Brown trout. Image: Jay Fle
Methamphetamine, a potent stimulant and a widely abused illicit substance, is a pervasive public health risk to communities that span the globe. Now, scientists have discovered that fish can also get addicted to meth that seeps into natural habitats through wastewater, a process of contamination that may disrupt entire ecosystems. Experiments with meth-exposed trout revealed that these “fish can display signs of addiction and withdrawal symptoms” that could result in “unexpected ecological consequences,” according to a study published on Tuesday in the Journal of Experimental Biology.
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“Such contamination could change the functioning of whole ecosystems as adverse consequences are of relevance at the individual as well as population levels,” said Pavel Horký, an ecologist at the Czech University of Life Sciences in Prague who led the study, in an email. “Furthermore, drug reward cravings by fish, as was documented in our results, could overshadow natural rewards like foraging or mating that provide homeostatic and reproductive success,” he added. Wastewater has become a valuable source of pharmaceutical and epidemiological information in recent years, and even during the COVID-19 pandemic, as scientists have developed novel methods to extract drug residue and other biomarkers from sewage samples. These techniques can track drug use in regional populations on large scales, and have already been used to confirm that consumption of amphetamines is rising in many parts of the world. Aquatic habitats in some areas of the Czech and Slovak Republics, for instance, have been contaminated with concentrations of meth that reach hundreds of nanograms per liter, according to the new study.
To assess whether fish can become addicted to stimulants at these concentrations, Horký and his colleagues studied two groups of trout, each with 60 individuals, that were placed into separate holding tanks for a period of eight weeks: one laced with similar meth concentrations to those found in contaminated freshwater rivers, and one with all the same water conditions, but minus the meth.
After their roughly two-month stint in the tanks, the researchers transferred trout from each tank into a shared freshwater environment. The dosed fish were less likely to move around the new tank compared to the control fish, suggesting that they may have been experiencing stress, anxiety, and other symptoms related to withdrawal from the drug. These behavioral changes lasted about 96 hours as the drug concentrations subsided in the exposed populations, a gradual decrease that was confirmed by later biochemical studies of their brains.Horký and his colleagues also created a “choice arena” in the new tank that contained water with similar meth concentrations to the previous dosed tank. The exposed trout were more likely to seek out this area compared to the control group, no matter where it was placed in the environment, which was “considered an indicator of addiction,” according to the study.
“Personally, I was mostly surprised by the fact that methamphetamine users can unknowingly cause fish meth addiction in the ecosystems around us”
“We predicted/expected that there would be some signs of addiction, however, we were quite surprised by how the whole system worked tightly,” Horký said, referring to the clear correlations between fish behavior, metabolic activity, and brain chemistry revealed in the study. “Personally, I was mostly surprised by the fact that methamphetamine users can unknowingly cause fish meth addiction in the ecosystems around us,” he added.Indeed, the new research is another example of the many unintentional effects that humans can have on surrounding wildlife. Horký and his colleagues plan to build on these troubling findings by designing new experiments to test wild fish outside of the laboratory, in their natural habitats, which could reveal insights about the broader ecological impacts of drug contamination in waterways.“The elicitation of drug addiction in wild fish could represent another example of unexpected evolutionary selection pressure for species living in urban environments, along with ecological side effects of human societal problems within aquatic ecosystems,” Horký concluded.