Thirteen Brazilian sharpnose sharks caught off Rio de Janeiro have tested positive for cocaine, marine biologists who carried out the tests have reported.

High concentrations of cocaine were found in the sharks’ muscles and livers by researchers at the Rio de Janeiro-based Oswaldo Cruz Foundation.

Benzoylecgonine was also detected in the same sharks. Benzoylecgonine is one of the byproducts produced when cocaine is consumed.

What did the study find?

The 13 sharks were three males and 10 females, five of which were pregnant. All the sharks were captured from fishing boats off the coast of the state of Rio de Janeiro from September 2021 to August last year.

Samples were taken from the sharks, and the concentration of cocaine was found to be more than 100 times those previously reported in any other aquatic animals, according to the study.

The study also found that cocaine levels were three times higher in muscle than the livers of the sharks.

Why are sharks testing positive for cocaine?

Although further research is needed, the study suggests this is mostly down to more people using cocaine.

A rise in cocaine use in the region, coupled with an inadequate drainage system, has caused higher quantities of cocaine to be present in seawater.

The study states: “Global COC [cocaine] consumption has increased exponentially in the last decades, as highlighted in the United Nations World Drug Report. About 22 percent (4.8 million) of the estimated 22 million COC users worldwide reside in South America as of 2021, with Brazil emerging as the second largest consumer market in this area.”

The study added that sewage analysis covering 60 million people in 37 countries from 2011 to 2017 had consistently revealed the presence of cocaine in aquatic environments.

Has the effect of cocaine in sharks been studied before?

In 2023, a documentary called Cocaine Sharks followed marine biologist Tom Hird and environmental scientist Tracy Fanara, who were studying what happens when sharks come in contact with cocaine in the Florida Keys.

The Florida Keys is a known hotspot for drug running. In July, according to federal authorities, the fifth largest load of illegal drugs ever was found by former police officer and Tampa Mayor Jane Castor on a fishing trip off the Florida Keys. Castor reported the find to United States Border Patrol.

According to Chief Patrol Agent Walter Slosar with the Border Patrol’s Miami sector, the 32kg (70lb) of cocaine had an estimated street value of $1.1m dollars.

Last month, a recreational boater in the Florida Keys found 30kg (65lb) of cocaine with a street value of $1m floating in the sea.

Recreational boater in the Florida Keys discovers 65 lbs. of cocaine floating at sea. The cocaine has an estimated street value of over $1 million & will be seized by the U.S. #BorderPatrol. We appreciate the support from Good Samaritan’s in our community.#floridakeys #drugs pic.twitter.com/g2hQYhLATG

— Samuel Briggs II (@USBPChiefMIP) June 18, 2024

The documentary was made after a rumour among fishermen that sharks were coming into contact with bales of cocaine due to the heavy prevalence drug traffickers dumping cocaine from planes off the Florida coastline. The belief was that it was due to cocaine leaking into the sea.

Furthermore, drug traffickers are known to dump cargoes of illegal drugs into the sea if they fear being caught by the coastguard.

In other instances, drug traffickers use a delivery system in which they wrap cocaine in multiple layers of plastic and other waterproof materials and drop it into the water to be picked up by another boat. In February 2023, police in New Zealand found 3.2 tonnes of cocaine worth $300m floating in the sea. In 2019, authorities intercepted a submarine carrying 3 tonnes of cocaine worth $110m off the coast of Spain.

One of the experiments conducted by Hird and Fanara involved dumping fake cocaine bales and plastic swans in the water to test which the sharks would approach first. Although it appeared that the sharks attacked the bundles first, it’s still unknown why.

It also remains unclear how cocaine affects sharks physiologically, experts say.

“A few studies done with cocaine show that it affects fish really differently than it affects humans,” Florida International University biological scientist Laura Garcia Barcia said in July last year on an episode of National Geographic’s When Sharks Attack and Why.

According to Barcia, cocaine in sharks acts as an anaesthetic while it is a stimulant in humans.

In a 2016 study, toxicology researchers from the Swiss Federal Institute of Aquatic Science and Technology (Eawag) and Zurich University found that giving cocaine to zebrafish did not have the same stimulating effect as it did on humans. By using a sophisticated imaging method to analyse tissue samples, researchers showed that cocaine accumulated in the eyes of zebrafish instead of the brain like humans.

“If we want to have a better knowledge of the effects of such chemicals on the ecosystem, we need a more detailed understanding of the processes of uptake through water. They’re quite different from when drugs are inhaled or injected,” said Eawag environmental toxicologist Kristin Schirmer, co-author of the study.

Have other animals or fish been affected by cocaine in the sea?

Biologists at the University of Naples Federico II conducted a study in 2018 in which they submerged European eels in water containing a small quantity of cocaine similar to the amounts found in many rivers.

The eels lived in the water for 50 days, and researchers found they exhibited hyperactive behaviour.

This has raised concerns among environmentalists and biologists that the presence of the drug in rivers could represent a major health problem for some species of fish.

In 2021, another study was carried out by researchers from the Universidade Estadual Paulista and the Universidade de Sao Paulo in Brazil on the presence of cocaine and benzoylecgonine in seawater, sediment and mussels from Santos Bay, Brazil.

The researchers found “widespread contamination by cocaine and its main human metabolite benzoylecgonine in Santos Bay” and said “mussels were able to accumulate” cocaine as a result.