Cooperation between dolphins and humans

A study reveals how cooperative hunting between dolphins and fishers can benefit both species—and why this behavior faces extinction

January 26, 2023

In the city of Laguna on Brazil’s southern coast, dolphins and humans have been helping each other hunt for over a century. In the practice, traditional net-casting fishers wait in the lagoon for wild bottlenose dolphins to appear. When they do, the fishers watch the cetaceans’ behavior, looking for cues that tell them when to cast their nets to catch fish. It’s a rare case of cooperation between humans and wildlife; and the benefits to fishers in the form of greater catches was well known. But what was unknown was how this cooperation unfolded underwater—and whether or not the dolphins also benefitted from the partnership. Now, a 15-year study of the practice by an international team from Brazil and the Max Planck Institute of Animal Behavior has revealed the behaviors of fishers and dolphins with unprecedented detail. By combining drones with underwater imaging, they found that dolphins and net-casting fishers synchronize their behavior, which leads to both catching more fish. This rare example of a mutually beneficial interaction by two top predators, however, is facing extinction. Predictive models run as part of the study show that the future of the practice could be threatened if populations of mullet—the type of fish both dolphins and people are seeking—continue to decline, or future generations of fishers lose interest in learning the art of this unique fishing practice. The researchers’ findings are published in the Proceedings of the National Academy of Sciences.

Synchronized movements of flocks of birds and schools of fish are a common yet striking behavior that can be key to the animals’ survival. However, synchronized behavior between species, like that between the bottlenose dolphins and the traditional net-casting fishers in Brazil, is much more rare.

The practice is considered a cultural tradition in Laguna, where it has occurred for more than 140 years and has been passed down through generations of fishers and dolphins. The cooperative fishing relationship is specific to this population of dolphins and is not a genetic trait in the animals, the study authors say.

“From the fishers’ perspective, this practice is part of the culture of the community in all kinds of ways,” said first author Mauricio Cantor who led the study while at the Max Planck Institute of Animal Behavior (MPI-AB) and is now a scientist at Oregon State University’s Marine Mammal Institute.

“They acquire skills passed down from other fishers and knowledge is spread through social learning. They also feel connected to this place and have a sense of belonging to the community.”

There are historical and recent accounts of similar behaviors in a handful of locations elsewhere in the world, but the practice is in decline or has disappeared completely in most places and remains almost completely unstudied in others. The rare nature of the practice is one reason it is being considered for a cultural heritage designation in Brazil, he said.

Cooperating is a win-win for dolphins and fishers

To better understand this cultural tradition, the researchers combined drones, hydrophones, and underwater cameras to capture the mechanics of the partnership. Together, the methods reveal that dolphins and humans are actively coordinating their behaviors to catch fish: dolphins herd mullet schools towards fishers, creating temporary high-quality patches just before giving a cue to fishers to cast their nets.

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

To measure the short- and long-term consequences for both fishers and dolphins, the researchers conducted long-term demographic surveys for dolphins and interviewed and observed the fishers. They found that foraging synchrony led to fishers catching nearly four times more mullet.

This cooperation also boosts the dolphins’ survival—dolphins who engage in cooperative fishing in this area have a 13% increase in survival rates—and the socioeconomic wellbeing of the fishers. “This shows that this is a mutually beneficial interaction between the humans and the dolphins,” said Cantor.

“Questionnaires and direct observations are different ways to look at the same phenomenon, and they match up well. By integrating these together, we could then get the most complete and reliable picture of how this system works and, most importantly, how it benefits both fishers and dolphins.”

A future hanging in the balance

Most interspecific interactions, including those between humans and other animals, are competitive rather than mutually beneficial, the researchers said.

“But not in this case,” says co-author Damien Farine who conducted the research while at MPI-AB and is now a scientist at the University of Zurich and the Australian National University.

“This makes this system of substantial scientific interest, as it can help us to understand under what conditions cooperation can evolve and—of growing importance in our rapidly changing world—under what conditions it might go extinct, or flip from a cooperative to a competitive interaction.”

Fábio Daura-Jorge, a co-author who has studied the practice for 15 years at the Universidade Federal de Santa Catarina in Brazil, said researchers are already seeing early signs of decline in the practice.

Predictive models run as part of the study support this observation. Both the dolphins and the fishers are reliant on a strong and healthy fish population for the cooperative relationship to succeed. In recent years, the region has seen reduced availability of fish. There is also reduced interest in learning the tradition, said Daura-Jorge.

“We don’t know what is going to happen in the future, but our best guess, using our best data and best models, is that 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.

Saving the practice

The researchers suggest several conservation measures may be necessary to secure the future of the practice. First is to try to identify the source of the mullet decline and take measures to better manage that species, such as reducing use of illegal nets through law enforcement, Daura-Jorge said.

Second, the researchers recommend steps to work with current and future artisanal fishers, stressing the cultural and economic importance of the net-casting practice. That might include offering incentives to encourage the traditional practice, such as setting a premium price for fish caught with this method.

“This phenomenon of mutually-beneficial interaction between wildlife and humans is getting more and more rare and seems to be at global risk,” Cantor said. “The cultural value and the biological diversity are important, and it’s important to preserve it.”

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