If the prospect of sharks lurking just off the beach wasn’t frightening enough, researchers in Brazil have discovered a new reason to be unnerved: Some of them have cocaine in their system.
In a study published last week, researchers tested 13 sharks off the coast of Rio de Janeiro and found that all had traces of cocaine in their liver and muscle tissues. The levels of cocaine found in these sharks were reported to be as much as 100 times higher than in previously observed marine life.
“We were actually dumbfounded,” said Rachel Ann Hauser Davis, a co-author of the study and a biologist at the Oswaldo Cruz Foundation in Brazil. “We were excited in a bad way, but it’s a novel report. It’s the first time this data has ever been found for any top predator.”
This was the first study to analyze cocaine in sharks, following various studies on smaller species, including mollusks, crustaceans and even eels. All 13 sharks examined were found to have unfiltered cocaine in much higher concentrations than in previous studies on other animals, indicating chronic exposure to the drug.
But the study examined only a small sample, leaving many questions about whether the exposure harms the sharks or the humans who eat them.
“I thought it was pretty remarkable that they got it published even with just 13 animals,” said Daniel Snow, the director of the Water Sciences Laboratory at the University of Nebraska, who did not participate in the research.
Over a decade ago, Snow was among the first researchers to measure an illicit drug, methamphetamine, in wastewater in Nebraska. “It’s not too big of a stretch to imagine that these chemicals that wind up in the water can affect aquatic organisms that live in that same water,” he said.
The study in Brazil was conceived earlier this year after researchers discovered high levels of cocaine in the rivers that form Rio de Janeiro’s watershed. Other marine experts had looked into whether sharks in the Gulf of Mexico were ingesting cocaine from the numerous packages lost or dumped in the waters in a 2023 documentary titled “Cocaine Sharks,” which served as an inspiration for the title of last week’s study.
The team of biologists from the Oswaldo Cruz Foundation — an organization affiliated with Brazil’s Ministry of Health — were particularly interested in testing top predators inhabiting these watersheds. Having previously conducted tests on sharks for other contaminants, they sent to a lab samples of the Brazilian sharpnose — a relatively small species of shark from Rio de Janeiro’s coastal waters often consumed by locals.
Hauser Davis said there were several hypotheses as to how cocaine found its way to the marine creatures, including illegal labs refining cocaine or cocaine packages lost or dumped by traffickers. But she believes these account for only a small amount of the drug found in the ocean.
“We feel that the major source would be excretion through urine and feces from people using cocaine,” she said. Most wastewater treatment plants worldwide cannot effectively filter these substances, leading to their release into the ocean.
João Matias, a scientific analyst at the European Union Drugs Agency, was part of a team that analyzed cocaine levels in wastewater from more than 100 cities, including in Brazil. Their findings revealed that the levels of cocaine in Brazil’s wastewater were similar to those in the European cities with the highest levels.
But he emphasized that the concentrations of cocaine were not very high. Referring to last week’s study, he said, “It’s very important to stress that I’m 100% sure that the concentrations they found are super low.”
Tracy Fanara, an environmental engineer in Florida who led the research team for the “Cocaine Sharks” documentary, noted that cocaine was still a small part of the larger problem of pollutants in natural habitats.
“Cocaine gets people interested,” she said. “But we have antibiotics, antidepressants, pharmaceuticals, sunscreen, insecticides, fertilizers. All of these chemicals are entering our ecosystem.”
Hauser Davis expressed similar concerns. “Why isn’t anyone surprised when you find metals, pesticides and PFAS?” she said.
But she hopes their research will open new doors to testing other animals for cocaine.
“We’re hoping to do other sharks, rays and even sea turtles,” she said.
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