Some waterways may look pristine, but there's more going on beneath the surface. In 2019, a population of freshwater shrimp in England tested positive for cocaine. The year before, researchers found mussels near Seattle with traces of opioids and other drugs in their systems. And now, researchers in the Czech Republic suggest that fish can show signs of addiction when exposed to methamphetamine in contaminated water.

In a lab-based experiment, the researchers mimicked the surroundings of brown trout swimming in drug-polluted rivers. As CNET reports, the fish were separated into two groups. The first group was kept for eight weeks in a tank with clean water, while the second group lived in a tank with a concentration of 1 microgram of meth per liter of water. After the eight weeks, the researchers moved all the fish into a tank with two streams of water: one clean, the other laced with meth.

They found that the fish from the meth-laced tank suffered withdrawal-like symptoms for about four days and were sluggishly moving around the tank. About half of the meth-exposed fish were attracted to the meth-tainted stream, while only 41 percent of the drug-free fish went to the tainted stream. The researchers also discovered that the trout from the meth-containing tank had traces of the drug in their brains 10 days after their last exposure to the polluted water. The findings appeared in the Journal of Experimental Biology.

The study aimed to examine how detrimental low-level drug pollution is to aquatic health. As Pavel Horký, the paper's lead author, said in a press release, the team is “concerned that drug addiction could drive fish to congregate near unhealthy water treatment discharges in search of a fix, as well as disturbing their natural tempo of life.” But since this study was conducted under lab conditions, more research is needed to know if brown trout in the wild would experience the same effects.