Technology and tradition team up to watch over a sacred island’s sea turtles

  • Uninhabited Poilão Island in Guinea-Bissau is Africa’s most important nesting ground for green sea turtles (Chelonia mydas).
  • Local people have long regarded the island as sacred and have limited access to it, which has kept its environment in a near-pristine state.
  • The island is now protected as part of a marine national park, a compromise in which local people granted researchers and some other outsiders access to the island in an exchange that included employment monitoring turtles.
  • Researchers and local people now work together to study the turtles with the aim of better protecting them.

POILÃO ISLAND, Guinea-Bissau — As night falls, the only lights visible on uninhabited Poilão Island are the nearly full moon and the eerie red glow beaming from a few headlamps.

The soft spotlight of a headlamp occasionally falls on a sea turtle and gives the beach scene an other-worldly feel befitting the island’s sacred status. It’s one of a handful of sacred islands dotting the 88-island Bijagós Archipelago off the coast of Guinea-Bissau in West Africa.

The turtles dig their nests, shoveling the beach with the same fore flippers they use to maneuver the ocean. The swoosh of flying sand harmonizes with the soft crash of waves to form the soundtrack to hundreds of sea turtles laying their eggs. Besides bridging the aquatic and terrestrial worlds, the turtles’ ancient physiology seems to bridge a time chasm between past and present. They have, after all, been around for 120 million years.

“I like the turtles,” says Carlito Sedja, his headlamp lighting a rosy path on the beach. “Its very nature fascinates me — how it lives in water and at the same time on land.”

A green sea turtle makes its way back to the Atlantic Ocean after laying eggs on Poilão Island, an important green sea turtle nesting ground in the Bijagós, an 88-island archipelago off the coast of Guinea-Bissau. Image by Ricci Shryock for Mongabay.
Carlito Sedja, a turtle monitor from the Bijagós island of Canhabaque. Image by Ricci Shryock for Mongabay.

Yet another gap is being bridged on the island between the sacred and science. Here on Poilão, workers from the Institute of National Biodiversity and Protected Areas (IBAP), residents of another Bijagós island, and a handful of outside researchers say tradition and technology can work together to reveal the habits of endangered green sea turtles (Chelonia mydas). As part of a project to better protect the turtles by identifying where they forage outside of nesting season, IBAP employs local young men with GPS trackers to monitor the number of nests on the island as well as the movements of adult turtles once they leave Poilão.

Sedja and two other men in their late teens or early twenties step gingerly around a turtle the size of a coffee table as she lays her eggs. Two teams split monitoring of Poilão’s 2-kilometer (1.2-mile) beach tonight; Sedja and his team count more than 150 nesting turtles in less than half an hour on their 1-km (0.6-mi) stretch. The turtles nest on Poilão year round, but most come between June and December, with a peak in August and September.

Project researchers estimate that Poilão hosts around 20,000 nesting females, each of which lays three to five clutches of eggs every two to three years, making the island the most important green sea turtle nesting ground in Africa. Although the species’ population as a whole is decreasing, largely because humans overharvest eggs and turtles and accidentally catch them in fishing nets, IBAP’s research shows the Poilão population is generally holding steady. Females have laid far fewer nests so far in 2021 compared to a record-breaking 60,000 nests in 2020; however, the number of nests has varied widely since IBAP began counting methodically in 2007, as is typical for the species.

A female green sea turtle digs a nest where she will lay eggs on Poilão Island. Image by Ricci Shryock for Mongabay.

Along with the green sea turtles, critically endangered hawksbill (Eretmochelys imbricata) and leatherback sea turtles (Dermochelys coriacea) also nest on Poilão and other Bijagós islands, albeit in lesser numbers. The vulnerable olive ridley (Lepidochelys olivacea) nests on Orango Island in the archipelago.

Sedja bends over and clicks his handheld GPS device to mark the location where the big turtle is laying her eggs.

He and the other turtle monitors are all from four villages on Canhabaque Island, about a two-hour speedboat ride away from Poilão.

For centuries, residents of the four villages have considered Poilão and a few neighboring islands as sacred spaces. On these islands, the communities hold elaborate and secretive coming-of-age ceremonies that are essential to each community’s identity, hierarchy and social order. Until recently, only initiated men could visit Poilão, and even they could not stay long.

These rules preserved the island’s pristine environment. Today, conservationists and scientists recognize Poilão as one of the world’s top five nesting sites for green sea turtles and the most important site in Africa. Around 5% of the global population nests on this tiny 43-hectare (106-acre) island, the southernmost in the archipelago.

“Bijagós [residents] have a special relationship with turtles,” says Castro Barbosa, head of sea turtle monitoring at IBAP. Some of the important cultural events held on the island traditionally called for the killing of a sea turtle — the way cultures around the world eat certain meats to celebrate certain occasions.

A team of green sea turtle monitors ending their shift on Meio Island greet the boat delivering a fresh team. Meio Island is another green sea turtle nesting ground in the Bijagós Archipelago. Image by Ricci Shryock for Mongabay.

In the 1990s, IBAP officials and the Canhabaque communities began negotiations toward establishing the João Vieira-Poilão Marine National Park, which now encompasses Poilão and three other islands, all of which hold sacred spaces for ceremonies, plus three islets. The parties discussed both the killing of turtles and access to the sacred areas. IBAP officials and conservationists hoped the communities could compromise on these points, so they could launch efforts to learn more about and protect the sea turtle population. The negotiations lasted years, says Barbosa, who assisted at the beginning, and they were often difficult because the communities were reluctant to allow outsiders to enter the sacred spaces.

