Human drug use can spill over into streams and rivers, because the chemicals pass through wastewater systems that weren’t designed to extract them. To study the effects of a common illicit drug on wildlife, Pavel Horký at the Czech University of Life Sciences Prague and his colleagues looked to brown trout, Salmo trutta.

For 8 weeks, the researchers held 60 trout in a tank spiked with methamphetamine at a concentration of 1 microgram per litre, and 60 control trout in a meth-free tank. The fish were then placed in a tank containing two separate streams of water — one with methamphetamine and one without — between which they could swim freely. Trout that had spent 2 months swimming in meth-spiked water were found on the meth side in 50.5% of observations, compared with only 41.5% for control trout. The authors interpret this preference as a sign of addiction.

Fish from the drugged tank were also markedly less mobile for the first 96 hours after their last exposure to meth, suggesting that they were experiencing withdrawal. The researchers warn that fish that become addicted to drugs could congregate around wastewater discharges, with unknown ecological effects.