For decades, a small group of fishers have gathered in waist deep water at the edge of a lagoon in southern Brazil and awaited the arrival of some unusual fishing partners: a group of bottlenose dolphins.
Both species are pursuing grey mullets, silvery fish as long as a person’s forearm that are locally known as tainha. But, in an unusual display of intraspecies cooperation, the people and dolphins have found a way to join forces to benefit both groups.
The phenomenon in the waters of Laguna has been known for years, charming people with this seeming tale of people living in harmony with nature. Now, scientists armed with high-tech gadgets have pieced together a detailed account of this relationship. It illustrates both how nuanced the choreography is, and how imperiled. That Disneyesque harmony, they warn, is in danger of breaking down.
“We don’t know what is going to happen in the future,” said Fábio Daura-Jorge, who has studied the practice for 15 years at the Universidade Federal de Santa Catarina in Brazil. But scientists’ best guess 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.”
The phenomenon appears, at first glance, relatively straightforward. Dolphins chase schools of mullet close to the shore, where individual fishers fling nets onto the water, hauling in whatever fish are trapped as the nets sink. But scientists wanted to understand just how this partnership works, and what both parties get out of it. The advantage to fishers is fairly obvious. The dolphins help corral the fish. But what do the dolphins get out of it?
To understand this, the scientists deployed an arsenal of electronics. Aerial drones recorded the fishing from above. GPS wristbands monitored fishers’ movements, such as when they waded into the water. Sonar gauged the number of mullet swimming through the murky waters. Underwater microphones recorded the echolocation clicks of the dolphins.
Researchers also watched the fishing for hours, tracking how many people and dolphins were at work and the number of fish trapped in nets. And they interviewed fishers to tap into their years of experience.
The scientists learned that success was not as simple as waiting for dolphins to show up and then hurling a net. Timing and reading the body language of the dolphins were both critical. Fishers caught far more fish when they timed their net throw to specific signals from dolphins, notably a sudden, deep dive after approaching the shore. Of the 2,800 interactions recorded by the drones, slightly less than half followed this script. Yet they accounted for three quarters of the incidents when fishers caught something, and 86% of all the fish caught, the scientists reported last week in the Proceedings of the National Academy of Sciences.
But things didn’t always go so smoothly. More than half the time, people tossed nets when there was no signal from the dolphin, failed to respond when dolphins did send signals, or threw the nets too late or in the wrong place.
Sonar confirmed what the contents of the nets were showing. When dolphin and person moved in tandem, more mullets were detected close to the fishers. In other scenarios, there were no additional fish. Clearly, the fishers benefitted from reading the signs and reacting accordingly.
So what did the dolphins get out of it? While it’s hard to count how many fish the dolphins caught in different circumstances, scientists did see clues suggesting the dolphins were hunting more actively when they moved in synchrony with the fishers. In those moments, dolphins more often made distinctive buzzing echolocation clicks as they homed in on prey. Drone shots also revealed that dolphins sometimes snagged a fish directly from a net.
“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 Mauricio Cantor, who led the study while at the Max Planck Institute of Animal Behavior in Germany and is now at Oregon State University.
Despite the benefits, this spectacle is in danger of vanishing. Not all dolphins in the area engage in this cooperative arrangement. And even the ones who do don’t cooperate all the time. The researchers found that the number of dolphins with very frequent interactions with fishers fell by roughly half between 2007 and 2018 . At the same time, the experience level of the fishers has fallen in recent years, from an average of nearly 30 years to 20. This coincides with signs that mullets are being overfished in the region by both industrial and small-scale fishers.
To better understand where this might be headed, the scientists modeled different scenarios, depending on the state of the mullet fishery. The results showed that as fish are depleted, it’s increasingly likely that people will abandon their teamwork. Likewise, dolphins that once could rely on fishers will instead turn to foraging elsewhere, putting them at greater risk of getting caught and killed in other types of nets. If the fishery collapses, the cooperative system will likely go extinct within 60 years, the scientists concluded.
To counter this, the researchers recommend giving fishers incentives to maintain their dolphin-centric techniques, such as setting a higher price for dolphin-caught fish. At the same time, they point to the need for measures to revive the mullet fishery, including cracking down on illegal nets that endanger dolphins and deplete the fish.
“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 biodiversity are important, and it’s important to preserve it.”
Cantor, et. al. “Foraging synchrony drives resilience in human–dolphin mutualism.” Proceedings of the National Academy of Sciences. Jan. 30, 2023.
Photo: Dr. Fabio G. Daura-Jorge, Departamento de Ecologia e Zoologia, Universidade Federal de Santa Catarina, Florianópolis SC, Brazil
