Are fish getting more aggressive because of prescription drugs?

SWEDEN — A new study, conducted by scientists in Sweden, suggests that freshwater fish are absorbing pharmaceutical drugs from waterways, making them more voracious and aggressive. While some anglers may consider this good news, the research raises serious questions about the ecological effect of drugs.

The study, reported in Science magazine, is based on a simple problem — what goes into your body must come out. Modern drugs are formulated to resist breaking down, helping them last longer on shelves and work more efficiently in a patient’s body. This hardiness poses problems, however, when the chemicals pass through patients’ bodies, through wastewater treatment plants (which are primarily designed to eliminate bacteria and nutrients), and into our waterways.

According to the study, scientists began noticing psychotropic drugs in our rivers and lakes in the 1990s. To begin evaluating how these chemicals affect wildlife, scientists with Sweden’s Umeå University tested perch living downstream from a wastewater treatment plant on a local river. They first analyzed the river water for Oxazepam, a popular anti-anxiety drug. Traces of the drug were present in the water, so the next question was whether the fish had absorbed it. When they examined muscle tissue from the perch, they found Oxazepam in six times the concentration it was in the river water, indicating the drug had accumulated over time.

For the next stage of the study, the scientists hatched a fresh batch of perch from eggs. Once grown, they used a series of tests to measure survival behaviors essential to perch.

First, they released the perch into the middle of a three-sectioned tank. One of the side compartments held another group of perch, the other was empty. For 10 minutes, the researchers tracked the swimming patterns of the perch in the central tank. Perch are schooling fish, and in this first round of tests, they all stayed close to the perch in the side compartment, seeking safety in numbers.

To measure feeding habits, each fish was placed in a tank with 20 zooplankton, which are their main food source. The scientists recorded how long it took for them to devour all the zooplankton.

As a final test, the scientists measured boldness by putting each fish in a small, dark box in a separate tank. After five minutes, they opened a side door in the box. None of the perch ventured out into the open water, preferring instead the shelter of the box.

The perch were then separated into three groups: the first group went into a tank with clean water, the second into a tank with water with the same amount of Oxazepam as had been found in many rivers worldwide, and the final group got tank water with 500 times the Oxazepam levels as the second group.

After a week in their respective environments, the perch were put through the same three survival behavior tests as before. In the schooling test, the fish from the clean water reacted just as expected, seeking safety in numbers by congregating near the side with other perch. The fish with river levels of Oxazepam spent significantly less time with the other fish, tending to dart around rapidly. And the fish with the elevated levels of Oxazepam avoided schooling altogether.

In the feeding test, the clean water perch took roughly the same amount of time they had when the test was administered a week earlier. The river water perch, after only a week of exposure to Oxazepam, devoured the zooplankton much faster than before. The fish with elevated levels of Oxazepam went on a feeding frenzy.

In the boldness test, none of the clean water fish left the safety of the box. Some of the river water fish were braver, venturing past the edge. All but one of the fish with elevated Oxazepam completely exited the box for open water.

“It’s an extreme effect,” Tomas Brodin, the study’s lead author, told Time magazine. “They get fearless.”

More research needs to be done to fully understand the effects of pharmaceutical drugs on wildlife, but several studies indicate that it is a potential threat for fish worldwide. For example, a recent study reported in the Environment Science and Technology journal found Prozac and Zoloft in brain tissue of fish from rivers in Colorado and Iowa. As additional data emerges, the EPA may enter the picture by regulating the chemical components in drugs. Or perhaps, new filtration systems at wastewater treatment plants are the answer. Hopefully there are answers on the horizon, because the idea of eating a Zoloft-tainted fish is just plain weird.

Grant Olsen joined the team in 2012. He covers travel, outdoor adventures, and other interesting things. Contact him at

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