A benzodiazepine drug seeping into waterways is causing juvenile Atlantic salmon to behave abnormally, and wild salmon are migrating faster and at greater risk on their journey from rivers to oceans. The drug even seemed to disrupt their social dynamics.
An international team of researchers, led by the Swedish University of Agricultural Sciences and including researchers from Australia's Griffith University and other institutions, found that the benzodiazepine clobazam - commonly used to treat anxiety, sleep disorders and to control epileptic seizures - had a particularly significant impact on salmon. This is the largest study of its kind and provides new insights into the impact of pharmaceutical pollution on marine life.
Dr Marcus Michelangeli, from Griffith University's Australian Rivers Research Institute, said: "This study is unique in that it investigates the impact of these pollutants on wildlife directly in the field, allowing us to better understand how exposure to pollutants affects wildlife in their natural environment. behavior and migration in salmonids exposed to clobazam appears to be a beneficial effect, it is important to recognize that any changes to the species' natural behavior and ecology may have wider negative impacts on the species and its surrounding wildlife communities."
The research team used slow-release drug implants and tracking transmitters to observe how clobazam and another common pharmaceutical contaminant - the opioid tramadol - affect juvenile Atlantic salmon (Salmo salar) during a critical stage of their life cycle, as they swim along waterways to the Baltic Sea.
Marcus Michelangeli conducts fieldwork on the Dal River in Sweden
Based on field observations, the researchers conducted further laboratory studies on clobazam to observe the drug's impact on the species' swarming behavior. School swimming (rather than schooling) is a social behavior that may affect reproduction and cohesion of fish schools. While that may not sound like a big deal, the species overall has been downgraded from "least concern" to "near threatened" on the IUCN Red List, the "bible" for tracking wildlife populations. In some areas, they are endangered, with freshwater fish populations in places such as Maine and the United Kingdom declining dramatically over the past decade.
Their population sizes are also becoming increasingly fragmented, meaning changes in their social behavior and more risk-taking behavior is a dangerous sign for a species in decline.
While benzodiazepines are often viewed as sedatives in humans, salmon taking clobazam actually traveled faster on their migratory route, zipping past two hydroelectric dams that often pose challenges to this important journey. This can have a variety of effects on a species - such as arriving in open water too early and fish being less careful on their way to the ocean to avoid being eaten by predators - but these effects are not yet fully understood.
Michelangeli said: "Pharmaceutical contaminants are an emerging global problem, with more than 900 different substances currently detected in waterways around the world. Of particular concern are psychoactive substances such as antidepressants and painkillers, which can severely interfere with brain function and behavior in wildlife."
He added: “When you consider that entire ecosystems are exposed to realistic exposure scenarios – spanning multiple species and multiple contaminants – the potential consequences become even more complex.”
Drugs cause Atlantic salmon to behave strangely - not a good sign for their populations
Indeed, there is evidence that these pharmaceutical contaminants can affect humans who eat pharmaceutically affected fish, however, the long-term health effects are not fully understood. Generally speaking, low concentrations of pollutants do not produce any direct, noticeable effects, but research in this area is still insufficient.
Scientists are calling on pharmaceutical companies to make their drugs more biodegradable and improve wastewater treatment to limit what we dump back into the wild.
"Advanced wastewater treatment methods are becoming increasingly effective in reducing pharmaceutical contamination, and green chemistry methods also have broad potential," Michelangeli said. "By designing drugs that break down more quickly or are less harmful after use, we can significantly mitigate the environmental impact of future pharmaceutical contamination."
The study was published in the journal Science.