Eventually, the communities agreed to increase access, permitting a small number of workers from the Canhabaque communities that traditionally took care of the islands, as well as park officials, researchers and a limited number of tourists onto the islands. They also agreed to reduce the number of sea turtles used in ceremonies.

In exchange, IBAP officials agreed to hire young men from these communities to monitor the turtles on a part-time basis, an element that was crucial to reaching a deal in an area where paid employment is scarce.

Young women from Canhabaque are traditionally barred from the sacred islands where male coming-of-age ceremonies take place; they have their own sacred areas for performing female rites of passage. This rule still holds. Barbosa jokes that not allowing women on Poilão for centuries kept the ecosystem from being spoiled by would-be settlers from Canhabaque, because no men would have wanted to live there without women.

The negotiations were beneficial to both parties, Barbosa says, and the park opened in 2000. “Our intention was to protect, and that is in the interest of the community, but we must make the people understand [our work] so that they can help us protect what is theirs,” he says.

Carlito Sedja and a colleague measure a green sea turtle on Poilão Island. Image by Ricci Shryock for Mongabay.

Barbosa, along with researchers from other institutions, completed the first nest survey that same year, and GPS-tagged 10 female turtles after they nested in 2001. Between 2018 and 2020, researchers from IBAP and Portugal’s ISPA University Institute satellite-tagged a total of 45 female turtles.

Since the park’s founding, researchers and park officials have worked in tandem with local residents. This began with food subsidies to compensate residents for work identifying and outlining local sacred spaces and ceremonies. Then, in 2010, IBAP began regularly employing Canhabaque youth to count nests and monitor the sites.

The tracking studies showed that the females forage in three main areas: some stayed in the Bijagós, while others went north to the Banc d’Arguin in Mauritania and the Saloum Delta in Senegal.

For the first time this year, the team tagged males as well.

The females are often easier to tag, because they come up on land to nest, whereas the males tend to stay in the water. This year, the researchers relied on fishermen from Canhabaque to gently catch the male turtles so they could place the GPS tags, says Rita Patrício, a sea turtle researcher with ISPA and the University of Exeter in the U.K. who has been working with IBAP for more than a decade. The team managed to tag 12 male turtles, and the fishermen gave them each a name.

One they called Minté, which means “Where are you from?” in the Bijagós language.

Another was Mennei, or “Where are you going?”

And another they named Poor Thing because its shell was slightly wounded.

Carlito Sedja examines a turtle egg that was ejected from its nest when another green sea turtle laid her eggs in the same spot. Image by Ricci Shryock for Mongabay.

Data from the tags will help identify where the male turtles are traveling to feed. “We know where the breeding females are going, but we didn’t know where the males were going,” Patrício says. “This part of the population is particularly important because in the future with climate change there will be less males being born.” Sea turtle gender is determined by the temperature of the sand in which the eggs are incubated; rising temperatures mean warmer beaches, and hence fewer males.

One of the most surprising recent findings from the tagging studies was that a third of the females are nearly resident, venturing only as far as 90 km (56 mi) from Poilão and remaining within the Bijagós.

“That makes the Bijagós even more important. We already knew Bijagós was really important as a breeding space for this species, but now we know it is important year-round as foraging, as well,” Patrício says.

Information about where the turtles travel after breeding and nesting allows researchers and activists to asses threats to the turtles in their foraging grounds, and to determine where protection is needed. The point is to provide the scientific basis for defining the limits of marine protected areas and supporting regulations if needed in the future.

Barbosa says the new data are helping not only scientists but also local residents understand the behavior of the sea turtles. Traditions have often provided the “how” when it comes to conservation methods in the Bijagós, by protecting the spaces from development and the destruction of plants and animals and by teaching the natural value of plants for traditional medicines. Now technology is helping answer the question of “why” conservation is important.

The sun sets over Poilão Island. Image by Ricci Shryock for Mongabay.

Seeing maps showing how some of the turtles travel to other countries to forage when they are not nesting on Poilão gives people a broader understanding of the connectedness of sea life beyond the islands’ borders, Barbosa says. “Now … the people can understand the importance of their island and the turtles even more,” he says. “In the village they talk about that.”

The maps are especially helpful for the residents back on Canhabaque who may not get the chance to work with the turtles, Sedja points out. “There is an evolution,” he says.

Sedja, who says he hopes someday to undergo his initiation ceremony, is confident that tradition and technology can co-exist.

Back on Canhabaque, Sana Mané, the head of the village that traditionally protects Poilão, says he agrees that the opportunity for both increased knowledge and employment for local youths is worth the price of opening access to the sacred island.

“Really I am OK with changing the attitude and the tradition a little bit in search of good things,” he says.

On Poilão, researchers and monitoring teams are limited to the beach. Their camp is a few meters inland, but no one ventures beyond into the sacred forest, where the ceremonies still take place to initiate some young men.

The people of Canhabaque have made compromises, but some secrets remain worth keeping.

Banner image: A team of green sea turtle monitors from Canhabaque Island rides out to the islands where they will complete 12-day shifts. One group will go to the island of Meio and the other to Poilão. Image by Ricci Shryock for Mongabay.


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Godley, B. J., Barbosa, C., Bruford, M., Broderick, A. C., Catry, P., Coyne, M. S., … Witt, M. J. (2010). Unravelling migratory connectivity in marine turtles using multiple methods. Journal of Applied Ecology, 47(4), 769-778. doi:10.1111/j.1365-2664.2010.01817.x

Patrício, A. R., Varela, M. R., Barbosa, C., Broderick, A. C., Catry, P., Hawkes, L. A., … Godley, B. J. (2018). Climate change resilience of a globally important sea turtle nesting population. Global Change Biology, 25(2), 522-535. doi:10.1111/gcb.14520

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