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What happens when cocaine makes its way into rivers and lakes? Scientists tested the answer on wild salmon in Sweden. Here\u2019s what they found.<\/span><\/p>\n getty<\/small><\/div>\n<\/div>\n<\/figure>\n You\u2019d probably never expect those words to form a meaningful sentence \u2014 let alone appear in the context of real, peer-reviewed biological research. But they did. In an April 2026 study<\/u><\/a> published in Current Biology<\/em>, researchers conducted one of the strangest-sounding ecological experiments imaginable: testing the effects of cocaine pollution on Atlantic salmon (Salmo salar<\/em>) in Sweden. <\/p>\n It sounds absurd. But from a biologist\u2019s perspective, it\u2019s a phenomenal piece of research that, hopefully, will shed light on yet another unexpected way human activity is severely impacting the ecosystems. Here\u2019s what the researchers actually tested, why they did it, and what it could mean for salmon populations around the world.<\/p>\n Cocaine pollution isn\u2019t what it sounds like. Most take the phrase at face value: they assume drug dealers are dumping their products into rivers and lakes and, in turn, contaminating the water. While this probably does happen occasionally, that\u2019s not what researchers mean when they use the term.<\/p>\n In actuality, the term refers to the trace levels of cocaine and its breakdown products that exist in waterways. The caveat, however, is that it\u2019s not directly from the drug entering the water itself, but via sewage systems<\/p>\n When humans do cocaine, the body metabolizes it incredibly quickly in the liver. The main byproduct of this process is a compound known as benzoylecgonine \u2014 the exact compound that drug tests screen for. That metabolite is then excreted in urine, enters sewage systems, passes through wastewater treatment plants and, eventually, makes its way back into local rivers, lakes and coastal waters<\/u>.<\/p>\n The problem is that wastewater treatment plants are very rarely successful in fully removing these compounds. And cocaine isn\u2019t the only issue, either; trace levels of THC, MDMA, methadone and antidepressants have also been detected in aquatic ecosystems around the world.<\/p>\n According to a 2024 study<\/u><\/a> published in Water Research<\/em>, average surface-water concentrations reach roughly 100 ng\/L for cocaine itself, and around 250 ng\/L for benzoylecgonine, with some pollution hotspots reaching concentrations in the thousands.<\/p>\n Of course, these levels are nowhere near high enough to affect humans negatively or to have psychoactive effects. But fish are far smaller and physiologically different, and they\u2019re also constantly <\/em>immersed in the contaminated water itself. Even tiny concentrations can alter their biology and behavior over time.<\/p>\n To clarify, this actually isn\u2019t the first time scientists have tested the effects of cocaine on aquatic animals. Previous studies have examined everything from crayfish to eel larvae. But until now, nobody had tested what these compounds do to fish living freely in a natural ecosystem.<\/p>\n<\/section>\n Atlantic salmon are both ecologically and economically important, yet populations have declined dramatically in recent decades due to habitat destruction, overfishing, disease, climate change and pollution.<\/p>\n Researchers also knew \u2014 according to prior global analyses, like the 2024 Water Research<\/em> study \u2014 that Swedish wastewater systems release measurable amounts of cocaine-related compounds into waterways within the salmon\u2019s native range. Specifically, roughly 3.6 grams of cocaine and 14.1 grams of benzoylecgonine enter waterways every day.<\/p>\n So, lead author Jack Brand and his colleagues performed a rare field experiment on Atlantic salmon smolts in Lake V\u00e4ttern, Sweden. The research team \u201chaphazardly selected\u201d 105 two-year-old salmon held at the Gammelkroppa salmon hatchery, and surgically implanted each fish with a tiny slow-release capsule through a small incision in the abdomen. <\/p>\n One-third of the fish received cocaine implants. Another third received benzoylecgonine implants. The remaining fish served as controls. Notably, the doses were designed to mimic environmentally realistic exposure levels, meaning they were similar to what a fish would experience in real-life contaminated waterways.<\/p>\n After implantation, all the fish were released into the lake and tracked for eight weeks using acoustic telemetry tags, which record each fish\u2019s location over time. This design allowed the researchers to compare how far and how actively each group of fish moved in the wild. <\/p>\n Perhaps the most fascinating part of the study is that scientists literally had to figure out how to give salmon cocaine in a way that realistically replicated long-term environmental exposure in the wild. That\u2019s what makes this study so important. Previous cocaine ecotoxicology research had only been conducted in laboratory tanks. This was one of the first attempts to see how these compounds influence fish behavior in a real ecosystem.