Along the coast of Santa Catarina in southern Brazil, fishing is a way of life. But not just any kind of fishing. For the last 150 years, the fishers of the city of Laguna have cultivated a unique relationship with the local Lahille’s bottlenose dolphins, working together with their cetacean comrades to catch the schools of mullet that frequent their lagoon. 

The dolphins herd the fish toward human fishers who wait by the shore, hip deep in water. Then, at exactly the right moment, the dolphins dive, a signal for the fishers to cast their nets. Working together, the fishers catch more fish, and the dolphins have a chance to snag a few from the net as well.

“The experience of fishing with dolphins is unique,” Laguna fisher Wilson F. Dos Santos told The New York Times. Both the humans and the dolphins have passed the practice down for generations, with the fishers giving their dolphin counterparts names like Scooby, Caroba, and Porquinho. Dos Santos learned the tradition from his father, bringing him coffee and food by the shore and watching the nets unfurl, he told the Times.

Now, in a study published this week in Proceedings of the National Academy of Sciences, scientists have zeroed in on just how the fishers and dolphins synchronize their hunts, and how both sides benefit in the short and long term. The team, led by marine biologist Mauricio Cantor of Oregon State University, concluded that the situation is one of true mutualism: The fishers catch four times more fish when they have dolphin help, and the dolphins that cooperate with them enjoy higher survival rates. But without intervention, they predict, both the interspecies partnership and the dolphins themselves are at risk. 

To understand the underlying mechanisms at work in Laguna, Cantor and his colleagues spent more than 10 years measuring the synchronized fishing from all angles. They tracked mullet schools with sonar cameras; documented dolphin behavior with photography, drones, and audio recordings; and captured the movements and success of 177 fishers with GPS wristbands, interviews, and regular weighing of the catch. The team then categorized the dolphins as “cooperative,” “occasional cooperative,” and “non-cooperative,” and analyzed the data for each.

Of the 65 dolphins observed, more than 40 collaborated to some degree. While helping the fishers, the cooperative dolphins would drive the mullet toward shore, then dive to signal to their human counterparts. And the data showed that the dolphins would stay underwater longer and modify their echolocation clicks and buzzes in tandem with the sinking nets–but only when the fishers had responded to their diving cues correctly. 

“We knew that the fishers were observing the dolphins’ behavior to determine when to cast their nets, but we didn’t know if the dolphins were actively coordinating their behavior with the fishers,” Cantor said in a statement.

The scientists hypothesized that the nets swishing through the water disrupted the fish schools, making individual mullet easier to catch. And underwater imaging captured what many fishers could feel: the dolphins snagging a few fish directly from the nets, a quick payoff for their collaboration. “The dolphins know what they are doing,” coauthor Fábio Daura-Jorge of the Universidade Federal de Santa Catarina told The New York Times. “They’re taking advantage of the fishers’ actions to actively forage.”

The dolphins seem to be the true experts here, according to Simon Ingram, a marine biologist at the University of Plymouth who has studied the human-dolphin dynamic in Laguna but was not involved in this study. Over the decades, the dolphins have shown the fishers “where to stand and when to get ready to throw their nets” in murky water where they might not be able to see otherwise, he told Science. “It’s almost as if the dolphins are training the humans.”

But really, everyone wins. The various tracking techniques showed that 86% of the mullet caught by human fishers came from successfully synchronized interactions with dolphins. In fact, fishers were 17 times more likely to catch fish, and caught nearly four times as much, when dolphins were present—regardless of how many fish were nearby. On the other side, besides their occasional snacks, the dolphins that cooperated gained a more dramatic advantage: a 13% higher survival rate over their non-cooperative peers. This difference may be related to illegal forms of fishing elsewhere in the lagoon that frequently cause dolphin entanglement. The researchers found that non-cooperative dolphins were much more likely to die as bycatch in a fishing net than their cooperative brethren.

“These dolphins and humans have developed a joint foraging culture that allows them both to do better,” Dalhousie University marine ecologist Boris Worm, who was not involved in the research, told The Associated Press.

University of Bristol biologist Stephanie King, who studies dolphin communication and was also not involved, agreed. “This study clearly shows that both dolphins and humans are paying attention to each other’s behavior, and that dolphins provide a cue to when the nets should be cast,” King told The AP. “This is really incredible cooperative behavior.”

Cooperation between dolphins and humans fishing with nets has occurred in communities from Mauritania to Myanmar, but over the centuries many of these relationships have died out. As overfishing and rising ocean temperatures cause a decline in the local mullet population, the researchers fear the Laguna partnership is also at risk. As mullet numbers drop, so do the benefits of cooperation, and the study authors calculate that this unique alliance may fade out in the next 40 to 60 years. “If things keep going the way they are right now, there will be a time when the interaction will no longer be of interest by at least one of the predators—the dolphins or the fishers,” Daura-Jorge said in the statement. He and his colleagues predict that that trend will mean fewer cooperative dolphins, ultimately leading to more dolphin deaths. 

As part of their study, the researchers used computer modeling to explore possible solutions. They concluded that the most effective response would be a combination of increased policing of the illegal techniques that lead to entanglement and economic incentives, such as premium prices, to encourage fishers to keep interacting with the dolphins, even if their fishing hauls decrease.

Stephanie King told Scientific American she considered the study to be “of huge importance,” since it shows not just “how human-wildlife cooperation is coordinated and achieved but also how it can be conserved in the face of large-scale environmental and social challenges.”

And Cantor considers the study itself a form of intervention. “Getting people excited about these rare phenomena is a way to add more value to these kinds of cultural practices that seem to be fading away,” he told Scientific American. “Preserving cultural diversity has been shown to promote the preservation of biological diversity as well.”