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A new study led by the Swedish University of Agricultural Sciences determined that Atlantic salmon were swimming faster as they migrated to the sea. Shockingly, researchers found that prescription drug pollution not only helped them swim faster, but it also increased overall migration success. As global populations of the endangered Atlantic salmon have decreased 23% from 2006-2020, any increase in migration success sounds like a win. Yet, researchers warn that artificial changes in the natural behavior of the species are likely to result in wider consequences for the entire ecosystem.
Migratory Patterns of Atlantic Salmon in the Baltic Region
Baltic Atlantic salmon only migrate to the Baltic Sea. They do not migrate further to the Atlantic Ocean.
©slowmotiongli/Shutterstock.com
Atlantic salmon are an anadromous species. This means they migrate from freshwater rivers to the sea, then back again to spawn in the rivers where they were born. There are three groups of Atlantic salmon: North American, European, and Baltic salmon. The North American and European Atlantic salmon migrate from various freshwater rivers to the Atlantic Ocean. However, Baltic Atlantic salmon are confined to the Baltic Sea and its tributaries.
Baltic Atlantic salmon face a number of obstacles on their migratory journey. Weirs, dams, and hydropower plants are often cited as reasons for migratory stoppage. While these barriers are generally navigable, passage often results in stress and injury. Researchers report injury and mortality due to changes in water pressure, moving parts in turbines, and hard surfaces in narrow bypass channels and spillways. They also note that reservoir conditions resulting from reduced water velocities upstream favor large predators, exposing juvenile salmon to predation. The slower water flow also delays their progress, exposing them to predators even longer.
What’s Happening to the Salmon?
Researchers explain that over 900 pharmaceutical substances have been detected in water sources all across the globe. Psychoactive substances, chemical compounds that alter brain functions, are particularly concerning. Scientists wanted to examine the effects of common pollutants such as clobazam, often prescribed for seizures and anxiety, and tramadol, which is a prescription painkiller. Using extended-release pharmaceutical implants and tracking transmitters, the researchers were able to monitor changes in the juvenile salmon as they migrated from the Dal River in Sweden to the Baltic Sea.
Their efforts discovered that clobazam increased migration success. It also reduced navigation times through hydropower dams that often stop or delay their progress. The medicated salmon saved an estimated five hours of traveling time. They were also 2.3 times more likely to reach the Baltic Sea than the untreated salmon. Considering that the highest predicted migration success rate was only 25.7% (74.3% loss rate) for the clobazam-tested group, an increase in success seems like a good thing. However, a follow-up experiment found that the drug affected shoaling. Scientists believe this suggests medically induced changes in group dynamics and risk-taking behavior may be the catalyst for changes in migration.
Researchers monitored juvenile salmon as they migrated from the Dal River in Sweden to the Baltic Sea.
©Chanonry/Shutterstock.com
The Wider Impact on the Ecosystem
The study results have implications for other species. Findings suggest how pharmaceutical pollution might affect important events in the lives of other migratory fish. The authors point out that any benefits resulting from such artificial changes will likely have ramifications for the entire ecosystem. However, researchers note that predicting the wider impact is especially challenging, due to the number of species that could be affected and the wide variety of possible contaminants.
The Atlantic salmon is considered a keystone species, so their health has a disproportionate effect on the ecosystem relative to their population size. Their existence is crucial to the survival of many other species along their migratory route. Atlantic salmon support plant growth, nutrient cycling, and predator populations in freshwater and marine environments. So, negative consequences for Atlantic salmon result in negative consequences for the other wildlife in the system.
What Does This Mean for Humans?
No doubt these results spark questions about whether we’re eating salmon contaminated with pharmaceuticals. It’s true that wastewater treatment processes do not always remove traces of prescription drugs excreted by humans. Yet, advanced treatment methods are becoming more effective. Unfortunately, pharmaceutical waste is also known to be highly resistant to biodegradation. However, the salmon in this study were deliberately medicated. Scientists wanted to better understand how wild populations might be impacted by common drugs found in our waterways.
It’s also important to note that only farm-raised Atlantic salmon is available in the public market in the U.S., although it may be imported from farms overseas. The European Union manages Baltic Atlantic salmon populations. The law restricts fishing to unavoidable by-catch and scientific investigation, with few exceptions. Recreational fishing is also banned. So, although pharmaceutical waste in our water remains an ongoing problem, the possibility of eating drug-laced Atlantic salmon is extremely low.
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