Cocaine in Water Makes Salmon Hyperactive: Study

What Happens When Salmon Get Cocaine?

Cocaine residues in rivers and lakes are making salmon hyperactive, according to a new study from the Swedish University of Agricultural Sciences. The research reveals that cocaine and its breakdown product benzoylecgonine accumulate in salmon brains, causing dramatic behavioral changes. The study, published in April 2025, shows that salmon exposed to environmentally relevant cocaine concentrations swim longer distances, expend more energy, and face higher predation risks.

How Cocaine Enters Aquatic Ecosystems

Drug residues enter waterways primarily through wastewater treatment plant effluent. While most cocaine is removed during standard water treatment, heavy rainfall events can overwhelm systems, causing untreated sewage—including drug residues—to flow directly into rivers and lakes. This phenomenon is not unique to Sweden; similar contamination has been documented worldwide, from the Thames in London to the Tiber in Rome. The environmental impact of pharmaceutical pollution has become a growing concern among ecologists.

The Swedish University Study: Methodology and Findings

Experimental Design

Researchers implanted salmon with slow-release devices that administered small amounts of cocaine, matching concentrations found in natural environments. A control group received implants without drugs. All fish were fitted with acoustic transmitters and tracked for two months in a large Swedish lake.

Key Results

• Increased activity: Cocaine-exposed salmon became progressively more active over time, while control fish became less active.
• Greater energy expenditure: Hyperactive salmon swam longer distances, burning more energy.
• Higher predation risk: Increased movement made them more vulnerable to predators.
• Altered feeding behavior: Fish needed to seek more food to compensate for energy loss.

The study's findings align with previous research showing that drug pollution affects aquatic wildlife in alarming ways.

Broader Context: Drugs in the Wild

This is not the first study to document drug-induced behavioral changes in aquatic species. Previous research has shown:

These findings raise questions about the long-term ecological consequences of drug contamination.

Why This Matters: Ecological and Human Implications

The behavioral changes observed in salmon could have cascading effects on aquatic ecosystems. Hyperactive salmon may deplete their energy reserves, reducing spawning success and population numbers. Increased predation could further destabilize food webs. Moreover, cocaine and its metabolites may bioaccumulate in the food chain, potentially affecting humans who consume contaminated fish.

"The levels we tested are comparable to what fish actually encounter in polluted waters," said a researcher involved in the study. "Even low concentrations can trigger significant behavioral changes."

FAQ: Cocaine in Water and Salmon

How does cocaine get into rivers and lakes?

Cocaine enters waterways through human excretion and improper disposal. Wastewater treatment plants remove most of it, but during heavy rain, untreated sewage can overflow into natural water bodies.

What concentration of cocaine was used in the study?

Researchers used concentrations matching those found in polluted natural environments, typically in the nanograms-per-liter range.

Can cocaine affect other fish species?

Yes. Similar studies have found behavioral changes in trout exposed to methamphetamine and crayfish exposed to antidepressants.

Is it safe to eat salmon from polluted waters?

While cocaine residues can accumulate in fish tissues, the concentrations are extremely low. However, the ecological effects on fish populations could impact fisheries over time.

What can be done to reduce drug pollution?

Improving wastewater treatment infrastructure, reducing illicit drug use, and developing better detection methods for pharmaceutical pollutants are key strategies.

Sources

This article is based on reporting by BNR Nieuwsradio and the Swedish University of Agricultural Sciences study. Additional context from National Geographic and previous research on pharmaceutical pollution in aquatic environments.