Test results have shown that sharks in Brazil are taking cocaine. Scientists say they have several theories about how the Class A drug made its way to the South Atlantic.
Scientists tested 13 Brazilian sharpnose sharks found along the coast of Rio de Janeiro and found high levels of cocaine in their bodies.
The Oswaldo Cruz Foundation, which conducted the tests, says the drug significantly coats the sharks' livers and muscles.
The study, published in the journal Science of the Total Environment, suggests that the drug has been filtered through laboratory pipes into the South Atlantic Ocean.
Other possibilities for exposure include drug user waste that has made its way into the ocean through untreated sewage.
Scientists have also considered the idea that drug traffickers dump the dangerous drug into the sea, leading to sharks catching it, but they believe this is less likely.
"The results show chronic exposure due to human cocaine use in Rio de Janeiro," British research team member Dr Rachel Anne Hauser-Davies told The Telegraph.
She suggested that chronic exposure could also be explained by "discharge of human urine and feces through sewers, as well as from illegal laboratories."
She acknowledged that there is no evidence that the drugs make the fish more aggressive or more likely to go into a feeding frenzy. However, she said it is possible that taking cocaine could be harmful to them.
Scientists are concerned that sharks consuming cocaine could damage their eyes, impair their hunting skills and reduce their life expectancy.
Dr Tracy Fanara of the University of Florida, who was not involved in the study, told The Telegraph she suspected that cocaine in sharks' diets could be affecting fertility.
Dr. Enrico Mendez Saggioro, of the research center, told the newspaper that other animals had shown erratic behavior after taking cocaine, and he did not rule this out for sharks.
But he also stressed that more studies were needed to prove the effect of cocaine on sharks.
The Brazilian sharpnose sharks used in the study were recovered from small fishing vessels traveling in coastal waters off Rio de Janeiro.
The scientists dissected them before examining them, and discovered concentrations of cocaine up to 100 times higher than those previously found in similar animals.