Cocaine pollution in rivers may be affecting salmon behaviour more than scientists expected

Lakes and rivers are becoming reservoirs for a broad variety of chemicals many of which result in human activities. Some of these pollutants, such as traces of illegal drugs like cocaine are drawing increasing research attention. Recent research has shown that even minor levels of these substances within freshwater ecosystems could cause unexpected or even harmful consequences for fish, specifically salmon.

Researchers have discovered that cocaine, as well as its principal break-down product benzoylecgonine could alter the normal behavior of fish. The findings raise questions not only with regards to individual species, but also the overall wellbeing of the aquatic ecosystems.

In the course of study, and it's roots


The study, which is published in Current Biology, was conducted by scientists from the Swedish University of Agricultural Sciences located in Sweden. The study was conducted by Dr. Jack Brand, the research team was tasked with determining the way that concentrations of cocaine impact the behaviour of fish on the open sea.

In order to simulate exposure in real life, scientists employed juvenile Atlantic salmon, which were 2 years old, and were reared in hatcheries. The fish were outfitted with implants that would gradually release controlled quantities of benzoylecgonine or cocaine over time. Another group of salmon were given implants that are drug-free to act as a means of control. Every fish was equipped with an acoustic receiver that allowed scientists to follow the movements of their fish with precision.

The study took place at Lake Vattern, Sweden's second-largest lake that covers nearly 2000 sq km and houses a range of species of fish, such as predatory pike. For two months, sensors installed in the water monitored the movement and activity that the pike.

Movement and behavior changes

The outcomes of the research are striking. As all salmon slowly diminished their activity and concentrated in certain regions of the lake, the ones exposed to cocaine and its metabolite showed distinct characteristics towards the end of the study.

The final two weeks of exposure, the salmon exposed to cocaine traveled five kilometers further than those in the control group. More surprisingly, those exposed to benzoylecgonine traveled nearly 14 km further, almost increasing the distance covered by fish that were not exposed. They also went further to the north of the lake being exposed to metabolism moving up to 12 kilometres further than the distance of the group that was in control.

According to Dr. Brand that the metabolite produced greater effects over cocaine in itself. This is especially important since benzoylecgonine tends to be present with higher concentrations in the natural water system that the cocaine itself. It is suggested that existing risk assessments of environmental risks may miss important aspects by not taking into account the decomposition substances.

The environmental trade-offs

While the research highlights obvious behavior changes, long-term impacts on the fish population remain unclear. Researchers believe that the increased mobility and dispersal might come at costs. Fish who travel further could use more energy, possibly impacting their growth as well as general well-being.

Dr. Brand observed that these behavioral modifications could lead fish to hunt more often to fulfill their needs for energy. The increased foraging activity could put them at risk from predators, specifically on open water, in areas where they're more noticeable and exposed. In time, these choices could affect the survival rate and stability of populations.

The consequences go beyond the the individual species. The behavior of an important species such as salmon may ripple throughout the entire ecosystem, altering the predator-prey relationship and the equilibrium of the aquatic community.

The increasing threat to the biodiversity

This research adds to the increasing body of evidence indicating that illicit and pharmaceutical pollution is a major risk to the biodiversity. Research has previously revealed unusual behavioral changes observed in different aquatic species. For instance trout were identified to have symptoms of methamphetamine addiction as well as perch exposed antidepressants show less anxiety towards predators.

The 2019 tests carried out on freshwater fish in the rivers of Suffolk discovered evidence of the presence of many substances like methamphetamine and cocaine as well as anxiolytics, antidepressants and anxiolytics. Although the study could not identify any direct damage, it did highlight the prevalence of these drugs in aquatic ecosystems.

Scientists are more concerned than ever about the potential for multiple effects from different contaminants can increase the risk and make it difficult to anticipate and control the effects they can have on wildlife.

The importance of wastewater as well as potential solutions

One of the main routes to introduce these contaminants into wastewater systems is the sewage. While modern wastewater treatment facilities are able of eliminating a wide range of illegal drugs like benzoylecgonine and cocaine, they can't always be 100% effective. One of the main sources of contamination is wastewater that is not treated or that is released in storms overflows, or because of plumbing issues.

Experts agree that better the management of wastewater is essential in limiting the amount of pollutants. The reduction of raw sewage discharges as well as modernizing the infrastructure for treatment could reduce the risk of exposure for the aquatic creatures to harmful chemicals.

Prof. Leon Barron, who leads the group of emerging chemical contaminants within Imperial College London, highlighted the significance of conducting further studies. He stressed the necessity to determine whether the behavioral impacts observed in controlled research are also observed in wild fish exposed to the contaminants in the natural environment. Furthermore, comparing the impacts of the cocaine-related chemicals with those of other commonly used chemicals in water bodies will be vital to have a full comprehension of the risks to our environment.

In the future

The fact that the presence of cocaine could affect the behaviour of salmon can be a stark reminder of the fact that human actions can cause a wide-ranging and unanticipated impact. The full impact of these activities on the environment is not yet known, this study highlights the necessity of looking at the effects of drugs as well as their metabolites as part of environmental assessments.

While research is ongoing, scientists seek to understand more about the effects that the contaminants, and devise strategies to safeguard the aquatic ecosystems. The findings for now are a warning about the potential dangers that lurk in the waters of our planet and the need to take action.