Illicit drug use is a growing global health concern that causes a financial burden of hundreds of billions of dollars in the US alone. But hidden beneath the societal costs of this human epidemic is a potential ecological crisis. As methamphetamine levels rise in freshwater streams, fish are increasingly becoming addicted.

“Where methamphetamine users are, there is also methamphetamine pollution,” says Pavel Horký at the Czech University of Life Sciences.

Humans excrete methamphetamines into wastewater, but treatment plants aren’t designed to deal with such substances. Because of this, as treated wastewater flows into streams, so do methamphetamines and other drugs.

In some streams in the Czech Republic, methamphetamine concentrations have been measured at hundreds of nanograms per litre, according to Horký and his colleagues, but the effect of these levels on aquatic animals has been unclear.

To investigate, they set up an experiment to detect possible adverse side effects of this hidden ecological epidemic. They divided 120 hatchery reared brown trout (Salmo trutta) into two 350 litre tanks. The water in one tank contained methamphetamines matching concentrations measured in wild streams while the other was left uncontaminated as a control.

After eight weeks, the researchers removed the methamphetamine from the experimental tank. During the following 10-day “withdrawal” period, Horký tested fish selected at random from both groups for signs of addiction and withdrawal. To do this, he constructed a tank in which water could flow in on one side and out the other as if a stream were passing through the enclosure. One side of the flow, however, contained the same level of methamphetamine that the experimental tank had contained.

The control fish showed no preference for one side of the simulated stream or the other, but the methamphetamine-exposed fish repeatedly chose to stay in the drugged water.

What’s more, the methamphetamine-exposed fish had elevated levels of methamphetamine in their brain tissue and were also less active than normal – which might reduce their chances of surviving and reproducing.

“Drug reward cravings by fish could overshadow natural rewards like foraging or mating,” says Horký. “Such contamination could change the functioning of whole ecosystems.”

Journal reference: Journal of Experimental Biology, DOI: 10.1242/jeb.242145

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