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Trout can become ‘addicted’ to meth. Here's why that’s so scary. - National Geographic

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Illegal drugs could be having a little-known—and disastrous—impact on freshwater wildlife, new laboratory experiments show.

By Carrie Arnold

Published July 7, 2021

5 min read

Traces of methamphetamine and other illegal drugs that enter waterways could cause addiction in fish, a novel study finds.

Recent laboratory experiments found that brown trout, a common fish in Eastern European rivers, exposed to methamphetamine at concentrations like those seen just downstream of wastewater treatment plants showed signs of addiction—such as being less active—and withdrawal. In the wild, meth-addicted fish could have difficulties reproducing and finding food.

“I was surprised that methamphetamine users can unknowingly cause fish meth addiction in the ecosystems around us,” Pavel Horký, a behavioral ecologist from the Czech University of Life Sciences in Prague, said by email.

Addiction to methamphetamine, a synthetic stimulant, is considered one of the most important global health threats, Horký says, in part because it can lead to mood swings, paranoia, elevated heart rate and blood pressure, and in some cases, death. During the pandemic and in the years before, rates of methamphetamine skyrocketed in the U.S. and Europe: Between 2011 and 2018, U.S. methamphetamine overdose deaths increased among all racial and ethnic groups, according to a January 2021 study . (Read how the pandemic may fuel another opioid epidemic.)

After a person’s body metabolizes the drug, it’s excreted in their feces and urine. Wastewater treatment plants remove many—but not all—contaminants from sewage water before it’s released back into waterways, which are affected by water pollution of all kinds.

The results build on growing evidence that many human-made compounds found in wastewater—from cocaine and heroin to antidepressants and birth control pills—are harming ecosystems, especially fish, says Horký, whose study appeared this week in the Journal of Experimental Biology . For instance, brown trout are vital prey species for many predators, and changes to their behavior or population could reverberate up the food chain. ( Read how antidepressants may crayfish bolder.)

Spillover effect in trout

For their research, Horký and colleagues dosed 60 captive-bred brown trout with methamphetamine-laced water for two months, while keeping another group of 60 control trout in a drug-free tank. To simulate wild conditions, the researchers ensured that the drug levels (one microgram per liter) matched the meth levels other researchers have documented just downstream of wastewater treatment plants in Czechia and Slovakia. 

In the first few days after being removed from the methamphetamine tank, the fish moved around less, which the team interpreted as stress from drug withdrawal. Analysis of brain tissue showed that the fish that moved the least had the most methamphetamine in their brains.

The researchers also gave the trout from both groups a choice to enter one of two streams of water: one with methamphetamine and one without. The meth-exposed trout preferred to swim in the meth-laced water, particularly in the four days after their drug supply stopped. Over time, the study trout’s preference for methamphetamine declined to match those of the control fish—a clear sign of addiction withdrawal, Horký says. (Read how rivers and lakes are the most degraded ecosystems on Earth.)

Disrupted freshwater ecosystems

The study shows wastewater is an underappreciated route through which drugs can hurt wildlife, says Emma Rosi, an ecosystems ecologist at the University of Georgia’s Cary Institute of Ecosystem Studies who wasn’t involved with the research.

“The way aquatic [animals] respond to an antidepressant is going to be different to humans, but that doesn’t mean that they’re not going to respond,” Rosi says.

In one study, scientists found that cocaine in European rivers could interfere with reproduction in critically endangered eels. In Ontario, young male fathead minnows exposed to synthetic estrogen from contraceptive pills didn’t develop testes, instead producing eggs. Other studies have found feminized fish and unnaturally hermaphroditic frogs due to high chemical residues in wastewater.  (Read why these male fish are growing eggs.)

Wildlife dependent on drugs may choose to spend more time near wastewater pipes or runoffs—“which disrupts the system’s whole ecology,” says Matthew Parker, a behavioral neuroscientist at the University of Portsmouth in the U.K. For instance, constantly moving fish help distribute nutrients throughout the environment, both through their own waste and foraging activities, so staying in one place could throw that out of balance.

Wild trout still unstudied

Because these studies were carried out in the lab—a necessary step to understand the potential effects of a chemical under controlled conditions, it’s still not clear how this might alter fish behavior in actual streams, Rosi cautions. Wastewater has a huge array of contaminants and nutrients that could have a different effect compared with methamphetamine alone.

Even so, Rosi says, these results should be a motivation for governments and conservation groups to improve the health of the world’s waterways, such as testing for and removing more contaminants, including pharmaceuticals and illegal drugs.

“Wastewater treatment plants are doing an amazing public service,” she says. “If we want them to do a better service, we need to invest in ways to deal with waste more effectively.”