Europe’s river-dwelling creatures are threatened by traces of cocaine in some of the continent’s most well-known rivers, according to a new study. Migratory animals like the eel are particularly at risk.

In the study, researchers at the University of Naples Federico II said they discovered that low levels of cocaine in rivers and other freshwater bodies are causing severe damage to eels.

An earlier study by the European Monitoring Centre for Drugs and Drug Addiction dating from 2015 showed that London has Europe’s highest concentration of cocaine in sewage, while a 2005 Italian report revealed that the River Po contains high levels of cocaine by-product.

Cocaine mostly gets into our rivers in the form of urine and benzoylecgonine, the metabolised form of the drug which drug testers look for when checking professional athletes.

Water purification procedures often fail to remove the substances from sewage, explaining the small but significant traces that end up in waterways.

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The findings prompted the Naples team to look into how the presence of cocaine and other drugs in waterways affects aquatic life, particularly the critically endangered European eel.

Their investigation studied 150 of the eels by observing the effects of cocaine-infused water on the animals, using tanks and quantities that replicate their environment. They compared those animals with a control group kept in tap water.

Coked-up eels displayed traces of hyperactivity and muscle damage, and even after ten days of rehabilitation, the effects were still present. Eels are a migratory species that can travel up to 6,000 km, meaning muscle damage is a particularly debilitating injury.

Anna Capaldo, leader of the Naples team, said that “we chose eels because they are considered to be at risk of extinction and because they are a fat fish, which favours the accumulation of substances”.

She added that other substances, such as medicinal drugs and metals, could also have a significant effect and that her team wants to embark on another study to look into that impact.

In terms of bioaccumulation in the food chain, Capaldo warned that the substances can build up in the edible flesh of the animals but added it was not clear what happens with the substances after the eels die and after cooking.

The European Commission has the European eel on its radar and in December 2017 the European Council agreed that limits should be put on fishing in order to safeguard the animal’s numbers.