Thirteen sharks caught in waters off Brazil have tested positive for cocaine, with particularly high concentrations detected in their tissues. The findings have renewed attention on the presence of illicit substances in marine environments and their possible effects on wildlife.
One factor behind the phenomenon may be the widespread use of cocaine. According to figures cited in the UN World Drug Report 2023, 5.8% of the global population used drugs in 2021, while around 22 million people reportedly used cocaine.
Consumption is not evenly distributed around the world. Around 22% of global cocaine users are reported to live in South America, with Brazil ranking second among the markets with the highest consumption of the substance.
Another factor may be inadequate wastewater treatment infrastructure, which can result in significant quantities of drugs being discharged into the ocean.
How cocaine-positive sharks were identified
Previous studies had already highlighted the possible effects of cocaine on marine wildlife. The substance had been linked to eyesight damage in bony fish, negative effects on mussels, toxicity in crustaceans and molluscs, and reduced cellular viability in zebrafish.
Until this research, however, no targeted study had examined the possible effects of cocaine on elasmobranchs, the group that includes sharks and rays.
The presence of cocaine in Brazilian sharks was reported by a research team led by biologist Rachel Ann Hauser-Davis and ecotoxicologist Enrico Saggioro of the Oswaldo Cruz Institute in Rio de Janeiro.
The analysis involved 13 sharpnose sharks accidentally caught in fishing nets between the summer of 2021 and the summer of 2023 off Recreio dos Bandeirantes, a densely populated neighbourhood of Rio de Janeiro.
The Oswaldo Cruz Institute carries out environmental testing in the area to identify heavy metals, viruses, bacteria and other potentially harmful substances in wastewater.
Very high cocaine concentrations in shark tissues
Laboratory tests detected cocaine in the muscles and livers of the sharks, at concentrations described as the highest ever observed in marine wildlife.
The cocaine concentration in the animals’ tissues was also around three times higher than that of the metabolite produced in the human liver after the substance is consumed.
One hypothesis is that some cocaine may enter the ocean directly from laboratories where it is produced. This would expose sharks to cocaine that has not previously been metabolised by the human body.
Another possible source is drug shipments abandoned at sea by traffickers during unsuccessful transport operations.
Effects on sharks remain unclear
According to the information reported, the risk to humans is considered minimal because of their limited exposure to seawater.
Researchers still need to determine how such high cocaine concentrations could affect sharks. The main areas requiring further investigation include behaviour, health and reproduction.
The findings therefore add another element to concerns over drug pollution in marine waters and raise questions about the possible consequences of this contamination for ocean ecosystems.