Perch swim more and eat faster when exposed to concentrations similar to those in rivers

By Erin Wayman

Web edition: February 14, 2013
Print edition: March 23, 2013; Vol.183 #6 (p. 14)

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European perch exposed to an antianxiety medication in the lab became bolder, more active and less social than unexposed fish, a new study finds.

Credit: Courtesy of Bent Christensen

Fish let their inhibitions go when exposed to a common antianxiety medication, a new study shows. Perch living in water spiked with the drug became bolder, less social and more active than unexposed fish. They also ate much faster.

These behavioral changes could make fish living in wastewater-polluted rivers more vulnerable to predators, the researchers argue in the Feb. 15 Science. And their speedy eating could disrupt the food chain.

Fish and other aquatic animals ingest a variety of pharmaceuticals that end up in the environment. People excrete drugs and flush unused pills down the toilet. Since wastewater treatment plants don’t filter these compounds out of sewage, the drugs end up in rivers.

Toxicologists worry about psychiatric drugs because they target brain molecules found not only in humans but in many other animals. In contaminated waterways, many species may feel the effects of such medications. Previous studies have shown that antidepressants, for example, slow a fish’s reaction time and decrease shelter-seeking behavior.

The new study looked at oxazepam, a type of benzodiazepine. Benzodiazepines are among the most frequently used antianxiety medications. A team led by Tomas Brodin of Sweden’s Umeå University measured concentrations of oxazepam in a Swedish river, five meters downstream of a wastewater treatment plant. The level of oxazepam was 0.58 micrograms per liter. European perch (Perca fluviatilis) that they collected from the river had six times that concentration stored in their muscles, an average of 3.6 micrograms per kilogram.

In the lab, the researchers placed young perch in tanks for seven days with either no oxazepam or with the drug at one of two concentrations: 1.8 micrograms per liter or 910 micrograms per liter. Dissections after the experiment revealed that fish exposed to the lower drug level accumulated amounts of oxazepam in their muscles similar to the wild perch’s supply.

Perch exposed to both low and high levels of oxazepam swam more, spent more time alone and gobbled up a meal of zooplankton faster than unexposed fish. At the highest concentration, fish became more adventurous, entering a new tank more quickly than unexposed fish.

These behaviors directly relate to survival and ecology, says aquatic toxicologist Heiko Schoenfuss of St. Cloud State University in Minnesota. Brazen, solitary fish are probably easy for predators to catch. And fish foraging at high rates may take food away from other animals, altering the makeup of the ecosystem. The next step, he says, is to investigate whether these behavioral changes actually influence wild populations.

That’s difficult to study, in part because wild fish encounter so many different drugs, he says. Some compounds may enhance or counteract other medications’ effects. Researchers need to look at how drug mixtures alter behavior in wildlife, he says.

Regulators assessing the environmental hazards of pharmaceuticals also need information about how drugs influence animal behavior, says environmental toxicologist Bryan Brooks of Baylor University in Waco, Texas. With many chemicals, regulators are concerned only about doses strong enough to kill an organism, he says. They don’t usually consider how smaller doses may change ecologically crucial behaviors.

“If your regulatory toolbox is not suitably developed,” he says, “you run the risk of applying the wrong tool to diagnose environmental perturbations.”