The water looks the same as yesterday, the lake too. From the outside you can’t see anything. Then the data arrives and the picture shifts: residues of drugs consumed by humans also end up in fresh waters, and those residues can change the behavior of wild animals. In the case of salmon, the change concerns movement, the distance they travel, the way they occupy space. It is a detail only in appearance. In an aquatic ecosystem, where a fish goes decides what it finds to eat, who it can prey on and how a population is distributed over time.
This is shown by a study published in Current Biology and signed by an international research group that worked between Australia, Sweden, the United Kingdom and Germany. The work is a collaboration between Griffith University, the Swedish University of Agricultural Sciences, the Zoological Society of London and the Max Planck Institute for Animal Behaviour. The fact that makes it particularly weighty lies here: it is the first evidence collected in nature, outside the controlled perimeter of the laboratory, which links exposure to cocaine residues to an alteration in the behavior of wild salmon.
The researchers followed 105 young salmon for eight weeks
To understand what really happens inside a natural ecosystem, the researchers chose Lake Vättern, in Sweden, and monitored 105 young Atlantic salmon for eight weeks. They used slow-release chemical systems and an acoustic telemetry system, which allows them to follow the movements of the animals in the lake. The fish were divided into three groups: one control, one exposed to cocaine and one exposed to benzoylecgonine, the main metabolite of cocaine, i.e. the substance resulting from its degradation and which often appears in wastewater.
The results all moved in the same direction. Salmon exposed to benzoylecgonine swam up to 1.9 times farther each week than unexposed fish, and dispersed up to 12.3 kilometers farther into the lake. As the weeks passed, this difference became clearer, a sign that the exposure had an impact on the way animals use space and move within a complex environment, made up of constraints, resources, predators and constantly changing trajectories.
The most interesting fact concerns the substance which often remains in the background. Benzoylecgonine showed a more marked effect than cocaine itself. For those involved in environmental risk this difference weighs heavily, because the assessments often focus on the main compound, while in watercourses the metabolites can be more common, more persistent and, apparently, also more influential on a biological level. Translated: some of the effects could be missed if you look at the wrong substance or only look at that one.
The residues end up in rivers via wastewater
The spectacular part of the news risks losing sight of the substance. The topic does not concern the grotesque anecdote of the “drugged” fish. It concerns pharmaceutical and chemical pollution that enters waterways through wastewater treatment systems incapable of completely eliminating these molecules. The authors recall that cocaine and metabolites are increasingly detected in rivers and lakes. From there begins an invisible pressure on aquatic animals, which receive small, continuous, realistic doses compared to those already found in polluted environments.
The researcher Marcus Michelangeli, involved in the study, recalled this passage: the movement of animals has a central role in the relationship with the surrounding environment. Changing course, covering more space, dispersing more means affecting nutrition, risk of predation and population structure. The long-term effects remain to be clarified, yet the direction of the problem is already clear. When a contaminant alters the movements of a species, the wave spreads far beyond the individual animal.
There is also a point that the authors wanted to make clear to avoid false alarmism. The results do not indicate a risk for those who consume fish. The study involved young specimens, under the legal catch size, and the researchers themselves explain that the exposure levels used reflect concentrations already observed in polluted waters, with compounds that degrade over time. In short, the problem lies in the balance of the aquatic environments, in the ecological health of the lake and in the mechanisms by which human waste re-enters wild life.
From here on the issue becomes less picturesque and much more serious. It will be necessary to understand how widespread the phenomenon is, which species are most vulnerable and whether this increase in movements translates into concrete effects on survival and reproduction. For now, a dry image remains: fresh water, young salmon, a huge lake and a substance that comes from outside and pushes those fish to go further than usual. The rest, as always, is paid by the ecosystem.
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