Scientists are increasingly detecting common human drugs making their way into rivers, lakes and oceans around the world, with largely unknown consequences for aquatic wildlife.
A first-of-its-kind study has revealed traces of cocaine polluting rivers and lakes may accumulate in the brains of salmon and disrupt their behaviour, with fish swimming further and dispersing more widely after exposure to environmental levels of the drug and its main metabolite.
Tracking ‘cocaine’ fish in the wild
This study, conducted in Lake Vättern in Sweden, set out to explore whether cocaine pollution could affect fish in the wild. Researchers fitted two-year-old hatchery-reared Atlantic salmon with implants that slowly released environmentally realistic levels of cocaine or its primary metabolite, benzoylecgonine (i.e. the main thing left over after our bodies break down cocaine). A third group of fish that received drugless implants served as controls. All were fitted with acoustic transmitters.
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.
Juvenile Atlantic salmon that were artificially exposed to benzoylecgonine swam up to 1.9 times farther per week than unexposed fish and dispersed up to 12.3 kilometres farther across the lake. Fish exposed to cocaine showed a similar pattern, but surprisingly the effect was weaker and less consistent.
Dr Jack Brand at the Swedish University of Agricultural Sciences concluded “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 warns the study’s findings “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.”
Alteration in behaviour could expose vulnerable fish to greater predation risk
For species such as Atlantic salmon, which are already under pressure from climate change, habitat loss and other pollutants, even subtle behavioural disruptions could compound the challenges they face.
Additionally, when contaminants alter behaviour, the consequences can extend out well beyond the individual. Small shifts in how animals move, feed or respond to threats can scale up to influence the dynamics of whole populations, interactions between species, and the way entire ecosystems work.
While the impact the pollutants have when they enter watercourses from sewage works remains unclear, the study’s researcher warn 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.
Common drugs pose major and escalating risk to biodiversity
After cocaine is consumed, the body rapidly breaks it down, mainly into benzoylecgonine. Both the original drug and the metabolite- leftovers from the body’s use of a different substance – are excreted and can then enter wastewater systems.
While existing wastewater treatment can remove many illicit drugs, including cocaine and benzoylecgonine, a main source in waterways is raw sewage that can come from storm overflows and misconnects in household plumbing. Cocaine is now one of the most detected illicit drugs in aquatic environments worldwide.
A global analysis found average surface water concentrations of about 105 nanograms per litre for cocaine and 257 nanograms per litre for benzoylecgonine, with maximum concentrations reaching into the thousands of nanograms. While these levels are low, they remain a concern because the compounds target brain systems shared across many animals, meaning even small amounts have the potential to affect wildlife.
Scientists have previously warned that pollution from common drugs poses “a major and escalating risk to biodiversity” and have called on pharmaceutical companies to focus on developing medicines that breakdown in the environment.
Professor Leon Barron, who leads the emerging chemical contaminants team at Imperial College London, told the Guardian “better wastewater management, particularly reduced raw sewage discharges, could help lower any risks to wildlife and their ecosystems”.