Discovery of Cocaine's Unexpected Effects on Wild Salmon

In a groundbreaking field experiment conducted in Sweden's Lake Vättern, researchers from the Swedish University of Agricultural Sciences (SLU) have uncovered startling behavioral changes in juvenile Atlantic salmon exposed to trace levels of cocaine pollution. The study, published in the prestigious journal Current Biology, marks the first investigation into how illicit drugs like cocaine and its primary metabolite, benzoylecgonine (BE), influence the movement patterns of fish in their natural habitat. Lake Vättern, Sweden's second-largest lake spanning 1,912 square kilometers, serves as a critical habitat for these wild salmon populations, making the findings particularly alarming for local ecology and conservation efforts.

Atlantic salmon (Salmo salar) are anadromous fish that migrate between freshwater lakes and rivers and the sea, playing a vital role in aquatic food webs as both predators and prey. Disruptions to their natural behavior could have cascading effects on biodiversity, fisheries, and even human communities reliant on healthy salmon stocks. This research highlights how pharmaceutical and illicit drug residues, often overlooked as environmental pollutants, are infiltrating even remote freshwater systems.

The Innovative Experimental Design Behind the Study

Led by Dr. Jack A. Brand from SLU's Department of Wildlife, Fish, and Environmental Studies in Umeå, the team devised a novel method to simulate real-world exposure. They implanted 105 hatchery-reared juvenile salmon smolts—each averaging 110 grams—with slow-release silicone implants containing either cocaine hydrochloride, benzoylecgonine tetrahydrate, or a control coconut oil matrix. The drug dose was calibrated to environmentally relevant levels: 50 micrograms per gram of implant, delivering approximately 27.5 micrograms per fish over several weeks.

After a recovery period, the tagged fish (equipped with acoustic transmitters and passive integrated transponder tags) were released into the southwestern part of Lake Vättern on April 12, 2022. Their movements were monitored for eight weeks using a network of 71 acoustic receivers strategically placed across the lake. This setup allowed precise tracking of positions, calculating centers of activity every 60 minutes and estimating in-water swimming distances while accounting for lake bathymetry to avoid overestimation.

Brain tissue analysis from a parallel laboratory group confirmed uptake: cocaine-exposed fish had mean brain concentrations of 42.85 nanograms per gram, while BE-exposed reached 33.74 ng/g—levels comparable to those in polluted urban waterways worldwide. Water sampling near the release site detected trace cocaine (0.11–0.85 ng/L) and BE (0.07–1.05 ng/L), underscoring the lake's vulnerability despite its size.

Key Results: Salmon Swimming Nearly Twice as Far

The data revealed profound alterations in movement ecology. In the final two weeks (weeks 7–8), BE-exposed salmon swam up to 1.9 times farther per week—approximately 13.7 kilometers more than controls (89% confidence interval: 3.0–25.3 km). Overall weekly distances for controls hovered around 7 km, while exposed fish pushed toward 14 km or more. Dispersion was equally dramatic: BE fish ventured up to 12.3 km farther from the release site (89% CI: 1.9–24.0 km), often heading northward into riskier open waters.

Cocaine itself induced milder effects, with about 5.3 km extra distance weekly in the same period (overlapping confidence intervals). Bayesian statistical models (using R's brms and mgcv packages) confirmed these trends, showing no significant survival differences (median apparent survival ~55–75 days across groups). However, the heightened activity suggests increased energy expenditure, potentially at the cost of fat reserves needed for marine migration.

• BE-exposed: 1.9x weekly distance, 12.3x dispersion in final weeks.
• Cocaine-exposed: Modest increase, less pronounced.
• Controls: Baseline movement ~7 km/week, limited dispersal.

