"Our first thought is, this is like a person diagnosed with ADHD," said Jonsson, referring to attention deficit-hyperactivity disorder. "They become asocial and more active than they should be."
Tomas Brodin, another member of the research team, called the drug's environmental impact a global problem. "We find these concentrations or close to them all over the world, and it's quite possible or even probable that these behavioral effects are taking place as we speak," he said Thursday in Boston at the annual meeting of the American Association for the Advancement of Science.
Most previous research on trace drugs and marine life has focused on biological changes, such as male fish that take on female characteristics. However, a 2009 study found that tiny concentrations of antidepressants made fathead minnows more vulnerable to predators.
It is not clear exactly how long-term drug exposure, beyond the seven days in this study, would affect real fish in real rivers and streams. The Swedish researchers argue that the drug-induced changes could jeopardize populations of this sport and commercial fish, which lives in both fresh and brackish water.
Water toxins specialist Anne McElroy of Stony Brook University in New York agreed: "These lower chronic exposures that may alter things like animals' mating behavior or its ability to catch food or its ability to avoid being eaten -- over time, that could really affect a population."
Another possibility, the researchers said, is that more aggressive feeding by the perch on zooplankton could reduce the numbers of these tiny creatures. Since zooplankton feed on algae, a drop in their numbers could allow algae to grow unchecked. That, in turn, could choke other marine life.
The Swedish team said it is highly unlikely people would be harmed by eating such drug-exposed fish. Jonsson said a person would have to eat 4 tons of perch to consume the equivalent of a single pill.
Researchers said more work is needed to develop better ways of removing drugs from water at treatment plants. They also said unused drugs should be brought to take-back programs where they exist, instead of being flushed down the toilet. And they called on pharmaceutical companies to work on "greener" drugs that degrade more easily.
Sandoz, one of three companies approved to sell oxazepam in the U.S., "shares society's desire to protect the environment and takes steps to minimize the environmental impact of its products over their life cycle," spokeswoman Julie Masow said in an emailed statement. She provided no details.
Overview of the drug: www.nlm.nih.gov/medlineplus/druginfo/meds/a682050.html