When Your Old Fishing Buddy Has a Snout and a Blowhole

Every summer in waters off the town of Laguna in Brazil’s southeast corner, fishers wade out into estuary canals to cast their nets in hopes of catching migratory mullet fish. The water is murky, and the fish are difficult to spot. However, the fishers have help from an unexpected quarter: bottlenose dolphins that push the prey toward the nets.

The two species of predators have coordinated their fishing for generations.

“The experience of fishing with dolphins is unique,” said Wilson F. Dos Santos, a Laguna fisherman who’s been casting nets alongside the dolphins for 50 years. He learned the practice when he was 15, taking his father coffee and food as he fished and watching the aquatic partners at work. He added that working with the dolphins “helps with our family income,” because human families eat what they catch.

In research published Monday in the Proceedings of the National Academy of Science, a team of Brazilian scientists reported that dolphins might benefit from the cooperation as much as their terrestrial mammal counterparts did. Those that fish with humans seem to live longer than other dolphins in the area do.

“Human-wildlife cooperation in general is a rare phenomenon at a global scale,” said Mauricio Cantor, a biologist at Oregon State University and an author of the paper. “Usually humans gain the benefit, and nature pays the cost. But this interaction has been happening for over 150 years.”

Humans have worked with other species to find food for millenniums, including honeyguides in southeast Africa and Indigenous American accounts of hunting alongside wolves. And net-casting fishers working with dolphins isn’t unique to Brazil — the practice has also been observed in Mauritania, Myanmar and India, but the bottlenoses of Laguna are the most famous. The local population of about five dozen cooperative dolphins has been systematically monitored since 2007, said Fábio G. Daura-Jorge, a biologist at the Federal University of Santa Catarina in Brazil and an author of the paper. In 2017, the team began surveilling both the fish and dolphins with GPS, drones and sonar.

The team found that the dolphins gave a cue — usually a sudden, deep dive — to signal that they’d driven prey within reach of fishers’ nets. Eighty-six percent of the successful catches during the study period came from fishers reading dolphins’ behaviors, Dr. Cantor said. Careful observation and timing were key: Those who cast nets too late or missed dolphin cues were less likely to catch anything.

The dolphins also time their foraging carefully. The team of researchers used hydrophones to measure the animals’ echolocation clicks, the rate of which increased as the nets hit the water. When a fisher made a successful cast, the dolphins homed in on disoriented mullet or plucked a few fish from inside the net. When fishers mistimed their cast or failed to respond to the dolphins’ cues, the dolphins didn’t strike.

“The dolphins know what they are doing,” Dr. Daura-Jorge said. “They’re taking advantage of the fishers’ actions to actively forage.”

The human participants proved to be keen observers, too. The fishers shared a wealth of experience with the researchers about how the dolphins and fish behaved, and they knew how to recognize the dolphins that made good fishing partners. The fishers identified their partners’clicking and told the researchers that they felt the buzz of the echolocation in their legs when the dolphins “cry out.”

The strategy has clear benefits for the dolphins, Dr. Cantor said: Those that hunted with humans were 13 percent more likely to survive to adulthood. Cooperative dolphins tended to linger near the fishing grounds that they shared with humans, while other dolphins that traveled more widely throughout the area’s water were three times more likely to end up tangled in illegal fish nets.

While this partnership helps both sides, the practice has been in decline over the last decade, Dr. Cantor said. Commercial operations overfish mullet stocks across southern Brazil. As fish numbers drop, individual dolphins and fishers hunt together less.

Because the whole system depends on both sides having a careful understanding of each other’s cues, Dr. Daura-Jorge said, it can easily break down. That could make Laguna’s bottlenose dolphins appear to be more competitors than partners, placing them under further threat.

“Safeguarding the cultural tradition of fishers and dolphins is critical to keep them cooperating and also important to conserve the population of dolphins,” he said.

Regulation of industrial mullet fishing and a crackdown on illegal fisheries could help to ensure that there are enough fish to go around, Dr. Cantor said. Many area participants like Mr. Dos Santos are also committed to the practice, which is a point of local pride and draws in tourists.

“It’s an interaction that goes beyond only the material benefits,” Dr. Cantor said. “Trying to preserve cultural diversity is an indirect way of preserving biological diversity, too.”