Dolphins and Humans Partner to Catch Fish

Source: Courtesy of Bianca Romeu, Universidade Federal de Santa Catarina.

For generations, bottlenose dolphins have cooperated with artisanal net-casting fishers in Laguna, Brazil, to catch migratory fish called mullet. The dolphins herd groups of fish toward shore, where the fishermen are waiting with nets. The fishers wait for a signal from the dolphins that the fish are close, like a tail slap against the water. At the sign, the fishers cast their nets and fill them with mullet. This also breaks up the schools of mullet and makes it easier for the dolphins to catch individual fish.

This unusual partnership has been going on for more than 100 years, and scientists have been studying it since the 1980s. In a new study, scientists investigated the mechanisms that allow this mutually beneficial relationship to exist and asked whether it can persist in the face of current and predicted environmental and cultural changes.

Synchrony Creates Benefits for Fishers and Dolphins

Mauricio Cantor, now at Oregon State University, and colleagues Damien Farine and Fábio Daura-Jorge, used GPS, drones, sonar, underwater microphones, and surveys to track the interactions among fishers, dolphins, and mullet.

“We combined about seven different types of data to record interactions simultaneously above and below the water,” says Cantor. “That fine-scale data gave us new insights on how the interaction works.”

The research team found that dolphins and fishers benefit the most when they modify each other’s behavior and coordinate their actions. Fishers were 17 times more successful when they timed their net casting to match the behavior of dolphins, which, in turn, timed their dives and echolocation to match the fishers’ net casting.

Source: Courtesy of Fábio Daura-Jorge, Universidade Federal de Santa Catarina.

Interviews with the fishers over the last 15 years revealed that, in addition to the economic benefits, fishing with dolphins creates a sense of belonging and cultural identity.

And the dolphins profit, too. From Daura-Jorge’s work surveying the dolphin population, the researchers learned that dolphins that interact with fishers are about 13 percent more likely to survive to adulthood, compared with dolphins that do not partake in cooperative fishing.

An Interspecies Relationship in Danger

Despite benefits for both dolphins and humans, cooperative fishing is in decline. Cantor and colleagues combined their data with a mathematical model to project what might happen in different future scenarios.

“Our model suggests that if things continue changing the way they are right now, it is possible that in the next four to five decades, this tradition could go extinct,” says Cantor. “It is a bit alarming, but it helps us to start thinking about what we can do now to avoid that and safeguard this interaction.”

Cantor says conserving cooperative fishing is challenging because of the number of moving components in the system. First, an abundant and stable population of mullet is required. However, mullet are locally overexploited by industrial fisheries. And factors like warming water temperatures could change the migration patterns of the fish, preventing them from entering the lagoons anymore.

The population of cooperative dolphins must also be protected. Currently, a major cause of dolphin mortality in the area is the illegal use of trammel netting by other fisheries.

Finally, Cantor says that the interspecific knowledge that enables this interaction must be maintained. The dolphins and humans need a mutual understanding of the signals in order to fish together.

Source: Courtesy of Fábio Daura-Jorge, Universidade Federal de Santa Catarina.

Conserving Cooperation

The researchers also modeled how different conservation actions might support this special relationship. For initial steps, they suggest working with local fishers to help prevent the extinction of this important cultural practice. Their model shows that by both enforcing the removal of illegal fishing gear that causes dolphin bycatch and incentivizing fishers to work together with dolphins (for instance, by putting a premium price on fish caught this way), cooperative foraging may persist into the future.

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Cantor says that one reason it is important to conserve this relationship is its importance as a source of identity and pride for the people of Laguna. Just as there is biological diversity associated with different places in the world, there are cultural traditions specific to each place, as well. Cantor hopes that this study shows the significance of conserving both biological and cultural diversity for future generations.

Additionally, positive interactions between humans and the natural world are relatively rare.

“Most of those interactions tend to be one-sided,” Cantor says. “They tend to be beneficial for humans, but not so much for the animals and the environment around them.

“This human–wildlife interaction, which is mutually beneficial for both parties, is inspiring.”