In southeastern Brazil, local fishers wade into murky waters in search of migrating mullet. On their own, it would be tricky to find the silvery fish. But the humans get help from an unusual ally: wild bottlenose dolphins.
With nets in hand, the fishers patiently wait as their cetacean partners drive the fish toward the shore. A cue from the dolphins—usually a deep dive—indicates when they should cast their nets.
This fishing partnership has passed down through the generations, enduring for more than a century. While researchers knew humans profited from this pairing, they couldn’t confirm it benefited the dolphins, too. Now, in a new study published Monday in Proceedings of the National Academy of Sciences, researchers suggest cetaceans that hunt with humans have increased survival rates over those that don’t.
“Human-wildlife cooperation in general is a rare phenomenon at a global scale,” Mauricio Cantor, a biologist at Oregon State University and an author of the paper, tells the New York Times’ Asher Elbein. “Usually humans gain the benefit, and nature pays the cost. But this interaction has been happening for over 150 years.”
Using drones, sonar and underwater sound recording, the research team recorded the interspecies fishing team in action. They found that when the fishers cast their nets in sync with the dolphins’ cues, the cetaceans increased their echolocation click rates to create a “terminal buzz”—a sign of them homing in on prey. When the fishers were not in sync with the dolphins, this response was less common.
The dolphins took advantage of disoriented fish after the fishers cast, even snatching some directly from the nets.
Fishers were also more successful when they worked with the dolphins. When dolphins were present, the fishers were 17 times more likely to catch fish and netted nearly four times more mullets when they timed their casting with the cetaceans’ signals. Eighty-six percent of all 4,955 mullets caught during the study period came from “synchronous interactions”—when both predators coordinated their actions perfectly with one another.
“Dolphins benefit fishers by herding mullet schools towards them, creating temporary high-quality patches just before giving a cue, and signaling when prey are within reach of fishers’ nets,” write the authors. “During interviews with the most experienced fishers, 98 percent reported that dolphins gain foraging benefits from synchronous interactions.”
The study also revealed that dolphins hunting with humans had a 13 percent increase in survival rate over other dolphins. These cooperative dolphins are more likely to stay near the shore, reducing their chance of entanglement in illegal fishing gear.
“This study clearly shows that both dolphins and humans are paying attention to each other’s behavior, and that dolphins provide a cue to when the nets should be cast,” Stephanie King, a biologist who studies dolphin communication at the University of Bristol in England and was not involved in the research, tells Christina Larson of the Associated Press. “This is really incredible cooperative behavior.”
But researchers say this partnership is in danger. Changing sea surface temperatures are leading to fewer mullets and affecting where they go, writes Andrew Jeong for the Washington Post. “More often than not, [sea temperatures] are rising, making it less likely for the mullet to come inshore,” says Damien Farine, a co-author and ecologist at Australian National University, to the Post. Additionally, the dolphins are threatened by pollution, and commercial fishing operations overharvest the mullet.
Still, the dolphin-human relationship could be protected if Brazil declares it a cultural heritage, reports Virginia Morell for Science.
“It’s an interaction that goes beyond only the material benefits,” Cantor tells the Times. “Trying to preserve cultural diversity is an indirect way of preserving biological diversity, too.”