The fishers wade through waist-deep seawater near the beaches of Laguna, Brazil, patiently watching for bottlenose dolphins to appear and send cues, which indicate where the mullets are. Those cues involve the dolphins making arch-shaped flips, quickly jumping out and back into the water. Upon the cues, the fishers cast their nets.

In return, dolphins earn an easy lunch, by gulping down some of the mullets that escape the nets. The dolphins that partake in this ritual have a 13 percent higher chance of survival than the dolphins that do not, according to a new study done by an international team of researchers and published in the Proceedings of the National Academy of Sciences on Monday.

For the research, scientists used drones and underwater sound recordings to document how the practice is carried out.

Dolphins in Brazil act like sheepdogs, helping fishermen catch mullets

But this tradition, a rare example of a mutually beneficial relationship between humans and nondomesticated animals, may soon go extinct due to declines in mullet populations triggered by climate change and excessive commercial fishing, according to the peer-reviewed study.

That, in turn, could cause significant drops in local dolphin populations, researchers said.

“A subpopulation of 60 dolphins could easily drop to a dozen or more in a few years, in a worst-case scenario,” Damien Farine, a professor of ecology at Australian National University and one of the three researchers who co-wrote the study, said in a phone interview.

Dolphins that participate in this unusual symbiosis are more likely to enjoy longer lives, partly because they don’t have to encounter dangerous fishing gear, where they can become the bycatch of often illegal, commercial fishing methods like drift or gillnetting, which can entangle dolphins and drown them.

Meanwhile, changing sea surface temperatures are not only resulting in fewer mullets; they are also driving schools of mullets to different parts of the shore. This means that the number of mullets available for local fishers to catch is also on its way down, Farine said.

Although scientists cannot agree on figures that show how fast seawater temperatures are changing, “more often than not they are rising, making it less likely for the mullet to come inshore,” Farine said.

That means fishers are catching fewer fish, and the dolphins are receiving smaller and fewer lunches.

The practice has worked for generations because it benefits both humans and dolphins, Farine said. Fishers can catch up to a half-ton of mullets in a single good netting, while the dolphins only need three or four mullets, which can grow to nine pounds or more, to be full.

“What makes this interaction so unusual is the fact it is mutually beneficial rather than competitive,” Farine said in a statement.

“This makes it of substantial scientific interest, as it can help us to understand under what conditions cooperation can evolve,” he said, “and under what conditions it might go extinct.”