For over a decade, researchers have been studying the fishing practices of two top predators in Brazil– humans and dolphins. The two mammals work together while fishing, when the dolphins herd schools of fish towards the coast and increase the fish that are available for the net-casting fishers. The practice appears to actually be beneficial to both species.
The fishing practice is a cultural tradition in the city of Laguna located on Brazil’s southern coast. For more than 140 years, it has been passed down through generations of fishers–and generations of dolphins.
A study published January 20 in the journal Proceedings of the National Academy of Sciences (PNAS) documents in depth how the two species work together, and how it leads to increased catches. The team used hydrophones, underwater cameras, and drones to watch the interaction in real time. They then conducted long-term demographic surveys of the dolphins, and interviewed and observed the fishers to better understand the short- and long-term consequences of this practice for both mammals.
The GPS wristbands worn by the fishers in the study showed that they rapidly move into the water when the dolphins arrive and they also cast nets at a higher rate when dolphins are present. While the dolphins might be attracted by the mullets, the fisher’s are attracted to the dolphins.
According to the results, working together raises the probability of catching fish and the number of fish caught. The dolphins who engage in cooperative fishing also have a 13 percent increase in survival rates. When the dolphins are present, the fishers are also 17 times more likely to catch mullet and they also catch nearly four times more mullet.
“We knew that the fishers were observing the dolphins’ behavior to determine when to cast their nets, but we didn’t know if the dolphins were actively coordinating their behavior with the fishers,” said study co-author Mauricio Cantor, a biologist from Oregon State University’s Marine Mammal Institute, in a statement. “Using drones and underwater imaging, we could observe the behaviors of fishers and dolphins with unprecedented detail and found that they catch more fish by working in synchrony.”
On the water, synchronized movements of flocks of birds and schools of fish are common, but this kind of synchronized behavior between species, like the Lahille’s bottlenose dolphins and Brazil’s net-casting fisherman is more rare. According to Cantor, the cooperative fishing relationship is specific to this population of dolphins and not a genetic trait.
“From the fishers’ perspective, this practice is part of the culture of the community in all kinds of ways,” said Cantor. “They acquire skills passed down from other fishers and knowledge is spread through social learning. They also feel connected to this place and have a sense of belonging to the community.”
In the study, an international team of researchers from the United States, Brazil, Switzerland, and Australia ran predictive models on populations of mullet, a type of fish that both dolphins and people fish for. However, populations of these silvery fish continue to decline, which could threaten the use of the unique practice.
Early signs of a decline of the practice are already being seen by researchers. “If we take steps to document and conserve the knowledge and the culture of the practice, we can indirectly and positively impact the biological aspects, as well,” said co-author Fábio Daura-Jorge, a biologist from Universidade Federal de Santa Catarina in Brazil, in a statement.
The researchers suggest that mullet conservation efforts and even offering incentives like placing premium price on fish caught using this method are needed to ensure that this symbiotic practice continues.
“We don’t know what is going to happen in the future, but our best guess, using our best data and best models, is that if things keep going the way they are right now, there will be a time when the interaction will no longer be of interest by at least one of the predators – the dolphins or the fishers,” said Daura-Jorge.