According to the results, approximately 1,000 pharmaceutical substances and their metabolites were detected in lakes, rivers and even polar regions such as Antarctica. They do this through several channels: untreated discharge of drug plants, human and animal excretion, inadequate disposal of unused drugs, which are often disposed of directly through the toilet. Because conventional wastewater treatment systems are not designed to completely filter these pollutants, they are released to natural water bodies where wildlife are exposed to them chronically.
We might ask: Do these drugs have a significant impact on animal behaviour? The answer, increasingly, seems to be yes. The Swedish study of salmon, one of the most important of its kind conducted outside the laboratories, showed that young salmon implanted with slow-release clobazam doses navigated faster in two hydroelectric dams and were more successful in reaching the Baltic Sea than in controlling fish. According to Dr. Marcus Michelangeli of the Institute of Australian Rivers at Griffith University, nature based on the field of study makes it exceptionally valuable. Most of the previous research on this topic depended largely on laboratory reservoirs, which did not simulate the dynamic encounter of stressful fish in real rivers.
What does the fish drug do? Clobazam, usually prescribed for human anxiety and seizures, works on the nervous system to reduce stress and promote calm. Exposed salmon appeared to have similar effects. They were less likely to snoop – i.e. swim in coordinated groups – and showed more risky behaviour, which could have allowed them to pass through prey more quickly. But it has a cost. As the main author of the Jack Brand study points out, schooling is a classic anti-predatory behavior. Fish alone are easier targets. Thus, while these bold salmon were fast, they could have been more vulnerable to downstream or high seas predators.
Concern is not limited to salmon. Behavioral Ecologist Michael G. Bertram, co-head of the study, noted that “bridge tails” of pharmaceutical compounds affect wildlife in several ways, many of which remain poorly understood. For example, previous research has shown that antidepressants such as fluoxetine (Prozac) and sertraline (Zoloft) can alter fish mating behaviour, reduce aggressiveness, and even cause morphological changes in male fish. Similarly, it has been shown that oestrogen-like compounds of contraceptive pills feminize male minnows, alter reproductive cycles and collapse populations in some cases.
Is this a general problem? Sure. According to Dr. Michelangeli, more than 900 pharmaceutical products have been documented in global water systems. These are not only psychotropic drugs, but also common substances such as caffeine, antibiotics, metformin (for type 2 diabetes) and acetaminophen. And since fish and other aquatic animals are generally much smaller than humans, even microdoses of these compounds can have a significant impact on their physiology and behaviour.
In one study, male Siamese fish exposed to environmental concentrations of metformin became less aggressive. Another found that oxazepam, another benzodiazepine, made it bolder and more exploratory. While some of these changes may seem neutral or even beneficial in isolation, ecologists warn that these changes are disruptions to fine-tuning the evolutionary characteristics that have developed over the millennia. In short, if a fish behaves in such a way that evolution has not prepared it – even if nothing is faster – it is not necessarily a good thing.
Could that help the fish survive? This is the provocative question at the heart of the salmon study. It is tempting to imagine that a small dose of an anti-anxiety drug is in a way to relieve the stress of salmon while navigating on industrial obstacles such as hydroelectric dams. But experts like Karen Kidd, an ecotoxicologist at McMaster University, are quick to clarify: just because a fish reaches the sea faster does not mean that it will survive long enough to free itself and complete its life cycle. And since the behavioural adaptations of salmon are perfectly suited to a complex food web, changing these behaviours could develop through ecosystems in an unpredictable way.
“The definition of pollution is that it causes damage,” said Kidd, noting that subtle changes in behaviour – such as reduced weakening – can affect not only the exposed individual, but the entire population and related predators or prey. Although modified salmon appear to be better in the short term, they may ultimately be disadvantaged in the high seas where predators abound. And we have little or no data on how these compounds could affect the ability of fish to return to the river as adults to reproduce.
Why not remove these drugs from wastewater treatment plants? It is a practical problem with profound consequences. Most traditional wastewater treatment systems are not designed to filter pharmaceutical compounds. Many of these drugs are chemically stable, which means they do not decompose easily in the environment. According to Michelangeli, this persistence is part of what makes pharmaceutical pollution such a thorny problem. However, it also noted that advanced wastewater treatment techniques are developing, including such methods as ozone, activated carbon filtration and membrane bioreactors, which could significantly reduce drug residues in treated water.
Another promising area is green chemistry: the idea of designing drugs that degrade faster in the environment or become inert after use. If these principles were applied more broadly, we could limit the long-term impact of pharmaceuticals on wildlife without compromising human health. But this will require industrial cooperation and regulatory support, which is still lacking in most countries today.
What can we do? Consciousness is the first step. According to Bryan Brooks, an environmental specialist at Baylor University, who did not participate in the study, the company must examine the consequences of our above behaviours. “If we put things in the environment, we have to understand what’s happening to it,” Brooks said. While the salmon study highlights a fascinating paradox – fish could benefit from our relief from stress in some narrow contexts – it also highlights how little we know about the vast cocktail of substances in our ecosystems.
In the broadest view, Atlantic salmon are already at risk, with population declines due to overfishing, climate change and habitat loss. The fact that even our drugs can now change your behavior during one of the most critical phases of your life cycle – migration from the river to the sea – must be a call to attention. We reshape not only the environment, but also the strategies of evolution and survival of the creatures within it.
What’s the big leak? The study of clobazam offers a look at a world where human habits – even something as harmless as taking a sleep pill – can mature in surprising and disturbing forms. It’s not because the fastest swimming salmon doesn’t mean it’s better. The natural behavior of a species exists for one reason, and when we begin to affect this behavior unintentionally, we risk changing the balance of all ecosystems.
It is not a question of satanizing drugs or discouraging their use, many are vital and essential. It’s about recognizing that our lives are linked to the natural world more and more than we know. And sometimes the invisible threads – like the chemicals we swallow – are the ones that tremble most in the seams of nature.
