Eels became hyperactive after time in water containing cocaine imageBROKER/Alamy Stock Photo

Eels exposed to low levels of cocaine in their water become hyperactive and suffer muscle damage. The eels may struggle to complete their migrations as a result.

The finding adds to the growing evidence that drugs in fresh water can cause harm, even if they are only found at extremely low levels.

Rivers and other water bodies often contain low levels of drugs, including both medicines and illegal drugs such as cocaine. These chemicals can make their way into drinking water and there is evidence that they affect wildlife.


Anna Capaldo of the University of Naples Federico II in Italy and her colleagues studied European eels (Anguilla anguilla). They have previously found that cocaine accumulates in the eels’ flesh and that it affects their skin and hormones.

Eels don’t like the drugs

The team kept 150 eels in tanks. Some of the tanks contained a low level of cocaine, just 20 nanograms per litre, while the rest held just tapwater. The eels were kept in these tanks for 50 days.

The eels exposed to cocaine were hyperactive, swimming noticeably faster than the control eels.

After the treatment was over, the eels were killed and dissected. The researchers found that their skeletal muscle, which powers their swimming, was damaged. The muscles were swollen and some of the muscle fibres were broken

This damage was still there if the eels had been allowed to recover in cocaine-free water for 3 or 10 days.

They just make the eels worse

European eels are critically endangered. It’s not entirely clear why, but there are many possible factors, from over-exploitation to habitat loss.

Capaldo points out that they famously make long migrations from Europe all the way across the Atlantic to the Sargasso Sea, near the Bahamas, to spawn. Eels with damaged skeletal muscle might not be able to make the trip.

What’s more, since the cocaine accumulates in the eels’ flesh, people eating eels will be consuming low levels of cocaine – although probably not enough to have any effect whatsoever.

To get rid of cocaine in surface waters, we would need to improve the efficiency of sewage treatment plants, and possibly also develop a system of water purification, says Capaldo. “The best thing would be to stop taking [the] drug, but I am afraid this is not realistic,” she says.

Journal reference: Science of the Total Environment, DOI: 10.1016/j.scitotenv.2018.05.357