The results showed “chronic exposure due to human use of cocaine in Rio de Janeiro and the discharge of human urine and faeces by sewage outfalls, as well as from illegal labs”, said Dr Rachel Ann Hauser-Davis, a British scientist and member of the research team from the Oswaldo Cruz Foundation in Rio de Janeiro.

‘We don’t get much coke at sea’

“We don’t usually see many bales of coke dumped or lost at sea here, unlike in Mexico and Florida,” she told The Telegraph, meaning it was unlikely the sharks had been eating discarded packages.

The cocaine was probably harmful to the health of the animals, she said. It was not known if it affected their behaviour, rendering them more aggressive and unpredictable or making feeding frenzies more likely.

“This may be the case, as cocaine targets the brain, and hyperactive and erratic behaviour has been noted in other animals. It’s a possibility and further studies are required,” said Dr Enrico Mendes Saggioro, an ecotoxicologist from the Oswaldo Cruz Institute.

The researchers acquired the small sharks from fishing boats that ply the coastal waters off Rio de Janeiro.

After dissecting them and subjecting them to tests, they found concentrations of cocaine up to 100 times higher than previously detected in other marine animals. The scientists described their findings as “worrying” for the marine ecosystem. 

Damaging to eyesight

One way cocaine may damage sharks’ health is by affecting their eyesight, affecting their capacity to hunt.

“You might start to see lower fecundity and growth rates,” Dr Tracy Fanara, an expert in ecotoxicology and environmental engineering from the University of Florida, told The Telegraph.

She agreed that there was no evidence, as yet, to suggest that cocaine encouraged crazed behaviour or feeding frenzies in sharks.

“They may not be going nuts from the cocaine but it could reduce their life expectancy,” said Dr Fanara. She took part in the Discovery Channel film Cocaine Sharks, which explored whether sharks in the Gulf of Mexico were feeding on cocaine packages dumped by traffickers.

Dr Fanara added: “This is representative of problems all over the world, whether it involves cocaine or fertiliser or herbicides. 

“We are releasing these chemicals into the environment and the question is, how are we affecting the ecosystem?”