Why Benzoylecgonine Proves More Potent Than Cocaine

Benzoylecgonine, cocaine's primary human metabolite, accumulates more readily in fish brains and exerts stronger psychoactive effects. Detected at higher environmental concentrations (global mean 257 ng/L vs. 105 ng/L for cocaine), BE blocks dopamine reuptake similarly to cocaine but persists longer in ecosystems. SLU's lab assays showed BE reaching comparable brain levels despite lower implant doses, indicating higher bioavailability.

This mirrors patterns in wastewater: Sweden discharges ~14 grams of BE daily versus 3.6 grams of cocaine from treatment plants ill-equipped for these polar compounds. The study's slow-release implants mimicked chronic, low-dose exposure, revealing subtler, ecologically relevant shifts absent in high-dose lab tanks.

Photo by Colin Davis on Unsplash

Ecological Implications for Lake Vättern's Salmon Populations

Lake Vättern hosts a recovering wild Atlantic salmon stock, vital for Sweden's biodiversity and recreational fisheries. Hyperactive, far-ranging juveniles may forage less efficiently, deplete energy for smolt migration to the Baltic Sea, or expose themselves to avian predators in open waters. Dr. Brand warns: "If contaminants alter how animals move, this could ultimately affect habitat use, predator-prey interactions, and even population connectivity."

Altered space use—fewer unique lake areas visited—suggests avoidance of optimal rearing zones, potentially reducing growth rates. Long-term, this could hinder recovery from historical overfishing and habitat loss, threatening the lake's €10 million annual fishery value.

Drug Pollution: A Growing Threat Across European Waterways

Europe faces rising pharmaceutical pollution, with cocaine use surging 25% post-pandemic per EMCDDA data. Swedish rivers show ng/L cocaine/BE, but hotspots like UK rivers hit 72 ng/L BE. Similar studies document oxazepam-dosed perch losing predator fear and methamphetamine-altered trout boldness.Guardian analysis highlights raw sewage overflows as culprits, bypassing treatment.

SLU's work builds on prior pharma-ecotox research, emphasizing wild conditions over aquaria. For Europe's 500+ salmon rivers, such pollutants compound climate stressors like warming waters.

Challenges in Wastewater Treatment and Monitoring

Conventional plants remove 50-90% cocaine but falter on metabolites like BE due to persistence. Advanced methods—activated carbon, ozonation—cut residues 95%, yet cost €0.01-0.10/m³ extra. Sweden monitors 73 pollutants quarterly, but illicit drugs lag. EU Water Framework Directive pushes upgrades, but funding gaps persist.

• Ozonation: 95% removal, but byproduct risks.
• Biofilters: Cost-effective for rural plants.
• Monitoring: Expand to metabolites via LC-MS/MS.

SLU's Role in Pioneering Aquatic Ecotoxicology Research

SLU Umeå leads Europe's pharma-pollution studies, with Prof. Tomas Brodin's group pioneering fish behavior assays. Collaborators from Griffith University, Max Planck Institute, and Baylor University underscore international scope. This study advances telemetry tech for large-lake tracking, informing EU Horizon projects on chemical stressors.

Funding from Swedish Research Council highlights higher ed's role in addressing One Health challenges—linking human drug use, wastewater, and wildlife.

Photo by Colin Davis on Unsplash

Global Parallels and Calls for Urgent Action

Beyond Sweden, US rivers show 100+ ng/L cocaine; Brazilian Amazon fish exhibit hyperactivity. WWF urges drug residue bans in effluents. Solutions: User education, take-back programs, policy like EU's Strategic Approach to Pharmaceuticals in the Environment.Read the full SLU study

Future Research Directions from the SLU Team

Brand et al. call for multi-species, multi-pollutant wild trials, mechanistic studies (dopamine pathways), and long-term migration tracking. SLU plans depth-profiling telemetry to probe vertical behaviors. Collaborative EU grants could scale to Baltic salmon runs, integrating AI for pollution forecasting.

This SLU study not only spotlights a quirky phenomenon but galvanizes action on invisible aquatic threats, positioning Swedish higher education at the forefront of environmental science.