When we talk about ocean pollution, most of us picture something physical we can actually see with our own two eyes: a plastic bottle bobbing through the waves, an oil slick spreading across the surface of the sea or fishing gear tangled around an animal. But, what if some of the most revealing pollution in the ocean is invisible Along the coast of Rio de Janeiro, Brazil, researchers with EcoShark have been uncovering exactly that. Since 2018, the project has been studying sharks from almost every angle imaginable, including their movements, diets, genetics, health and exposure to contaminants. And recently, that work made international news when they announced that antidepressant sertraline was detected in the shark tissue they had tested around Brazil’s waters, including brain tissue from hammerhead sharks.

“Honestly, my first reaction was surprise,” Adjunct Professor Dr. Mariana Batha Alonso of the Instituto de Biofísica Carlos Chagas Filho, Universidade Federal do Rio de Janeiro said. “We already know that sharks are exposed to many different contaminants, and our previous work has detected compounds such as sunscreen filters, insecticides and other pollutants in marine organisms. But finding sertraline — an antidepressant — in shark tissue was unexpected. What really caught our attention was where we found it.” Now, that does not mean the drug is harming sharks; detection is not the same thing as demonstrating a biological effect, and the researchers are careful to make that distinction! But because sertraline acts on the serotonergic system in humans, its presence in shark brains has left the scientists with more questions than answers. “At that point, the question became much bigger than simply ‘Is there a pharmaceutical in the ocean?’” she said. “We had to ask: How is it getting there? How is it reaching a shark’s brain? And, most importantly, could it have any biological consequences?”

Those questions sit at the heart of EcoShark’s broader approach. Researchers here want to understand how these animals interact with an increasingly human-dominated coastal environment. “Sharks are complex animals, and no single method can tell us the whole story,” Alonso explained. Contaminant analysis can reveal chemical exposure. Satellite telemetry can show where sharks travel and which habitats they use. Genetics can reveal population structure and help distinguish species. Dietary studies illuminate relationships within the food web, while health and physiological analyses can show how animals are responding to their environment. Put all those pieces together and the questions become much bigger: Where are sharks moving? Which habitats matter most? What pollutants are they encountering? Where do those threats overlap? “We are trying to understand the shark as a whole animal living in a complex ecosystem, rather than studying each threat in isolation.”

This sort of approach is particularly important in Brazil, a country with an extraordinary diversity of sharks and rays and a coastline stretching roughly 7,000 miles (11,000 kilometers). While Alonso says there is enormous potential for shark research, she also reminds us of the significant challenges faced here. Overfishing, bycatch, habitat degradation, pollution, inadequate health assessments, limited long-term monitoring and fragmented funding all complicate local conservation efforts. And one recurring problem is continuity.

“Shark research often depends on short-term projects and limited funding, while ecological processes happen over decades,” she said. “To understand population trends, movements, reproductive success or cumulative exposure to pollutants, we need long-term datasets.” Movement ecology especially is one area where long-term datasets could tell us a lot about sharks here, but satellite tags, acoustic receivers, vessels and field teams are expensive, making it difficult to follow individual animals for months or years. And sharks don’t care about our administrative boundaries. “A shark does not recognize the boundaries of a protected area, municipality or state. Ultimately, movement ecology allows conservation to follow the animal rather than simply following the map.”

This is where another critical piece of the EcoShark puzzle comes into play — people. EcoShark researchers work directly with fishing communities along the Rio de Janeiro coast, including places such as Marambaia, Recreio dos Bandeirantes, Barra da Tijuca, Barra de Guaratiba and Grumari. Much of their biological sampling comes from sharks accidentally captured by fishers, making those relationships essential to the research itself. “Fishermen have an enormous amount of practical knowledge about the ocean,” Alonso explained. Fishers tend to spend hundreds of days at sea each year, observing seasonal changes and animal movements that a scientific survey conducted over a few weeks could easily miss. They can notice changes in shark abundance, size, distribution and behaviour over periods researchers may not otherwise be able to capture due to lack of funds, resources, or time. “The important point is that local ecological knowledge and scientific knowledge are not competing forms of knowledge. They answer different questions and can complement each other.” But conservation in Brazil, Alonso argues, has to acknowledge that fishing is a livelihood. She believes that researchers need to understand what fishers are seeing and experiencing before asking them to participate in conservation efforts: “The first step is listening. If you approach a fishing community simply by saying, ‘You need to stop catching sharks,’ you are unlikely to build a meaningful relationship.” She continues on to say that shark fishing restrictions need to be accompanied by greater support for fishing communities and points to gaps in enforcement and the challenge of translating scientific evidence into practical policy.

The next phase of EcoShark’s research will continue pulling all of these threads together. The team of researchers want to investigate whether pharmaceutical exposure is associated with changes in sharks’ serotonergic systems, physiology or behavior. They also hope to examine more animals and different life stages, including whether contaminants can be transferred from mothers to offspring. At the same time, they are expanding their integrated approach to include telemetry, genetics, virome, health biomarkers, contaminant analysis, microbiome research and other molecular tools.

After all these years of studying sharks, what keeps Alonso excited about going into the field? “The fact that there is always something unexpected.”

That might be one of the best things about science: there is always something new to uncover. And with sharks, understanding what happens to them means looking beyond sharks themselves. Their movements intersect with fisheries. Their bodies record the chemicals flowing through our cities. And their survival depends not only on healthy habitats, but also on the people who share those waters. And the more we understand those connections, the harder it becomes to see sharks as isolated creatures beneath the waves. “They are part of our ocean,” Alonso said. “They are important predators, and protecting them means protecting the health of the ecosystem that we all depend on.”

The next time researchers go to sea, they don’t know exactly what they will find. That is precisely why they keep going…