Some dolphins help humans catch fish – but perhaps not for much longer. An uptick in the levels of noise pollution from nearby ships is changing the way the dolphins communicate. It is possible this may reduce the dolphins’ ability to coordinate their behaviour to their – and the humans – advantage.
Near the city of Laguna in southern Brazil, a group of Lahille’s bottlenose dolphins (Tursiops truncatus gephyreus) work together to drive schools of fish like mullet into the shallow water where fishers cast handheld nets, catching fish and breaking up the schools. While the water is murky, the dolphins are believed to take advantage of the chaos, snatching up disorientated fish missed by the nets.
The relationship is at least a century old, and depends on the dolphins communicating between themselves to coordinate the group’s behaviour. But this delicate rapport might be imperilled by the increasing noise made by ships that pass nearby.
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Bianca Romeu at the Federal University of Santa Catarina, Brazil, and her colleagues analysed the whistles made by the dolphins with and without noisy boats passing. They also counted how many whistles the dolphins made at these times.
They found that, during noisier periods, the dolphins produced fewer whistles and increased their pitch. In other words, the dolphins were communicating differently when boats were around, which could affect the way they get organised to forage with the fishers, says Romeu.
“Probably the noise isn’t interrupting the activities, but it can disturb them, can change their behaviour,” says Romeu. She adds that it may make the dolphins worse at working together to drive the fish towards the fishers.
Aside from meaning less food for the dolphins, this could also lower the fishers’ catches, possibly affecting their bottom line. It also threatens the future of a relationship central to the cultural identity of Laguna. The fishers know the dolphins well enough that many of the marine mammals have been given names. “These dolphins have economic and cultural importance,” says Romeu.
The behaviour is presumably important for the dolphins as well. There is one other place in south Brazil where humans cooperate with dolphins, but there are far fewer of the mammals there – only nine compared with the roughly 20 that cooperate with fishers in Laguna.
“It is a small population, genetically isolated, and it is sensitive to changes and disturbances in environment,” says Romeu, adding that there are only about 60 individuals in the population, including the 20 that cooperate with humans at Laguna.
The problem isn’t easily solved due to the local marine landscape. Boats travelling to and from Laguna must pass through a channel where the dolphins and fishers work together. But Romeu says that the noise may be reduced by implementing rules to reduce speed, or by limiting the number of boats that can pass through at any one time.
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