Researchers say dolphins and humans both benefit from working together to catch fish. 

Accounts of people and dolphins working together to hunt fish go back millennia, from the time of the Roman Empire up to 19th century Queensland, Australia. 

Historians and storytellers have detailed the collaboration from the human point of view, but until recently, it has been impossible to confirm if the dolphins benefit too.

But now, using drones, underwater sound recordings and other tools, researchers have documented how local people and dolphins coordinate actions and benefit from each other’s labour. 

They found that the most successful humans and dolphins are skilled at reading each other’s body language. 

In the seaside city of Laguna, southern Brazil, residents have worked with wild bottlenose dolphins for over a century to catch schools of migratory mullet.

The two species - humans and dolphins - collaborate by combining their strengths. 

“The water is really murky here, so the people can’t see the schools of fish. But the dolphins use sounds to find them, by emitting small clicks,” much as bats use echolocation, says Mauricio Cantor, an Oregon State University marine biologist and study co-author.

The dolphins herd the fish toward the coast, and people run into the water holding hand nets.

“They wait for dolphins to signal exactly where fish are – the most common signal is what locals call ‘a jump,’ or a sudden deep dive,” says Dr Cantor.

Sonar and underwater microphones were used 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 closely the people synchronised their net-casting to the dolphins’ signals, the more likely they were to trap a large catch.

From the dolphins perspective, the descending nets startle the fish, which break into smaller schools that are easier for them to hunt. 

“The dolphins may also take one or two fish from the net – sometimes fishers can feel dolphin tugging a little on the net,” said Dr Cantor.

The Laguna residents are familiar enough to categorise the individual dolphins as “good,” “bad,” or “lazy” — based on their skill in hunting and affinity for cooperating with humans.

The researchers observed people getting most excited when they saw a “good” dolphin approaching shore.

There is no clear evidence of how the Laguna cooperation first emerged, but it has survived multiple human and dolphin generations – with knowledge passed down by experienced fishers and dolphins to the next generation of each species.

The scientists hope that greater awareness of the unusual interspecies cooperation can help drive support to protect it, as pollution threatens the dolphins and artisanal fishing gives way to industrial methods.

The research was published in the journal Proceedings of the National Academy of Sciences.