A global conservation group called ReShark says it has launched what it describes as the world's first shark rewilding programme, releasing captive-bred leopard shark pups into protected waters near Kri Island in Raja Ampat, Indonesia, and in Phuket, Thailand.
The Indo-Pacific leopard shark, also known as the zebra shark, has historically lived in reefs across the western Indo-Pacific Ocean, the Red Sea, East Africa, Japan and Australia. Years of habitat destruction and overfishing have cut its population by 50 to 79 percent over the last three generations, roughly 57 to 72 years, and the species is classified as endangered by the International Union for Conservation of Nature (IUCN).
While the shark's historical decline was driven by the shark fin trade, pressure from overfishing continues through bycatch and trade in its exotic skin. In Raja Ampat, the sharks had nearly vanished less than a decade ago. The IUCN Shark Specialist Group estimated that, left alone, the archipelago's population could take between 100 and 200 years to recover naturally.
How the ReShark project works
The initiative, known as the Stegostoma tigrinum Augmentation and Recovery project, or StAR, after the shark's scientific name, involves leopard sharks bred in aquariums around the world, whose pups are later released into protected waters in Indonesia and Thailand.
Mark Erdmann, executive director of ReShark, said that in 2019 in Raja Ampat, the group estimated only about 20 adult sharks remained. He said most shark conservation work typically centres on establishing marine protected areas or fisheries management, but ReShark's niche is active recovery.
The idea first came to Erdmann during a visit to the SEA Aquarium in Singapore in 2015, where staff showed him tanks full of leopard shark pups that had been successfully bred in captivity. He recalled that the aquarium team told him they were producing too many pups and did not know what to do with them.
That encounter set off an eight-year effort involving governments, aquariums, non-profit organisations, academics, local communities and conservation scientists, who worked together to release captive-bred pups into Indonesian waters.
Why leopard sharks were chosen
Like many sharks, Indo-Pacific leopard sharks are harmless to humans. But unlike most sharks, they reproduce by laying eggs, and they are in fact the largest of the egg-laying sharks, a trait that makes them well suited to this rewilding effort.
David Ebert, programme director of the Pacific Shark Research Center at San Jose State University, who is not affiliated with ReShark, said about 43 percent of sharks lay eggs. He said live-bearing species come with a completely different set of challenges, making egg-laying sharks a good species to start with.
Ebert's global efforts to track down elusive sharks have earned him the online nickname "Lost Shark Guy." He said the leopard shark has another advantage: what he called its "cute factor," which can help win public support for conservation efforts.
Aquarium partners and 'shark nannies'
ReShark, which is backed by Rolex's Perpetual Planet Initiative, has partnered with nearly 60 aquariums around the world. About a dozen of them, in the United States, Australia, Hong Kong and Singapore, are actively breeding sharks for the programme, according to Erdmann.
Viable eggs from aquariums around the world are transported to ReShark's two nurseries, in Raja Ampat and in Phuket, Thailand. There, specially trained aquarium managers, nicknamed "shark nannies," monitor the eggs, care for the pups and prepare them for life in the wild. Once the sharks reach one metre in length and can forage for food on their own, they are tagged and released into protected waters.
The nickname may sound playful, but the work is highly technical. Shark nannies undergo months of training in animal husbandry, often with support from major public aquariums. An internship programme hires local residents rather than international volunteers, to involve local communities in the effort.
Erdmann said it was very important that people in Raja Ampat and in Phuket supported the programme, so that the sharks did not go back to being caught and killed as they were 10 or 15 years ago.
Experts remain cautious
Some shark experts remain cautious about the programme's long-term effectiveness. Ebert, who has spent more than four decades studying sharks and rays around the world, described the programme as innovative but still largely experimental.
He called it an interesting kind of science project, but questioned how much it would actually impact the shark population, calling that the big question.
Among his concerns is whether the released sharks will survive long enough to reproduce in the wild. Because leopard sharks are believed to take between five and 12 years to reach maturity, the earliest ReShark will know whether its first released pups are reproducing is 2028.
He also questioned whether releasing more sharks while fishing pressure still exists could unintentionally make them easier targets, noting that sharks released in a marine protected area will still swim wherever they want, raising the question of whether the programme is simply increasing the population for people to catch.
What comes next
Erdmann said ReShark only operates in sanctuaries committed to conservation. Raja Ampat, for example, is a 46,000 square kilometre (17,760 square mile) shark and manta ray sanctuary.
ReShark's own satellite data on released pups shows that most stay, on average, within 100 to 150 kilometres (62 to 99 miles) of the protected waters where they were released. By engaging local communities in the rewilding effort, fishermen have begun actively protecting leopard sharks, according to the group.
Combined with efforts to secure legal protections across Indonesia and Thailand, Erdmann said he remains confident about the sharks' survival. With 82 pups released since 2023 and 15 healthy sightings recorded in Raja Ampat this year, he said early results suggest the strategy is working.
