Importance Score: 55 / 100 🔵
Salmon migration, a remarkable journey from freshwater to saltwater, is increasingly threatened by pharmaceutical pollution. New research reveals that exposure to drugs like clobazam alters salmon behavior, prompting them to reach the ocean faster. While seemingly beneficial, this drug-induced change raises concerns about the long-term effects on salmon health and survival in the face of growing environmental contamination from human medications.
Pharmaceutical Pollution Alters Salmon Migration Behavior
Originating in tranquil streams and rivers, salmon embark on a hazardous migration toward the open ocean to mature. For millennia, generations of young salmon have undertaken extensive journeys, sometimes traversing hundreds of miles from freshwater habitats to the sea. However, contemporary salmon encounter a threat unknown to their predecessors: pharmaceutical contamination that modifies their migratory patterns.
Recent scientific investigations have demonstrated that the accumulation of clobazam, a pharmaceutical compound, in salmon brains causes migrating fish to reach the ocean more rapidly and navigate dam obstacles with increased speed. Although this alteration might initially appear advantageous, experts caution that any human-induced deviation from normal animal behavior, especially involving psychoactive substances, is alarming. The complete consequences of drug pollution on salmon health, behavior, and reproductive success remain largely unknown, according to findings published in the journal Science on April 10.
The Impact of Benzodiazepines on Aquatic Wildlife
Clobazam, a frequent contaminant in wastewater, is classified as a benzodiazepine, a class of drugs that depress the central nervous system. This medication is prescribed to manage epileptic seizures, for short-term anxiety relief, and to treat sleep disorders related to anxiety. Due to the biological similarities between fish and mammals, particularly in neural structure, fish exhibit high sensitivity to drugs that affect human neurochemistry, explained Dr. Christopher C. Caudill, a professor specializing in fish and wildlife sciences at the University of Idaho.
“Humans and fishes share considerable biological similarities—our physiology and anatomy display remarkable parallels. Therefore, it is logical that psychoactive drugs can modify behavior in both fishes and humans,” Dr. Caudill, who was not involved in the featured research, stated in an email to CNN.
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Prior studies suggested that benzodiazepines could impact the behavior of Atlantic salmon (Salmo salar), but these were often conducted under conditions not reflective of wild salmon environments, noted Dr. Marcus Michelangeli, a coauthor of the study and lecturer at Griffith University in Queensland, Australia.
“Previous research largely took place in controlled laboratory environments, monitored movement across limited distances—less than 100 meters (328 feet)—or employed drug concentrations significantly higher than those typically encountered by salmon in natural settings,” Dr. Michelangeli explained via email.
“Our investigation employed a distinct methodology. We tracked the entire river-to-sea migration of juvenile salmon within a natural riverine ecosystem, using drug concentrations consistent with actual environmental exposure levels.”
The outcomes of this field study emphasize the escalating dangers that pharmaceutical pollutants present to global wildlife populations, according to Dr. Michelangeli.
Navigating Migration: The Role of Drugs
For this study, scientists conducted experiments involving over 700 young salmon, known as “smolts,” both in laboratory and natural settings. The research team utilized sound-transmitting tags to monitor hundreds of smolts remotely in 2020 and 2021 as they migrated through the Dal River in central Sweden.
Smolts migrate downstream into a reservoir, traverse rapids, and pass over two dams before ultimately reaching the Baltic Sea. This migration typically spans 10 to 13 days.
Dr. Michelangeli noted that two major classes of pharmaceuticals—benzodiazepines and opioids—“are frequently detected in rivers and streams globally, including in Sweden, where our research was based.”
Time-release implants administered two drugs from these classes to the smolts: clobazam and tramadol. Fish were given clobazam, tramadol, or a combination of both. A control group of smolts received drug-free implants.
“These two drugs are known to interact chemically in humans when taken together, and they often coexist in the environment,” Dr. Michelangeli stated. “This made them suitable for examining how pharmaceutical mixtures might affect animal behavior.”
In conjunction with field tests, laboratory studies on 256 smolts were performed to verify implant functionality and confirm drug presence in fish tissues and brains.
Tracking migrating salmon with transmitters revealed that a higher proportion of clobazam-exposed salmon reached the Baltic Sea compared to other groups. Over twice as many salmon with clobazam implants reached the sea compared to the control group.
Laboratory analyses indicated that clobazam influenced shoaling behavior, where smolts group closely for predator avoidance. Under clobazam’s effect, fish swam more distantly even near predators, “suggesting that the drug may diminish natural fear responses,” Dr. Michelangeli explained.
The Trade-off: Reduced Fear, Increased Risk
Salmon with clobazam implants also navigated hydropower dams along their migration route more quickly—approximately two to eight times faster than other groups. These dams are recognized as hazardous zones due to turbine-induced mortality.
While clobazam-induced fear reduction might temporarily aid migration success, Dr. Caudill suggests it could also elevate vulnerability to ocean predators, decreasing long-term survival prospects for spawning.
“The transition from freshwater to saltwater is a particularly vulnerable period in a salmon’s life cycle due to exposure to novel ocean predators,” he stated. Drug-exposed, risk-tolerant salmon might reach the Baltic more readily but face diminished chances of long-term survival.
Dr. Caudill’s research focuses on environmental change impacts on fish ecology and evolution. He anticipates incorporating the potential behavioral consequences of pharmaceutical pollution in future studies.
Further research is essential to understand how drug-induced behavioral changes affect long-term survival, reproduction, and population dynamics—not only in salmon but also in other wildlife susceptible to pharmaceutical contaminants.
“While more drug-affected salmon may reach the sea, it does not guarantee their health or long-term population benefits,” Dr. Michelangeli cautioned.
“It is crucial to interpret these findings cautiously. Unintentional behavioral modifications caused by pharmaceuticals could fundamentally reshape entire populations in presently unforeseen ways.”