Off the southern coast of Brazil, humans and animals interact in an unusual way: fishermen and dolphins have been fishing together here for more than a century. Biologists have been studying the unique hunting community for 15 years and are now reporting on their observations in the specialist journal "PNAS". However, according to model calculations, the mutually beneficial cooperation could soon be a thing of the past.
A special cultural fishing tradition has been cultivated off Laguna, a city in southern Brazil, for more than 140 years: Here people and bottlenose dolphins (Tursiops truncatus gephyreus) hunt together for mullet (Mugiliza). Both the fishermen and the dolphins pass the practice on from generation to generation.
A video published for the study by the researchers led by Mauricio Cantor from the Universidade Federal de Santa Catarina in Florianópolis shows how the cooperation works: The marine mammals drive schools of fish towards the fishermen who are waiting in the water with cast nets. Then they give them a signal - usually a distinctive flapping of their fins or by appearing briefly. This is where the fishermen cast their nets.
"Using drones and underwater footage, we were able to observe the behavior of fishermen and dolphins in unprecedented detail and found that they catch more fish when they work in sync," Cantor said in a statement from the Max Planck Institute (MPI) for quoted behavioral biology. Cooperation is a special feature of this dolphin population.
The study shows that both sides win: when bottlenose dolphins are present, fishermen are 17 times more likely to catch mullets. In addition, they would catch almost four times more fish than without animal helpers.
Hunting together also offers advantages for the dolphins, as fishermen report: "61 percent stated that they feel when dolphins pull 'a mullet or two' out of their nets," it says. The animals also benefit in the long term: according to the study, dolphins that fish cooperatively in this area have a 13 percent higher survival rate. This is probably due to the fact that, by working together with the net fishermen, they avoid becoming bycatch with other fishing methods.
The scientists emphasize that cross-species cooperation, including between humans and animals, is characterized more by competition than by mutual benefit. But things are different here, emphasizes co-author Damien Farine: The system can help to understand “under what conditions cooperation can develop and – which is becoming increasingly important in our rapidly changing world – under what conditions it dies out or from a cooperative one could turn into a competitive interaction".
In fact, model calculations suggest that this type of fishing could be threatened: both dolphins and fishermen depend on a strong and healthy fish population. But in recent years, the number of mullets in the region has declined. "Interest in learning the tradition has also fallen," says co-author Fábio Daura-Jorge. "Our calculations show that if current developments continue, the cooperation could become uninteresting for either the dolphins or the fishermen."
The biologists propose several steps to preserve the common hunt of fishermen and dolphins. In this way, the causes of the decline in mullet numbers should be determined and measures taken to ensure sustainable fishing. In addition, traditional practice could be encouraged by setting a higher price for fish caught using this method.
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