Image caption Tomas Brodin and his lab fish

Psychiatric drugs taken by humans to cope with anxiety are being excreted and flushed into waterways where they are probably affecting fish behaviour.

This is the conclusion of a new Swedish study reported in Science magazine.

The Umea University-led investigation exposed laboratory perch to the drug Oxazepam at levels equivalent to the residues found in rivers and streams.

The fish were found to eat faster, to be less social and to be bolder than animals that were not dosed.

Umea’s Tomas Brodin said the behavioural changes seen in their lab animals could have unexpected evolutionary and ecological consequences if reflected in wild populations.

“The strong behavioural modifications we saw were at low concentrations, and I firmly believe we could go even lower and still see these effects,” the researcher told BBC News.

Media playback is unsupported on your device Media caption Tomas Brodin: "They became more efficient feeders"

“We looked at just one benzodiazepine-type drug but there are many others out there, and they probably all have the same effects on fish and other vertebrates. So we may be underestimating what is happening in nature.”

Dr Brodin was speaking here in Boston at the annual meeting of the American Association for the Advancement of Science (AAAS). His address was timed to coincide with the journal publication.

The fish in the Umea study were measured to have concentrations of the drug in their muscle tissues that were comparable to those found in wild animals.

This suggests wild fish ought to exhibit similar modified behaviours to the dosed lab animals.

However, unlike the oestrogenic residues from contraceptive pills that have been shown to produce physical changes in male fish, documenting the altered patterns of free-swimming animals will be much harder.

Image caption The notion that the drug affects perch has implications for the fish's entire ecosystem

Difficult, also, will be establishing the precise ecological consequences of the modified behaviours.

For example, perch eat zooplankton, which in turn consume algae. If perch become more efficient eaters, the numbers of zooplankton will be suppressed which could lead to more frequent algal blooms in waterways.

But the modified behaviours would also make the perch more vulnerable to predation themselves by the likes of pike. In that case, zooplankton populations would rise and the numbers of algae would fall.

“It’s really complex trying to work out what would happen on a community level, on the species level and on the individual level,” Dr Brodin said.

The solution to the problem, he argued, was not to stop medicating people but to find more efficient ways of capturing the chemicals as they go through sewerage plants. In addition, trials might be able to identify those drugs that had least ecological impact. The medical profession could then be encouraged to preferentially prescribe these products when appropriate.

Dr Brodin also spoke about the general problem of drug residues in waterways.

“It’s scary when you think about it," he told BBC News. "It’s a huge cocktail of drugs and fish are experiencing it 24/7 – breathing it, drinking it and eating in it.”

Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter: @BBCAmos