Dolphins and fishermen who use nets catch more fish when they work together, an international team of researchers have found.
The study, which examined fishing in Laguna, Brazil, highlighted a rare example of cooperation between two apex predators that benefited both.
Humans and dolphins working together substantially increased both the probability of catching fish as well as the number of fish caught.
The study was led by Mauricio Cantor from Oregon State University, whose team included researchers from the Australian National University (ANU), the University of Zurich and the Federal University of Santa Catarina in Brazil.
ANU biologist Damien Farine said the dolphins in Laguna had fished cooperatively with humans for the past 140 years.
While the researchers knew the fishermen were watching the dolphins' behaviour to know when to cast their nets, they did not know whether the dolphins were also observing and referring to the humans.
"On the fishers' side it's quite simple, because of course we can ask them questions," Professor Farine said.
"From our observations, we could quantify that they catch many, many more fish when the dolphins are present and when they're synchronising their behaviour with the dolphins than when they're just casting regularly.
"The really big questions were on the dolphins' side, because in Laguna the water is actually quite murky, so you can only see a few centimetres under the surface."
Cooperation increases dolphin survival rates
Unlike other cases of dolphins helping fishermen, Professor Farine said the Laguna dolphins were not rewarded with fish that they helped catch.
To understand what the dolphins got out of cooperating, the researchers used drones, underwater cameras and sonar to watch the animals interacted with the humans and what they did with the fish they herded.
They found the dolphins picked their own fish as a reward, taking them from the net after their herding was done.
It also found cooperation with fishermen was associated with a 13 per cent increase in the dolphins' survival rates.
"What makes this interaction so unusual is the fact it is mutually beneficial rather than competitive," Professor Farine said.
"This makes it 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, like the historic cases from Australia and elsewhere."
Mullets are the common factor
Professor Farine said there were several examples, contemporary and historic, of dolphins and humans fishing together, in India, Myanmar, the west coast of Africa, and in Australia with some First Nations people.
"These records are quite anecdotal. This case in Brazil is the first that has been explained in as much detail as we have, but they do all have something in common … they all involve catching mullet," he said.
Mullet migrate in very large schools and are a large fish, weighing up to 4 kilograms. This makes them a trickier fishing target but provides a better pay-off when successful.
"They're quite difficult to catch because they're in these big schools, but if they do manage to find a way to catch them there's plenty of food available for everyone involved," Professor Farine said.
"It means when they do make a catch there's not much competition among individuals for access to the resource because there is so much.
"One of the key factors … is that there is a superabundance of food, so there's enough for everybody to share, in this case dolphins and humans."
The professor said the other factor consistent across historical cases of cooperation was the underwater landscape.
"All these cases – in Myanmar, in the Irrawaddy river in India, in Brazil, and historical cases here in Australia — are in places where there's a deep channel that goes very shallow very quickly," he said.
"The mullets normally move through the middle in the deep part of the channel, which is too far for the fishers to reach, but the dolphins can come out and can actually herd the fish towards the edge of the channel to make it reachable by the fisher.
"The fishers catch the fish and then this makes it easier for the dolphins to get a feed."
The study's findings were published in the Proceedings of the National Academy of Sciences.
