How did this shark swim a record-breaking 4,000 miles—a journey once thought impossible
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.
Another study author and marine biologist at the Nigerian Institute for Oceanography and Marine Research, Dunsin Abimbola Bolaji, confirmed Hakeem's story.
In the year after she was tagged, the female bull shark was detected 567 times along the east coasts of South Africa and Mozambique by an array of 43 different underwater receivers.
Then she disappeared on March 25, 2022 and wasn't seen again until Hakeem's crew caught the shark on July 11 last year.
As part of their shark migration research, Daly and his colleagues also tagged and tracked 102 bull, blacktip, tiger and reef sharks in southern Africa. The longest recorded migration among these sharks was 1,400 miles, just one-third the distance traveled by the female bull shark that ended up near Lagos.
Bull sharks are coastal species, not known for long-distance travel in the open ocean. They prefer shallow waters where freshwater meets the sea and need water temperature warmer than 65°F.
During her voyage north, the female bull shark had to navigate the Benguela upwelling, one of the world's largest cold-water currents that extends along the west coasts of South Africa and Namibia. This upwelling has formed a cold barrier separating Africa's bull shark populations for at least the past 55,000 years.
Scientists think this bull shark bypassed the cold water by swimming out around the upwelling, which can extend up to 90 miles offshore. It's also possible she rode pockets of warmer water around South Africa into the Atlantic Ocean during a Benguela Niño event.
This climate pattern is similar to the El Niño events that influence sea temperatures off the west coast of the Americas. Certain cold-water fish, like mackerel and sardines, have also been pushed north during Benguela Niño events.
As waters warm and upwellings shift due to climate change, Daly says the Benguela's cold water barrier may break down more often, allowing ocean animals to move to different latitudes. These Niño-related water temperature changes can change the entire species makeup of certain marine areas, impacting everything in the food web from algae to plankton to sharks.
For bull sharks, however, more movement is likely a positive sign. 'If it means more gene flow, then typically that's a good thing,' Daly points out. 'We need to adapt to survive in a changing world.'
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.'
Hakeem says his crew didn't hook the tagged female bull shark on purpose. She took the bait meant for more lucrative grouper and snapper.
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.
Warming water may allow tropical species to expand their range polewards, which can relieve fishing pressure or allow them to spread to new homes. But simultaneously, climate change is also creating more intense cold events in their historic ranges, such as an extreme upwelling along the southeast coast of South Africa that killed individuals from 81 species in 2021, including sharks.
'It's kind of like this bait and switch,' Daly says. 'It gets warmer but then these intense upwelling events increase, so they might get trapped down there, at the end of their range for a tropical species and then die off.'
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