Humans and marine mammals are by no means always competitors when it comes to fish — they can also work closely together to their mutual benefit, scientists have found. Accordingly, there is such a symbiosis between fishermen in southern Brazil and the dolphins living there. The marine mammals help people catch mullets (Mugiliza) by herding schools of fish towards fishermen and signaling when to cast their nets. This help increases the fishing success of the fishermen considerably, but also increases the survival rate of the dolphins, as evaluations have shown. However, this rare example of cooperative collaboration between two species is under threat.
Whether in swarms of birds, hunting prides of lions or foraging ants: in many animal species, the survival and success of the species depends on the close cooperation of the individual individuals. When the animals work together to catch prey or search for food and suitable habitats, they benefit, but so do their fellow animals. But there are also forms of cooperation and symbiosis between different species, such as in hermit crabs and sea anemones or corals and their algae. However, such cross-species cooperation is much rarer among predators that are at the top of their food webs. Their relationship is mostly characterized by competition for prey – this also applies to us humans and our relationship to other large carnivores in the animal kingdom.
Man and dolphin fish together
But there are exceptions. Because under certain circumstances cooperation has also developed between humans and animals competing for the same food. Biologists led by Mauricio Cantor from the Max Planck Institute for Animal Behavior in Radolfzell and the Marine Mammal Institute at Oregon State University have now examined one such example in more detail. On the coast of Laguna in southern Brazil, net fishermen have been practicing close cooperation with the bottlenose dolphins (Tursiops truncatus) that live there for more than 140 years. Both mainly catch mullet (Mugiliza), a species of fish that is common in tropical and subtropical waters. The fishermen stand in the shallow water waiting for the dolphins to surface. “Fishermen have been known to monitor dolphin behavior to determine when to cast their nets,” Cantor explains. This usually gives the fishermen a more plentiful catch.
However, it was previously unclear how humans and marine mammals synchronize in this cooperation, whether the dolphins also adapt their behavior to the fishermen and whether they also benefit from the cooperation. To find out, Cantor and his team studied the behavior of fishermen and bottlenose dolphins over 15 years. They observed and questioned the fishermen and tracked the behavior and reactions of the dolphins with the help of drones, microphones and underwater recordings. Additionally, they used sonar equipment to monitor the mullets as well. In this way, the team was able to unravel the details of this unusual human-dolphin collaboration.
Dolphins show the fishermen where to cast their nets. (Video: Max Planck Society)
Active cooperation from both sides
The evaluations showed: While the fishermen only go into the water when they see the dolphins, the dolphins are primarily guided by the presence of the mullets. This shows that it is no coincidence that both species are in the water together: people use the dolphins as an indicator that there is worthwhile prey in the water. But in the next step, the dolphins also adapt their behavior to the presence of the fishermen: They drive the schools of mullets towards the fishermen standing in the water and come within about twelve meters of the people, as Cantor and his team determined. This increases the density in the school of fish and causes unrest among the mullets. Now the dolphins give the signal to the fishermen to throw out the nets – usually by abruptly diving down. “We didn’t know that the dolphins were actively coordinating their behavior with the fishermen,” says Cantor.
But what advantages do marine mammals have from this cooperative behavior? The balance for the fishermen is clear: In cooperation with the dolphins, they catch on average almost four times more mullet than alone. But the marine mammals also benefit, as Cantor and his team found out. When the dolphins hunt alongside the fishermen, their survival rate increases by 13 percent compared to uncooperative fellow dolphins. In addition, the observations suggest that the dolphins benefit from the confusion of the mullets trapped in front of the nets during their hunt. “Our footage also reveals that the dolphins are also catching some netted mullets,” the team said. The fishermen also reported something similar: 61 percent of them stated that they sometimes feel that the bottlenose dolphins take a mullet or two from the net. “So the collaboration is mutually beneficial,” Cantor said.
As such, this cooperation represents a rare example of mutually beneficial collaboration between two species at the top of the food chain. “That’s what makes this behavior so interesting. It teaches us under which conditions cooperation can develop and under which it could die out or switch from a cooperative to a competitive relationship,” says co-author Damien Farine, who conducted the research during his time at the Max Planck Institute of Animal Behavior and now works at the University of Zurich and the Australian National University. However, the traditional cooperation between net fishermen and dolphins in Brazil is on the wane. Because the stocks of mullet in this area have continued to decline in recent years, interest in this traditional collaboration is declining. “Such mutually beneficial collaborations between wildlife and humans are becoming rare and endangered globally,” Cantor said. “Their cultural value and the biodiversity on which it is based are priceless and must be preserved.”
Source: Mauricio Cantor (Max Planck Institute for Animal Behavior, Radolfzell / Oregon State University) et al., Proceedings of the National Academy of Sciences, doi: 10.1073/pnas.2207739120