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Impact of Pharmaceutical Pollution on Salmon Migration Investigated
Juvenile salmon undertaking their perilous migration from freshwater rivers to the saltwater sea face numerous challenges, including formidable dams, turbulent rapids, and menacing predators. However, a recent study reveals that a specific type of pollution – pharmaceutical pollution – might inadvertently provide some of these fish with an advantage, at least during certain segments of their journey. This research, published in Science, indicates that young salmon exposed to anti-anxiety drugs in the water navigate dams at a swifter pace. Yet, ecologists express reservations regarding whether this translates to a genuine survival benefit.
Ubiquitous Pharmaceutical Contamination in Waterways
Pharmaceutical pollution stands as a widespread environmental issue, with nearly 1,000 different medications and their byproducts detected globally in aquatic systems, even reaching remote locations like Antarctica. These substances enter the environment through various pathways, encompassing direct discharge from pharmaceutical manufacturers, improper disposal of unused medications by individuals, and excretion from both human and animal populations.
Exploring the Effects of Psychoactive Drugs on Wildlife
Scientists have dedicated years to studying the effects of these chemical compounds on wildlife. However, considerable gaps remain in our understanding of how animals react to the complex mixture, or “cocktail,” of diverse pharmaceuticals they encounter, according to Dr. Michael G. Bertram, a behavioral ecologist from the Swedish University of Agricultural Sciences and the lead investigator of the new study.
Psychoactive medications, such as anti-anxiety and antidepressant drugs, are of particular interest due to their inherent design to alter behavior.
Prior research has demonstrated that benzodiazepines, a class of drugs prescribed for anxiety and insomnia, can influence the migratory patterns of Atlantic salmon. Furthermore, these drugs can diminish stress responses in various fish species. In essence, fish experience a reduction in anxiety, mirroring the effects observed in humans.
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Bridging the Gap Between Lab and Wild: Investigating Salmon Smolt Migration
Nevertheless, these prior findings predominantly originated from controlled laboratory settings. Real-world scenarios present greater complexity. “We have substantial knowledge from laboratory experiments indicating that drugs can modify animal behavior, but predicting how these effects manifest within natural ecosystems proves extremely challenging,” explained Dr. Bertram.
Dr. Bertram and his research team sought to ascertain whether laboratory observations held true in natural environments. Their focus centered on the smolt phase of salmon development, a critical period when juvenile fish migrate towards the ocean.
In their experiment, they introduced salmon smolts to either a common anti-anxiety medication, an opioid analgesic, or a combination of both, at concentrations approximating levels that fish might encounter in the wild. A control group was also included, swimming in purely uncontaminated water. Subsequently, they equipped the fish with tracking devices and released them into a river in Sweden. Over a span of one to two weeks, the smolts traversed approximately 17 miles, navigating a reservoir populated by predators, swift rapids, and two hydroelectric dams before reaching the Baltic Sea.
“It’s a notably treacherous journey,” Dr. Bertram emphasized.
Faster Dam Passage for Salmon Exposed to Anti-Anxiety Medication
Smolts exposed to the anti-anxiety drug exhibited a higher likelihood of reaching the sea compared to the other experimental groups. Intriguingly, no statistically significant difference emerged in the overall time taken to reach the sea across all groups. However, a notable disparity surfaced in the speed at which smolts navigated the dams. Fish in the anti-anxiety group traversed these structures several hours faster than the control group.
Researchers hypothesized that this accelerated dam passage might stem from altered risk-taking behavior. Previous laboratory studies had indicated that anti-anxiety medications promote boldness. The present study connects this boldness to survival implications in the wild. “Here, we are directly demonstrating that a behavioral change influences survival outcomes,” Dr. Bertram stated.
“It’s compelling to observe how one environmental issue affects their response to another challenge,” commented Olivia Simmons, a salmon ecologist at the Norwegian Institute for Nature Research, who was not involved in the study. “These bolder fish might simply be moving more rapidly due to reduced inhibition.”
Impact of Anti-Anxiety Drugs on Schooling Behavior and Predation Risk
The research team further investigated whether heightened boldness altered the smolts’ schooling behavior, a natural defense mechanism against predators. They administered drugs to a separate batch of fish and recorded their schooling behavior in a tank, both with and without the presence of a predator. Smolts treated with the anti-anxiety medication formed less cohesive schools, even when a predator, a northern pike, was present.
Unnatural Selection and Ecosystem Consequences
These findings reinforce the notion that anti-anxiety drugs may induce young salmon to become more inclined towards risk-taking, potentially leading them to navigate dams independently. Dr. Bertram expressed his intention to conduct further research in the wild using more sophisticated tracking technology to discern whether smolts are traveling in groups or solitarily, and to determine if predation occurs.
Scientists voice concerns about the apparent influence of pharmaceuticals in waterways on crucial fish behaviors. The cascading effects of this “unnatural selection,” as characterized by Dr. Bertram, within ecosystems remain largely unknown.
For instance, salmon could modify their migration timing, potentially arriving at inopportune times when temperatures are unsuitable or resources are scarce. An increase in salmon populations could place excessive strain on their prey species, disrupting ecosystem equilibrium. Moreover, persistent risky behavior in the smolts’ new environment, particularly amidst abundant predators, could diminish their long-term survival prospects and reproductive success.
“It’s analogous to playing poker,” remarked Giovanni Polverino, a behavioral ecologist at the University of Tuscia in Viterbo, Italy, who was not affiliated with the recent study but has collaborated with Dr. Bertram on prior research.
“The more risks you undertake, the greater the potential for complete loss,” he cautioned. “In this case, the fish’s life itself.”
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