The finding by researchers from Umea University in Sweden may have significant implications for existing standard ecotoxicological tests, which predominantly focus on harmful effects of water contaminants and ignore the potential benefits.
The researchers retrieved two-year-old Eurasian perch from a lake in Sweden and randomly exposed them to high and low concentrations of Oxazepam.
Oxazepam is a benzodiazepine which is commonly used to treat anxiety and insomnia in humans and regularly contaminates surface waters via treated wastewater effluent.
The researchers have previously shown that the drug can increase the activity and boldness of Eurasian perch.
In this study, the low concentration of Oxazepam was below that measured in treated effluent water in Europe.
The researchers also collected eggs, or roe strings, from a separate population of perch and exposed them during the first nine days of embryonic development to three different concentrations of Oxazepam.
After hatching, a random group of the fry were collected and analysed.
Results showed that mortality rates were high among hatched fry - corresponding to mortality rates found among perch fry in natural populations - and relatively high among the two-year-old perch, but were significantly reduced by Oxazepam exposure in comparison to the control group of fish who were not exposed.
In the hatched fry, mortality was lower in the high concentration treatment than in the control and low concentration treatments. In the two-year-old perch, mortality was lower in both the low and high concentrations compared to the control.
"A therapeutic effect leading to increased survival of one species may generate a proportional increase in mortality of that species' prey, which may have cascading ecological consequences that need consideration," co-author of the study Tomas Brodin, said.
The study was published in the Institute of Physics (IOP)'s journal Environmental Research Letters.