Dolphins Tell Humans When to Cast

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Quick Take

• Fishers in Brazil partner with Lahille’s bottlenose dolphins to catch mullet more efficiently.

• Dolphins herd fish and signal when to cast nets, benefiting both species.

• Collaborative fishing boosts catches by up to 4x and improves dolphin survival.

• Declining mullet populations threaten this rare human-wildlife relationship.

For over 140 years, traditional fishers in Laguna, Brazil, have hunted for fish as they work together with local Lahille’s bottlenose dolphins. In a rare case of mutualism between humans and wild animals, the dolphins and the fishers both benefit from this cooperative relationship. It’s one of only three instances that remain where humans collaborate with wild species.

The Dolphin Herders of Laguna

From May to July, mullet fish migrate north during their spawning season from shallow lagoons to colder waters along the coast of southern Brazil. The fishers catch the fish in shallow waters using large nets that they cast out into the canals. But seeing the fish through the murky water is nearly impossible. The fishers who rely on help from their dolphin guides have much better luck catching fish than those that do not.

The dolphins round the mullet towards the shore where the fishers stand with their nets. Once the fish are pushed toward the coast, the dolphins dive in a signal to the fishers. The fishers benefit by easily catching large schools of mullet, while the dolphins benefit by grabbing stray fish from the nets. Both parties get more fish by cooperating with each other than they would on their own.

The fishers pass down this human-dolphin fishing knowledge from generation to generation. Scientists wondered if, in a similar fashion, generations of dolphins were passing down the knowledge to their young.

What Scientists Discovered About the Relationship Between Humans and Dolphins

Scientists knew the fishers were watching the dolphins for signals to throw their nets. But what they were unsure of was whether or not the dolphins were acting on purpose. Did the dolphins know what they were doing? A recent study observing 65 dolphins found that over 40 were collaborating with the fishers.

Author of the study, and biologist, Mauricio Cantor, said in a statement, “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.” He continued, “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.”

Both Humans and Dolphins Benefit

The research showed that 86% of the mullet caught by the fishers were from a collaborative hunt with the dolphins. In fact, when dolphins were around, the fishers caught nearly four times as many fish as when they were absent. And they were more than 17 times likely to catch fish at all. Likewise, the dolphins that participated with the fishers had a 13% higher survival rate than individuals that did not.

About Lahille’s Bottlenose Dolphins

Lahille’s bottlenose dolphins (Tursiops truncatus gephyreus) are a subspecies of the common bottlenose dolphin. Lahille’s dolphins live to be around 50 years old, and travel together in social groups of about five individuals. They grow to be about 13 feet long and can weigh up to 880 pounds.

Scientists estimate there are around 400 to 600 individuals remaining. They only live along the Atlantic coast of South America. Lahille’s dolphins are classified as endangered and are vulnerable to climate change, pollution, and bycatch by fisheries.

Like other dolphin species, Lahille’s dolphins are incredibly intelligent. Dolphins’ brains contain specialized brain cells called spindle neurons, which are only found in humans, primates, and several large-brained mammals. Dolphins have been observed using tools, communicating with each other through vocalizations, and working together to hunt fish.

A Shortage of Mullet Fish Threatens the Collaboration

The mullet fish caught only in the summer is incredibly important to the economy of Laguna. Along with a few other species, the catches of mullet may account for up to 70% of a fisher’s annual income. Unfortunately, the mullet fish are in decline, threatening the livelihoods of the fishers and the collaborative relationship between humans and dolphins.

Human-Wildlife Cooperation Is Declining Around the World

Humans once hunted with wolves and orcas. In fact, scientists are aware of six known instances of human-wild animal mutualism. Today, there are three known cases of cooperation between humans and wild animals. In addition to the dolphin herders of Brazil, Irrawaddy dolphins and local fishers in Myanmar also hunt cooperatively for fish. The third instance is the honey hunters of sub-Saharan Africa who hunt for honey in trees by following honeyguides. At the end of the hunt, the birds get to eat the leftover wax from the honeycomb.

Cantor said, “This phenomenon of mutually beneficial interaction between wildlife and humans is getting more and more rare and seems to be at global risk. The cultural value and the biological diversity are important, and it’s important to preserve it.”

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