Drugs to treat anxiety in people may alter the behaviour of fish when the chemicals are flushed into rivers, according to scientists. Swedish researchers found that European perch exposed to tiny concentrations of a drug became less sociable, ate more and became more adventurous – all changes in behaviour that could have unexpected ecological impacts on fish populations.

When scientists at Umeå University in Sweden screened rivers for pharmaceuticals they found that a drug for treating anxiety, called oxazepam, was accumulating in fish. Many drugs and other synthetic chemicals used by humans in everything from pesticides to cosmetics can pass through waste water treatment and end up in wildlife, potentially accumulating to toxic levels.

But until now scientists had never studied the behavioural impacts of small quantities of contaminants. Tomas Brodin led a team that mimicked in the lab the concentrations of oxazepam found in the wild – around a microgram per kilogram of fish body weight – and watched for changes in how bold, sociable and active the fish were.

"Normally, perch are shy and hunt in schools," said Brodin. "This is a known strategy for survival and growth. But those who swim in oxazepam became considerably bolder."

The results are published this week in Science and were announced at the annual meeting of the American Association for the Advancement of Science in Boston.

Jonatan Klaminder, an ecologist at Umeå University and an author of the paper, said the effect of the drug on fish was similar to its effect on people. "What the drug does is remove some of the fear that the very small fish experience," he said. "[They] become less interested in staying close with others – staying close to others is a well-known defence system to avoid predators. They become less afraid of exploring new areas, so they just go out to search for food and become more effective in finding and consuming food."

This change in behaviour could have evolutionary consequences. Adventurous or antisocial fish are more likely to be eaten by larger fishes but are also the ones that will explore new areas and, over time, alter the genetic diversity of future populations.

The solution, according to the researchers, is not to stop medicating people who need drugs such as oxazepam but to improve sewage treatment plants to capture the drugs and reduce their contamination of water systems in the wild.

The research also has implications for the way ecologists monitor pollutants in the environment, said Klaminder. "We're still deeply rooted in what a pollutant is and it goes back to the 1970s and 1980s where we had heavy rain, acid rain, organic pollutants that definitely cause harm and physiological effects. When it comes to drugs, there is a new area of contamination research that doesn't really fit with this old conceptual view." Focusing on the potential negative physiological impacts of an environmental contaminant could miss the subtle behavioural changes that may also occur.

He added: "Hopefully it will make researchers rethink what they are looking for."