Traces of cocaine that pollute rivers and lakes may accumulate in the brains of salmon and disrupt their behaviour, according to researchers who warn of unknown consequences for fish populations.
Juvenile Atlantic salmon that were artificially exposed to the drug and its main breakdown product swam further and dispersed more widely across a lake, suggesting the substances can affect where the fish go, what they eat and how vulnerable they are to predators.
What impact the pollutants have when they enter watercourses from sewage works is unclear, but the fish may pay a price if they burn more energy, or face greater risks from predators if they have to forage for more food to keep their energy up, scientists say.
“Largely, we don’t know the consequences, but I expect there to be trade-offs,” said Dr Jack Brand at the Swedish University of Agricultural Sciences. “They may end up in worse condition or have to offset it by foraging a lot more, meaning they spend more time out in the open.”
Scientists have said before that pollution from common drugs poses “a major and escalating risk to biodiversity” and have called on pharmaceutical companies to make greener medicines that breakdown in the environment. Concerns over the contaminants have been fuelled by reports of trout “addicted” to methamphetamine and perch losing their fear of predators because of antidepressant medications.
In 2019, tests on freshwater shrimp in rivers in Suffolk found traces of dozens of different drugs, including cocaine, methamphetamine, antidepressants, anxiolytics for anxiety and antipsychotics, but the researchers did not draw any conclusions about their potential to cause harm.
To explore whether cocaine pollution could affect fish in the wild, Brand and his colleagues fitted two-year-old hatchery-reared Atlantic salmon with implants that slowly released environmentally realistic levels of cocaine or its primary metabolite, benzoylecgonine. A third group of fish that received drugless implants served as controls. All were fitted with acoustic transmitters.
The fish were released into the south-west corner of Lake Vättern, which at nearly 2,000 sq km (772 sq miles) is the second-largest lake in Sweden. Among other species in the lake are large predatory pike. Using sensors around the lake, the researchers tracked the salmon for two months.
All the salmon became less active and more settled in part of the lake over time, but those exposed to cocaine and its metabolite were more active towards the end of the study.
In each of the last two weeks, salmon exposed to cocaine swam 5km further than controls, while those exposed to the metabolite swam nearly 14km more, or twice as far. The fish exposed to cocaine and the metabolite also ventured farther north into the lake. Surprisingly, the metabolite had the greatest impact, with exposed fish moving 12km farther north than the unexposed salmon, according to the journal Current Biology.
“It was really the metabolite, which we know occurs at higher concentrations in the wild, that had the much more profound effect on fishes’ behaviour and movement,” Brand said. “That suggests that if we’re doing risk assessments and not including compounds like these metabolites and derivatives, we may be missing a big chunk of the environmental risk we’re exposing these animals to.”
Prof Leon Barron, who leads the emerging chemical contaminants team at Imperial College London, said it was important to establish whether the effects were seen in fish that were naturally exposed to the pollutants in the wild. Any effects should also compared to those driven by many other common chemicals detected in aquatic organisms.
“Better wastewater management, particularly reduced raw sewage discharges, could help lower any risks to wildlife and their ecosystems,” he said.
Existing wastewater treatment efficiently removes many illicit drugs, including cocaine and benzoylecgonine, but a main source in waterways is raw sewage that can come from storm overflows and misconnects in household plumbing.