A fishing community in southern Brazil has an unusual ally: wild dolphins.
Records of humans and dolphins working together to hunt fish go back millennia, from the days of the Roman Empire near what is now southern France to 19th-century Queensland, Australia. But while historians and storytellers have told the human point of view, it’s been impossible to confirm how the dolphins benefited — or if they were taken advantage of — before sonar and underwater microphones could track them underwater.
In the seaside town of Laguna, scientists have used drones, underwater sound recordings and other tools for the first time to document how local humans and dolphins coordinate actions and benefit from each other’s work. The most successful humans and dolphins are adept at reading each other’s body language. The research was published Monday in the journal Proceedings of the National Academy of Sciences.
Laguna residents work with wild bottlenose dolphins to catch schools of migrating silverfish called mullet. It is a locally known alliance dating back 150 years in newspaper accounts.
“This research clearly shows that both dolphins and humans pay attention to each other’s behavior and that dolphins signal when to cast the nets,” said Stephanie King, a biologist who studies dolphin communication at the University of Bristol and was not involved in the study.
“This is really incredibly cooperative behavior,” she added. “By working with the dolphins,” people catch more fish, “and the dolphins are also more successful at foraging.”
Dolphins and humans are both highly intelligent and long-lived social animals. But when it comes to fishing, they have different skills.
“The water here is very murky, so people can’t see the schools of fish. But the dolphins use sounds to find them, by emitting little clicks,” much like bats use echolocation, said Mauricio Cantor, a marine biologist at Oregon State University and a co-author of the study.
While the dolphins drive the fish to shore, the people run into the water with hand nets.
“They wait for dolphins to signal exactly where fish are — the most common signal is what the locals call ‘a jump’ or a sudden deep dive,” says Cantor, who is also affiliated with the Federal University of Santa Catarina in Florianópolis, Brazil. . .
The researchers used sonar and underwater microphones to track the positions of the dolphins and fish, while drones recorded the interactions from above and GPS devices attached to residents’ wrists recorded when they cast their nets.
The more accurately the humans synchronized their nets with the dolphins’ signals, the more likely they were to catch a big catch.
So what’s in it for the dolphins?
The descending nets startle the fish, which break up into smaller schools for dolphins to hunt more easily. “The dolphins may also pull a fish or two out of the net — sometimes fishermen can feel dolphins pulling the net a bit,” Cantor said.
Laguna residents categorize the individual dolphins as “good,” “bad” or “lazy” — based on their skill at hunting and affinity for cooperating with humans, Cantor said. People get most excited when they see a “good” dolphin approaching the shore.
“These dolphins and humans have developed a collaborative foraging culture that allows them both to do better,” said Boris Worm, a marine ecologist at Dalhousie University in Halifax, Canada, who was not involved in the study.
It’s not clear how the Laguna partnership came about, but it has survived several generations of humans and dolphins – with knowledge passed down from experienced fishermen and dolphins to the next generation of each species.
Still, the researchers in Brazil worry that the Laguna Alliance, perhaps one of the last of its kind, could also be endangered as pollution threatens dolphins and artisanal fishing gives way to industrial methods.
“The collaboration between humans and animals is disappearing because we are decimating wild populations,” said Janet Mann, a dolphin researcher at Georgetown University, who was not involved in the study.
Scientists hope that greater awareness of the unusual cooperation between species could help boost support to protect it. “It’s amazing that it’s lasted over a century — can we keep this cultural tradition alive in the midst of many changes?” said Damien Farine, a University of Zurich biologist and co-author of the study.
Follow Christina Larson on Twitter at @larsonchristina.
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