<\/p>\n<\/section>\n The results were sobering \u2014 no pun intended. The fish that were exposed to benzoylecgonine (i.e., the compound most common in treated wastewater) swam much farther and more widely than the unexposed control fish that received no drug. More specifically, on average, the benzoylecgonine-treated salmon swam about 1.9 times the distance per week of the controls.<\/p>\n By the end of the experiment, individual benzoylecgonine-fish had dispersed up to 20 miles (32 kilometers) from the site where they were initially released. This is staggering compared to the control fish, which only swam about 12 miles (19 kilometers). In other words, benzoylecgonine-exposed fish traveled roughly 60% farther in the lake.<\/p>\n The cocaine-exposed fish showed a similar trend, but with a much weaker, more variable effect. They tended to move more than controls, but the increase was smaller, and it wasn\u2019t always consistent. Brand et al. note that only the benzoylecgonine group had a clear, statistically robust increase in movement rate and dispersal distance.<\/p>\n With the same acoustic telemetry data, the researchers were also able to visualize how the fish distributed themselves throughout the lake. While control fish and cocaine-exposed fish remained relatively clustered, the benzoylecgonine group spread much more broadly across the ecosystem.<\/p>\n Overall, this means that, ironically, the metabolite produced after<\/em> humans use cocaine has a much stronger ecological impact on salmon than cocaine itself.<\/p>\n<\/section>\n The biggest surprise of the study was that benzoylecgonine (not cocaine itself) resulted in the greatest behavioral changes. But in retrospect, and in consideration of the environment, it makes perfect sense.<\/p>\n Benzoylecgonine is generally more abundant and persistent than cocaine in local waters. Humans metabolize cocaine quickly, meaning much of what ultimately enters wastewater is the metabolite rather than the original drug. And because wastewater treatment plants struggle to remove it completely, benzoylecgonine often accumulates at higher concentrations in rivers and lakes.<\/p>\n Still, many people might wonder why it matters if a salmon \u201ctakes a hit\u201d of cocaine pollution. The answer is that movement behavior is critical to a fish\u2019s livelihood. It affects how they search for food, avoid predators, find suitable habitat and ultimately survive and reproduce. Even the subtlest of changes to their normal movement patterns can cascade through the ecosystem and food chain.<\/p>\n Fish that swim more will burn more energy; as a result, they\u2019ll need to forage harder. Swimming farther than normal may also lead them into open areas or unfamiliar habitats, which puts them at greater risk of predation in turn. This could affect local populations and food webs. This is a behavioral change at the individual level that could shift where fish eat and are eaten.<\/p>\n But if many<\/em> fish in a population have altered movements due to pollution, it could have detrimental impacts on the species as a whole: spawning migration<\/u>, gene flow, population distribution and more. Atlantic salmon are already <\/em>facing threats like habitat loss, overfishing, disease and climate change. Adding subtle chemical exposures to the mix only makes matters worse. <\/p>\n The authors emphasize that while they didn\u2019t measure survival or reproduction, they\u2019re confident that the behavior changes they observed are a good enough warning sign: aquatic organisms are living in a dilute cocktail of human-derived chemicals and drugs, and we\u2019re only just now starting to understand the impacts. <\/p>\n Before this year, \u201cdrugs in the water\u201d would\u2019ve sounded like a crazy conspiracy theory. But now, it\u2019s a real ecological challenge that regulators and engineers are going to have to start tackling. So, if you\u2019ve ever needed a sign not to do cocaine, this is it \u2014 if not for you, then for the fish.<\/p>\n Cocaine pollution may sound bizarre, but it highlights a serious truth: humans and ecosystems are inseparable. Take the <\/em>Connectedness to Nature Scale<\/u><\/em><\/a> to explore your relationship with the natural world.<\/em><\/p>\n<\/section>\n<\/div>\nWhat Is \u2018Cocaine Pollution\u2019?<\/h2>\n
Why Give Cocaine To Salmon?<\/h2>\n
What Happened To The Salmon On Cocaine?<\/h2>\n
What Implications Does \u2018Cocaine Pollution\u2019 Have For Salmon? <\/h2>\n