By Brianna Randall, published in National Geographic. Photo by Ryan Daly.
When Turawa Hakeem caught a bull shark near Lagos, Nigeria last summer, the Ghanaian captain had no idea his crew was reeling a record winner onto his wooden fishing boat.
The eight-foot-long female had made an epic journey of at least 4,500 miles, the longest known movement of its species and the first time a bull shark was documented swimming through two oceans. The shark traveled from the Mozambique Channel in the Indian Ocean, swam around the southern tip of Africa, and then voyaged north through the Atlantic to Nigeria, according to research published this month in Ecology.
“Wow, I was surprised,” says Hakeem. “I didn’t know they could travel that far.”
When his crew began butchering the shark to sell its meat at a local market, Hakeem found a black finger-length cylinder inside its body that read: ‘Research: Reward if returned.’ Curious, Hakeem emailed the address. He reached Ryan Daly, the paper’s lead author and a shark ecologist at the Oceanographic Research Institute, a marine science and service facility that leads research projects in the western Indian Ocean. He implanted the acoustic transmitter in the bull shark in South Africa in 2021.
Daly was equally shocked—and very skeptical at first. “I thought it might be a scam,” Daly admits. “The chances of this happening are like one in a million.”
This lucky catch is providing new insights into how bull sharks move and shows how climate change may break down the environmental barriers that historically limited the migration of certain ocean animals.
Daly thinks that perhaps she was an immature shark who was “just exploring”. Females don’t reach sexual maturity until they are around 20 years old. Then they repeatedly return to the same estuary to reproduce. Until then, however, they may head out to “find their groove and the pattern that works for them,” Daly says.
It’s possible that this female’s extraordinary journey “might not be unusual at all”, says Rachel Graham, a shark biologist who was not involved in this study and executive director of MarAlliance, a conservation nonprofit based off the west coast of Africa.
Bull sharks may have always traveled farther than scientists realized, or perhaps this female was the “the black sheep in the family, the one who does something completely and utterly different to keep our gene pool robust,” Graham suggests.
Despite her long journey, this female won’t pass on her genetics after befalling a common shark fate. Globally, sharks’ numbers have been halved since 1970. Overfishing drives 90 percent of the decline in sharks—but three-quarters of the estimated 100 million sharks that are caught each year are killed accidentally.
As stocks of other fish plummet globally, more people are turning to shark meat for protein—especially in countries in sub-Saharan Africa like Nigeria where people depend on fishing for their livelihoods.
“It had a one-way ticket there because fishery pressure is so extreme,” Daly says. “Sharks are running the gauntlet. In every country, they’re facing different types of threats on top of climate change.”
To ensure sharks—including future record breakers—survive, Graham says that scientists need to rely more on fishers like Hakeem to track sharks and to learn whether other marine species are making transoceanic journeys.
“Small-scale fishers are our allies in science,” Graham says. “They have PhDs of the sea.”
These sorts of novel partnerships may help scientists better understand how and where marine species are moving into new habitats.