The predators were consuming the potent stimulant due to its continuous release from inadequate sewage treatment facilities and clandestine refining operations, scientists wrote in a study published in Science of the Total Environment.
Some may also have attacked bricks of cocaine which traffickers had lost at sea off the coast of Brazil, one of the world’s largest markets for the drug. Of the 13 specimens of Brazilian sharpnose shark scientists tested over almost three years, all presented cocaine in their muscle and liver tissue, according to the study by the Oswaldo Cruz Foundation (Fiocruz), an institute of science, technology and health.
A staggering concentration
The drug was found in all 13 wild Brazilian sharpnose sharks tested, with a concentration as much as 100 times higher than previously reported for other aquatic creatures.
Scientists collected the samples between September 2021 and August 2023 near Recreio dos Bandeirantes, a neighbourhood in the West Zone of Rio de Janeiro, as they monitored environmental impacts of pollution on marine life. Rachel Ann Hauser-Davis, a biologist from the Environmental Health Assessment and Promotion Laboratory, said that because sharks were predators, they were central figures in the food chain and were considered “sentinel species” that could provide early warnings about environmental threats to humans.
The findings were particularly shocking because cocaine was found in higher concentrations in muscle tissue than in the liver. Typically, the liver is the organ responsible for filtering toxins, so the accumulation in muscle suggests an abundance of the drug in the marine environment that the animals could not process fast enough.
Behavioural changes
The scientists did not list what effects cocaine and other recreational drugs might have on sharks, although other studies have shown brown trout can become addicted to methamphetamine.
In that Czech study, trout exposed to methamphetamine suffered withdrawal symptoms and actively sought out drug-laced water over clean water. For sharks, experts fear the drug could impair their vision and hunting ability. Since cocaine targets the brain, it may cause erratic behaviour, potentially making them less effective hunters or impacting their survival instincts in the wild.
Risks to humans
The study raises concerns not just for marine life, but for humans as well. Brazilian sharpnose sharks are commonly eaten in Brazil, where they are sold under the name cação. The high concentrations of cocaine and its main metabolite, benzoylecgonine, in the muscle tissue — the part humans consume — could pose health risks, though the extent of this danger requires further research.
Scientists elsewhere have documented that other pollutants like antidepressants, heavy metals and birth control medications are also making their way into rivers, lakes and oceans. These “invisible pollutants” are altering ecosystems in ways we are only just beginning to understand.
