By the shoreline of Laguna in southern Brazil, there is a cemetery with a difference: each cross marks the life of a dolphin that helped the local fishing community by driving mullet towards their nets.
The unusual partnership between bottlenose dolphins (Tursiops truncatus) and humans in this area has existed for more than a century. Now, a long-term study has shown that the dolphins involved have a higher rate of survival than those that don’t work with people.
While dolphins that practise the technique seem to catch more fish, some of the survival benefit happens because the dolphins avoid being caught in trammel nets used for fishing in other areas by the coast, says Mauricio Cantor at Oregon State University, whose team has been studying the behaviour with underwater cameras and microphones for 12 years.
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Traditional cooperative fishing between bottlenose dolphins and artisanal net-casting fishers in Laguna, Brazil Mauricio Cantor, Oregon State University.
Between May and August, dolphins drive migrating schools of mullet (Mugil liza) towards the coast, where fishers are waiting. When the dolphins do a final deep dive to drive the fish closer to shore, fishers drop their nets – but they have to act at exactly the right time or lose their chance.
The fishers watch for the dolphins arching their backs, which reveals they are about to dive deeply. Less often, the animals may seem to signal to the humans by slapping the water with their heads or tails. Both fishers and dolphins need to learn the right techniques to work together, says Cantor. “It’s tricky [to see] for the untrained eye,” he says.
If the fishers drop their nets on the dolphins’ cue, they are 17 times more likely to catch mullet than if they do so at other times, Cantor’s team found.
It is harder to work out how much more successful at catching fish the partnership makes the dolphins, says Cantor. But the locals report that the dolphins often dive into their nets to remove some fish. The nets could also aid the dolphins by forming a barrier that stops the mullet fleeing, he says.
About 10 dolphins work with humans out of a group of about 60. The team calculated that dolphins that worked with people were 13 per cent more likely to survive through the study period than ones that didn’t.
The other dolphins may have to catch fish without human help because there are only a few places near this coast where the technique is possible, says Cantor. But the number of cooperating dolphins used to be higher – between 12 and 18 – perhaps because of recent declines in mullet numbers.
The fishers can recognise individual dolphins by their dorsal fins, which have unique markings, and their behaviour. Last year, the community mourned when a particularly skilful dolphin they called Caroba died, probably from an infection. She was thought to be about 60 years old.
Bruno Díaz López at the Bottlenose Dolphin Research Institute in Galicia, Spain, says dolphins are one of the few animals that have unique cultures, where behaviours are shown by only certain groups and are shared between individuals. “They need to teach each other this,” he says.
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