Cocaine pollution in rivers and lakes may disrupt behaviour of salmon, study finds | Science

Traces of cocaine that pollute rivers and lakes could accumulate in the brains of salmon and disrupt their behavior, according to researchers who warn of unknown consequences on fish populations.

Juvenile Atlantic salmon that were artificially exposed to the drug and its main breakdown product swam farther and dispersed more widely across a lake, suggesting the substances may affect where fish go, what they eat and their vulnerability to predators.

The impact of pollutants when they enter waterways from sewage treatment plants is unclear, but fish may pay a price if they burn more energy, or face greater risks from predators if they have to search for more food to conserve energy, scientists say.

“Largely we don’t know the consequences, but I expect trade-offs,” said Dr Jack Brand of the Swedish University of Agricultural Sciences. “They may end up in worse condition or have to compensate for this by foraging a lot more, which means they spend more time out in the open.”

Scientists have previously said pollution from common medicines poses “a major and growing risk to biodiversity” and called on pharmaceutical companies to make greener medicines that break down in the environment. Concerns about contaminants have been fueled by reports that meth-addicted trout and perch were losing their fear of predators because of antidepressant medications.

In 2019, tests on freshwater prawns in Suffolk rivers revealed traces of dozens of different drugs, including cocaine, methamphetamine, antidepressants, anti-anxiety medications and antipsychotics, but researchers drew no conclusions about their harmful potential.

To determine whether cocaine pollution might affect fish in the wild, Brand and colleagues fitted two-year-old hatchery-raised Atlantic salmon with implants that slowly released ecologically realistic levels of cocaine or its main metabolite, benzoylecgonine. A third group of fish that received implants without medication served as a control. All were equipped with acoustic transmitters.

The fish were released in the southwest corner of Lake Vättern, which at almost 2,000 km² (772 square miles) is Sweden’s second largest lake. Other species in the lake include large predatory pike. Using sensors installed around the lake, researchers tracked the salmon for two months.

All salmon became less active and settled more in one part of the lake over time, but those exposed to cocaine and its metabolite were more active toward the end of the study.

In each of the last two weeks, salmon exposed to cocaine swam 5 km further than controls, while those exposed to the metabolite swam almost 14 km further, or twice as far. Fish exposed to cocaine and the metabolite also ventured further north into the lake. Surprisingly, it was the metabolite that had the greatest impact, with exposed fish moving 12 km further north than unexposed salmon, according to the journal Current Biology.

“It’s really the metabolite, which we know is present at higher concentrations in nature, that had the most profound effect on fish behavior and movements,” Brand said. “This suggests that if we conduct risk assessments and do not include compounds such as these metabolites and derivatives, we could miss much of the environmental risk to which we expose these animals.”

Professor Leon Barron, who leads the Emerging Chemical Contaminants team at Imperial College London, said it was important to determine whether the effects were seen in fish naturally exposed to pollutants in the wild. Any effects should also be compared to those caused by many other common chemicals detected in aquatic organisms.

“Better wastewater management, particularly a reduction in raw sewage discharges, could help reduce risks to wildlife and their ecosystems,” he said.

Existing wastewater treatment effectively removes many illicit drugs, including cocaine and benzoylecgonine, but raw sewage that can come from storm overflows and poor household plumbing connections is a primary source into waterways.