Tomas Brodin and colleagues from Umea University in Sweden discovered that the a wild European perch, a fish species ate faster, became bolder and acted less social after being subjected to an anxiety-moderating drug, known as Oxazepam.
The psychiatric drug is used to treat anxiety in humans. But, Oxazepam residues often wind up in natural aquatic systems, downstream from sewage treatment plants, where their effects on eco-systems have been unknown, the journal Science reports.
Now, Brodin and the other researchers have dosed wild perch with amounts of Oxazepam equivalent to those found in Sweden's rivers and streams, and their results suggest that even small amounts of the drug can alter the behaviour and the foraging rates of these fish, according to an Umea statement.
"While alone, fish that were exposed to Oxazepam dared to leave safe refuge and enter novel, potentially dangerous areas," explained Brodin.
"In contrast, unexposed fish stayed hidden in their refuge. The exposed fish seemed much less stressed and scared, behaving calmer and bolder."
Perch that were exposed to the drug also devoured their food quicker than unexposed fish-a behavioural quirk that the researchers say could alter the composition of species in the water and lead to ecological events, such as increased algal blooming, over time.
Since fish are generally integral pieces of their food webs, changes in their eating patterns could disturb ecological balances, according to the researchers.
The fish that were given Oxazepam during the study also became anti-social, distancing themselves from other perch and putting themselves at greater risk of predation.
"Perch that were exposed to Oxazepam lost interest in hanging out with the group, and some even stayed as far away from the group as possible," explained Brodin.
The fish in the study accumulated concentrations of the drug in their muscle tissues that were comparable to those found in wild fish, said the researchers.
But, a veritable cocktail of drugs can be found in waterways worldwide, making the discovery of Oxazepam's effects on fish that much more important.