How solving the mystery of pregnant tiger sharks in the Maldives could help save the apex predator

Within minutes the sharks, with their characteristic stripes and sharp, jagged teeth, appear from the depths of the Indian Ocean. They follow the scent of fish blood and oil coming from tuna heads a research team has hidden under a pile of rocks in a shallow sandy area.

The tiger sharks, perhaps eight or nine and up to four metres long, circle the divers, at times only an arm’s length away. All are females, two with bite marks on their flanks and fins, indicating recent mating. Some have remarkably fat bellies.

Tiger sharks, native to tropical and temperate seas, are responsible for most – though extremely rare – shark attacks on humans after the great white. A few years ago, these solitary hunters were discovered in large numbers off Fuvahmulah, a small island in the south of the Maldives that now attracts divers from all over the world . At the Tiger Zoo dive site, at the harbour entrance, encounters with more than 20 sharks are not uncommon.

• Tiger sharks circle at the Tiger Zoo dive site

Earlier this year, scientists, two local dive schools and the Maldivian shark organisation Miyaru teamed up for a two-week field study to answer the question: why is it that so many tiger sharks gather near the island? The findings could help locate what is considered to be the holy grail of shark research: the place where the sharks go to breed and life begins.

“To protect sharks, we need to know the habitats that are critical for their reproduction,” says Lennart Vossgaetter, the co-founder of Ocean Collective, a German shark research and expedition company. So far, these places are mostly unknown for migratory species such as tiger sharks, which roam the oceans over thousands of kilometres.

Work began back in 2021 when Vossgaetter, a student with a passion for sharks, came to the island. For his master’s thesis, he continued what a local dive school had started: documenting which sharks showed up on the dives using photo-IDs.

• A dive guide hides tuna heads under a pile of rocks at the site to attract the sharks

• Lennart Vossgaetter, co-founder of Ocean Collective, dives to see the sharks. ‘To protect sharks, we need to know the habitats that are critical for their reproduction,’ he says

After a year, and more than 300 dives, he had a database of 220 individuals. With the help of three other biologists carrying out identifications, this has grown to 266, “making it the largest known population of tiger sharks in the ocean”, says Vossgaetter. Surprisingly, 85% are females. He noticed that many got fatter and fatter over time – only to disappear for months and return with flat bellies: did the tiger sharks spend their pregnancy in these waters? This would make it a key region for the species’ survival in the Indian Ocean.

To confirm their suspicions, the research team enlisted the help of James Sulikowski. He co-leads the Big Fish Lab at Oregon State University and studies shark migration and reproduction.

It is Sulikowski and his team who planted the tuna heads as bait to enable them to carry out an important but delicate procedure. A shark approaches and inspects the bait, its tail pointing towards the surface. Sulikowski then reaches out with a telescopic pole and presses an ultrasound probe against the shark’s belly. After a few seconds, the shark moves on, seemingly indifferent to its examination.

• James Sulikowski and the team examine sharks for pregnancy with the underwater ultrasound device

To avoid the continuous decline of shark populations, we need to protect their critical habitats and migration routes

Lennart Vossgaetter

Back at the dive club, Sulikowksi displays an ultrasound scan of a shark named Aaya: she is indeed pregnant. The vertebrae of her embryos, their gills and pectorals are clearly visible. “The embryos measure 40 to 45cm,” he says, “Aaya carries about 40 of them.” When she gives birth after about 16 months – nobody knows for sure how long the gestation is – they will have reached 75cm.

During their study, the researchers scan 35 female tiger sharks. More than two-thirds are pregnant. Why the females visit this particular region during their pregnancy is unclear. “Perhaps because they are safe here from aggressive males,” Sulikowski says. “They could also seek out this warm region to accelerate the growth of the embryos, as their metabolism depends on the water temperature.”

Whatever the reason, it is good news for tiger sharks that one of their reproduction sites in the Indian Ocean, a hotspot of global shark mortality, falls within the protected area of the Maldives.

• Top: the island of Fuvahmulah. Clockwise: a boat at the entrance to the harbour dive site; a beach popular with locals and tourists at the northern end of the island; and fishermen selling tuna, caught by handline, at the fish market

The government of the Maldives, an archipelago of nearly 1,200 coral islands, realised that sharks are worth more alive than dead. In 2010, it was one of the first countries to declare its waters a shark sanctuary in response to a decline in dive tourists after shark populations were decimated by overfishing. In 2023, the country welcomed a record high 1.9 million tourists, with a primary attraction being the high number of sharks in its waters

In 2017, Tatiana Ivanova and her husband opened the first dive centre on the island, Fuvahmulah Dive School, and sparked a boom in the trade. Now there are about a dozen such centres. The reef-fringed island offers high chances to see big fish such as manta rays, thresher sharks and hammerheads. But its main attraction is the tiger sharks – at Tiger Zoo, sightings are guaranteed. “Ninety-five per cent of our guests come for them,” says Ivanova.

• Tatiana Ivanova and Ibrahim Siyan, owners of Fuvahmulah Dive School

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Where they mate and give birth the team can still only speculate. “Our guess is that pregnant females visit another atoll in the Maldives,” says Vossgaetter. But they could also swim to the Chagos archipelago, a marine reserve where sharks are caught illegally, or to Sri Lanka, where they are not protected at all.

“To avoid the continuous decline of shark populations, we need to protect their critical habitats and migration routes,” says Vossgaetter. He also calls for better management of existing protected areas, where increasing numbers of people are diving with sharks. Although a keen diver himself, he doubts whether shark conservation through dive tourism is a sustainable long-term solution. For now, however, he believes it is the best option as “without tourists sharks in areas like the Maldives would not be protected”.

In autumn, the researchers plan to return to Fuvahmulah to follow the progress of the tiger sharks’ pregnancies. In the next phase, they would like to tag females using a newly developed “birth alert tag”, a chicken egg-sized transmitter that is inserted into the uterus. At birth, the tag is released together with the baby sharks and transmits its position via satellite. If successful, it would help better understand the life of tiger sharks and protect this apex predator.