There is a special relationship between the local net-casting fishers and a group of wild Lahille’s bottlenose dolphins (Tursiops truncatus gephyreus) that inhabit the coastal region around the city of Laguna, on the south Atlantic coast of Brazil. This association dates back generations and involves dolphins that enter the bay and help drive schools of mullet towards fishers that wait eagerly on the shore with their throw-nets. Although it is well known that the fishers benefit from this help, the reason why the dolphins behave in this way had not been scientifically tested.
However, the results of a long-term study by researchers on this unusual alliance have now been published in the Proceedings of the National Academy of Sciences. The researchers collected data on the co-operative fishing that takes place in the bay by combining drones, hydrophones and underwater cameras to capture the details of the partnership. They also conducted long-term demographic surveys of the dolphins and interviewed and observed the fishers.
The study was led by Mauricio Cantor of the Marine Mammal Institute at Oregon State University. “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 Cantor.
“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. This shows that this is a mutually beneficial interaction between the humans and the dolphins.”
The traditional, net-casting fishers and the dolphins are both after the same fish for food – shiny, fat mullet. The fishers don’t even bother to throw their nets if there are no dolphins present; instead, they simply stand in the shallows, waiting for dolphins to arrive. The fishers recognize each dolphin and refer to it by name. The dolphins drive the mullet closer to the waiting fishers and, when a group of mullet is close enough, the dolphins signal – with a slap of the tail or a specific dive – and the fishers cast their nets.
Catches of mullet in this bay are far bigger than in other areas where dolphins do not help. Furthermore, the researchers found that the dolphins benefit because, as a net hits the surface, the fish respond by turning away from it, dashing to escape it – straight into the mouths of the waiting dolphins. In this way, dolphins in this group catch more fish and shown a 13 percent increase in survival rates, when compared to populations of non-cooperative dolphins.
Synchronous fishing, between these two top predators, is thus beneficial to both parties. The practice is considered a cultural tradition in the city of Laguna, where it has occurred for more than 140 years and has been passed down through generations of fishers and dolphins. The cooperative fishing relationship is specific to this population of dolphins is not a genetic trait in the animals, but is learned, Cantor said.
“From the fishers’ perspective, this practice is part of the culture of the community in all kinds of ways,” Cantor explained. “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.”
Sadly, the practice is in decline and may well cease to exist if populations of mullet decrease, or future generations of fishers lose interest in learning the art of this unique fishing practice. There are historical and recent accounts of similar behaviors in a handful of locations elsewhere in the world, but the practice is becoming less common or has disappeared completely in most places. The rare nature of the practice is one reason it is being considered for a cultural heritage designation in Brazil, he said.
Predictive models run as part of the study show that the future of the practice could be threatened in the Laguna area as well, if fish populations decline. “The practice is unlikely to continue if either the dolphins or the fishers no longer benefit from it,” said co-author Professor Damien Farine of the University of Zurich and the Australian National University.
Professor Fábio Daura-Jorge of the Universidade Federal de Santa Catarina, Brazil said researchers are already seeing early signs of decline in the practice. “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.”
The experts suggest that conservation action is needed to ensure the future of the practice. Both the dolphins and the fishers are reliant on a strong and healthy fish population for the cooperative relationship to succeed. In recent years, the region has seen reduced availability of fish. There is also reduced interest in learning the tradition, said Daura-Jorge, who has been monitoring this population for the past 15 years.
“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.
The researchers noted that several conservation measures may be necessary to secure the future of the practice. The first is to try to identify the source of the mullet decline and take measures to better manage that species, such as reducing use of illegal nets through law enforcement, said Daura-Jorge.
Second, the researchers recommend steps to work with current and future artisanal fishers, stressing the cultural and economic importance of the net-casting practice. That might include offering incentives to encourage the traditional practice, such as setting a premium price for fish caught with this method.
“This phenomenon of mutually-beneficial interaction between wildlife and humans is getting more and more rare, and seems to be at global risk,” said Cantor. “The cultural value and the biological diversity are important, and it’s important to preserve it.”
By Alison Bosman, Earth.com Staff Writer
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